Waves Revision
9th March 2026
DO NOW
1. Explain, in words, the difference between Transverse and Longitudinal waves.
2. Name examples of each
3. What are the 3 key properties of waves
4. What is the wave equation and its units?
5. What does ‘polarised’ light mean?
Challenge : Explain some applications of polarisation
What is diffraction?
• What is diffraction? Use a diagram to help you
• What is the amount of diffraction dependent on?
• Explain how diffraction affect the TV reception homes receive
What is interference?
• Explain how two waves can interfere with each other, using the following words: phase, constructive
/ destructive, amplitude
• What is the definition of light intensity?
• Explain why the fringe pattern for two waves
interfering looks like this:
• What is the difference in the fringes formed between diffraction of white light and monochromatic
light?
What is single and double-slit interference?
• Write down the double-slit formula and define each part
• How can interference happen with a single slit as well as a double slit?
• Explain using n and λ , the conditions needed for constructive and destructive interference to occur.
A point that is 2.4 m away from the first slit and 2.2 m away from the second slit. If the wavelength is 0.4 m,
what kind of superposition will be there?
A point lies on the fourth maximum D, that is 20 cm away from slit one and 12 cm away from slit two. What
is the wavelength?
What is a diffraction grating?
• Label each part of this formula and describe how you could measure each part
• How would you work out the maximum order, n, of a diffraction grating?
• How can you work out the % error in d, the line spacing?
• Make a list of all the factors that affect the width of the fringes, and how each factor affects it.
What is refraction?
• Describe and Explain what refraction is, using a labelled diagram.
• Write down Snell’s law, and label each part.
• How does the ratio of the speeds of the wave relate to Snell’s Law?
What is the critical angle?
Use Snell’s Law to derive the critical angle (Think: what angle does the refracted ray emerge at)
What is Total Internal Refection?
Draw it on the diagram above
What are optical fibres?
• Explain how an optical fibre works.
• What are the uses and advantages of an optical fibre?
• What is the purpose of the cladding? What refractive index does it have?
• Describe the different ways signal can be affected inside an optical fibre, and how these problems
can be addressed.
EXAM PRACTISE
Q1.
This question is about the measurement of the wavelength of laser light.
The light is shone onto a diffraction grating at normal incidence.
The light transmitted by the diffraction grating produces five spots on a screen. These spots are
labelled A to E in Figure 1.
Figure 1
A student uses a metre ruler with 1 mm divisions to take readings. He uses these readings to
obtain measurements a, b and c, the distances between centres of the spots as shown in
Figure 1.
Table 1 shows his measurements and his estimated uncertainties.
Table 1
Measurement Distance / mm Uncertainty / mm
a 289 2
b 255 2
c 544 2
(a) Explain why the student’s estimated uncertainty in measurement a is greater than the
smallest division on the metre ruler.
You should refer to the readings taken by the student in obtaining this measurement.
___________________________________________________________________
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___________________________________________________________________
___________________________________________________________________
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___________________________________________________________________
___________________________________________________________________
(2)
(b) The distance between the centres of spots A and C and the distance between the centres
of spots C and E are equal.
That is:
a+b=c
Calculate the percentage uncertainty in the sum of a and b.
percentage uncertainty = ____________________
(2)
(c) Discuss why the experimental measurements lead to a different percentage uncertainty in
c compared to that in a + b.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(d) Eye protection should be used to prevent eye damage when using a laser.
Describe one other safety measure to minimise the risk of eye damage when using a
laser in the laboratory.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(1)
(e) Figure 2 shows the experimental arrangement with y, the perpendicular distance
between the diffraction grating and the screen, equal to 1.280 m.
Table 2 shows some of the data from Table 1.
Table 2
Measurement Distance / mm
a 289
b 255
c 544
Figure 2
Calculate the angle θ shown on Figure 2.
θ = ____________________ degrees
(1)
(f) Spot E is the second-order maximum.
The diffraction grating has 3.00 × 105 lines per metre.
Calculate the wavelength of the laser light.
wavelength = ____________________ m
(1)
(g) The student plans to repeat the experiment using the same diffraction grating and laser.
State and explain one way the student can change the experimental arrangement to
reduce the percentage uncertainty in the measurement of the wavelength.
Assume the percentage uncertainty in sin θ is the sum of the percentage uncertainties in
y and c.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(Total 12 marks)
Q2.
A student removes the reflective layer from a DVD. She uses the DVD as a transmission
diffraction grating.
Figure 1 shows light from a laser pointer incident normally on a small section of this diffraction
grating. The grooves on this section act as adjacent slits of the transmission diffraction grating.
A vertical pattern of bright spots (maxima) is observed on a circular screen behind the disc.
Figure 1
(a) Light of wavelength λ travels from each illuminated slit, producing maxima on the screen.
State the path difference between light from adjacent slits when this light produces a first-
order maximum on the screen.
___________________________________________________________________
(1)
(b) Explain how light from the diffraction grating forms a maximum on the screen.
___________________________________________________________________
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___________________________________________________________________
(3)
The student has three discs: a Blu-ray disc, a DVD and a CD. She removes the reflective
coating from the discs so that they act as transmission diffraction gratings. These diffraction
gratings have different slit spacings.
The student also has two laser pointers A and B that emit different colours of visible light.
Table 1 and Table 2 show information about the discs and the laser pointers.
Table 1
Disc Slit spacing / µm
Blu-ray disc 0.32
DVD 0.74
CD 1.60
Table 2
Laser pointer Wavelength of light emitted / 10−7 m
A 4.45
B 6.36
(c) Deduce the combination of disc and laser pointer that will produce the greatest possible
number of interference maxima.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(d) The student uses the CD and laser pointer B as shown in Figure 2. A diffraction pattern
is produced on the screen. Laser pointer B and the CD are in fixed positions. The laser beam is
horizontal and incident normally on the CD. The height of the screen can be adjusted.
Figure 2
The screen has a diameter of 30 cm and is positioned behind the CD at a fixed horizontal
distance of 15 cm.
The student plans to adjust the height of the screen until she observes the greatest
number of spots.
The student predicts that, using this arrangement, the greatest number of spots on the
screen will be 3.
Determine whether the student’s prediction is correct.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(3)
(Total 9 marks)
Q3.
The diagram shows Young’s double-slit experiment performed with a tungsten filament lamp as
the light source.
(a) On the axes in the diagram above, sketch a graph to show how the intensity varies with
position for a monochromatic light source.
(2)
(b) (i) For an interference pattern to be observed the light has to be emitted by two
coherent sources.
Explain what is meant by coherent sources.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(1)
(ii) Explain how the use of the single slit in the arrangement above makes the light from
the two slits sufficiently coherent for fringes to be observed.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(1)
(iii) In this experiment light behaves as a wave.
Explain how the bright fringes are formed.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(3)
(c) (i) A scientist carries out the Young double-slit experiment using a laser that emits
violet light of wavelength 405 nm. The separation of the slits is 5.00 × 10–5 m.
Using a metre ruler the scientist measures the separation of two adjacent bright
fringes in the central region of the pattern to be 4 mm.
Calculate the distance between the double slits and the screen.
distance = ____________________ m
(2)
(ii) Describe the change to the pattern seen on the screen when the violet laser is
replaced by a green laser. Assume the brightness of the central maximum is the same for
both lasers.
______________________________________________________________
______________________________________________________________
______________________________________________________________
(1)
(iii) The scientist uses the same apparatus to measure the wavelength of visible
electromagnetic radiation emitted by another laser.
Describe how he should change the way the apparatus is arranged and used in order to
obtain an accurate value for the wavelength.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(3)
(Total 13 marks)
Q4.
(a) State what is meant by coherent sources of light.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(b)
Figure 1
Young’s fringes are produced on the screen from the monochromatic source by the
arrangement shown in Figure 1.
You may be awarded marks for the quality of written communication in your answers.
(i) Explain why slit S should be narrow.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(ii) Why do slits S1 and S2 act as coherent sources?
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(4)
(c) The pattern on the screen may be represented as a graph of intensity against position on
the screen. The central fringe is shown on the graph in Figure 2. Complete this graph to
represent the rest of the pattern by drawing on Figure 2.
Figure 2
(2)
(Total 8 marks)
Q5.
Figure 1 shows two prisms A and B of different refractive indices joined to make a block.
A ray of monochromatic light is shown entering and then leaving the block.
Figure 1
(a) Complete, on Figure 1, the path of the ray of light inside the block.
(1)
(b) Deduce which prism, A or B, has the greater refractive index.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
The block is used with a telescope to investigate stars.
The block can be replaced with a diffraction grating.
(c) Describe one non-astronomical application of a diffraction grating.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(1)
(d) Figure 2 shows a spectrum of light. Two lines in the spectrum are labelled X and Y.
Figure 2
The light passes at normal incidence through a diffraction grating. The number of lines
per metre for the grating is G.
The first-order diffraction angle of X is at 28.2° to the normal.
Calculate G.
G = ____________________ m−1
(3)
(e) A scientist wants to obtain an accurate value for the difference in wavelength between
line X and line Y.
She has two options:
• option 1: to analyse the second-order spectrum from the original grating
• option 2: to analyse the first-order spectrum from a grating with 2G lines per metre.
Discuss which option she should choose.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(3)
(Total 10 marks)
Q6.
In 1870 John Tyndall sent a beam of light along a stream of water.
Figure 1 shows a modern version of Tyndall’s experiment using a laser beam.
Water has a refractive index of 1.33
Figure 1
(a) Explain why the laser beam stays inside the stream of water.
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___________________________________________________________________
(2)
(b) Calculate the speed of the laser light in the water.
Give your answer to an appropriate number of significant figures.
speed = ____________________ m s–1
(3)
(c) Calculate the critical angle for the water−air boundary.
critical angle = ____________________ degrees
(1)
(d) Tyndall’s experiment led to the development of optical fibres.
Figure 2 shows a step-index optical fibre.
Figure 2
Discuss the properties of a step-index optical fibre.
Your answer should include:
• the names of part X and part Y
• a description of the functions of X and Y
• a discussion of the problems caused by material dispersion and modal dispersion
and how these problems can be overcome.
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(6)
(e) Scientists use optical fibres to monitor earthquakes. Light travelling through an optical
fibre can be reflected by impurities in the fibre, as shown in Figure 3.
Figure 3
Earthquakes bend the optical fibre slightly, as shown in Figure 4. This changes the
amount of reflected light.
Figure 4
Suggest why the amount of reflected light changes as the fibre bends.
You may draw on Figure 4 as part of your answer.
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(2)
(f) The waves caused by earthquakes can be longitudinal or transverse.
Describe the difference between longitudinal waves and transverse waves.
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(2)
(Total 16 marks)
Q7.
Figure 1 shows a type of refractometer.
A semi-circular glass block is arranged so that its semi-circular faces are vertical. A drop of
liquid is placed at the centre of the flat horizontal surface of the block.
Figure 1
Light enters the block through the curved surface and is incident on the midpoint of the
horizontal surface at angle of incidence θ.
Light that reflects at the glass–liquid boundary is detected on a screen that lies parallel to the
horizontal surface.
(a) Explain why the light ray in Figure 1 does not change direction as it enters the block.
___________________________________________________________________
___________________________________________________________________
(1)
(b) The refractometer is calibrated using a drop of liquid.
When θ = 15°, light is partially refracted at the glass–liquid boundary.
Calculate the angle of refraction at this boundary.
refractive index of glass block = 1.84
refractive index of liquid = 1.33
angle of refraction = ____________________ °
(2)
The refractometer is used to determine the critical angle θc at the glass–liquid boundary.
Figure 2 shows dimensions of the arrangement.
Figure 2
The intensity of the light ray on the screen is observed as θ is increased from 15°. When θ = θc
the intensity of the light ray is seen to increase sharply at a point T on the screen.
The distance between the left-hand edge of the screen and T is x.
(c) Explain why the intensity of the light ray on the screen increases at T.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(d) The liquid is replaced with a drop of sugar solution.
The refractive index of the sugar solution is greater than 1.33
Deduce how this change affects the position at which the sharp increase in intensity is
observed on the screen.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(e) The refractometer in Figure 2 is used to determine the concentration of a sugar solution.
Figure 3 shows the variation of refractive index with concentration of sugar solution.
Figure 3
For a drop of a particular sugar solution, x = 69 mm.
Determine the percentage concentration of the sugar solution.
refractive index of glass block = 1.84
percentage concentration = ____________________
(3)
(Total 10 marks)
Q8.
Porro prisms are used in binoculars to reverse the path of the light. The prism is in the shape of
a right-angled isosceles triangle.
Figure 1 shows a ray of light, at normal incidence on the longest side, passing through a glass
Porro prism.
Figure 1
The critical angle for light in the prism is 41.5°.
(a) Show that the glass used to make the prism has a refractive index of about 1.5
(1)
(b) Explain why the ray emerges parallel to the incident ray.
___________________________________________________________________
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___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
Figure 2 shows a ray of light entering the prism at an angle of incidence θ and reflecting off
one of the shorter sides.
Figure 2
θ is the largest angle of incidence for which all of the light leaves through the longest side.
(c) Draw on Figure 2 the path of the ray of light as it continues inside the prism and emerges
from the longest side.
(3)
(d) When the angle of incidence is greater than θ, some of the light escapes the prism
through one of the shorter sides.
Assume that the refractive index is 1.5 and the critical angle is 41.5°.
Show that θ is about 5°.
You can use Figure 2 in your answer.
(4)
(e) A manufacturer wants to make a prism with a larger value of θ.
Two alternative changes to the original design of the prism are suggested:
1. use a prism of the original glass in the shape of an equilateral triangle, as shown in
Figure 3
2. use a prism of the original shape made from glass with a smaller refractive index,
as shown in Figure 4.
Figure 3 Figure 4
Discuss whether either of the two suggestions would work.
1 _________________________________________________________________
___________________________________________________________________
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___________________________________________________________________
2 _________________________________________________________________
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(4)
(Total 14 marks)
9th March 2026
DO NOW
1. Explain, in words, the difference between Transverse and Longitudinal waves.
2. Name examples of each
3. What are the 3 key properties of waves
4. What is the wave equation and its units?
5. What does ‘polarised’ light mean?
Challenge : Explain some applications of polarisation
What is diffraction?
• What is diffraction? Use a diagram to help you
• What is the amount of diffraction dependent on?
• Explain how diffraction affect the TV reception homes receive
What is interference?
• Explain how two waves can interfere with each other, using the following words: phase, constructive
/ destructive, amplitude
• What is the definition of light intensity?
• Explain why the fringe pattern for two waves
interfering looks like this:
• What is the difference in the fringes formed between diffraction of white light and monochromatic
light?
What is single and double-slit interference?
• Write down the double-slit formula and define each part
• How can interference happen with a single slit as well as a double slit?
• Explain using n and λ , the conditions needed for constructive and destructive interference to occur.
A point that is 2.4 m away from the first slit and 2.2 m away from the second slit. If the wavelength is 0.4 m,
what kind of superposition will be there?
A point lies on the fourth maximum D, that is 20 cm away from slit one and 12 cm away from slit two. What
is the wavelength?
What is a diffraction grating?
• Label each part of this formula and describe how you could measure each part
• How would you work out the maximum order, n, of a diffraction grating?
• How can you work out the % error in d, the line spacing?
• Make a list of all the factors that affect the width of the fringes, and how each factor affects it.
What is refraction?
• Describe and Explain what refraction is, using a labelled diagram.
• Write down Snell’s law, and label each part.
• How does the ratio of the speeds of the wave relate to Snell’s Law?
What is the critical angle?
Use Snell’s Law to derive the critical angle (Think: what angle does the refracted ray emerge at)
What is Total Internal Refection?
Draw it on the diagram above
What are optical fibres?
• Explain how an optical fibre works.
• What are the uses and advantages of an optical fibre?
• What is the purpose of the cladding? What refractive index does it have?
• Describe the different ways signal can be affected inside an optical fibre, and how these problems
can be addressed.
EXAM PRACTISE
Q1.
This question is about the measurement of the wavelength of laser light.
The light is shone onto a diffraction grating at normal incidence.
The light transmitted by the diffraction grating produces five spots on a screen. These spots are
labelled A to E in Figure 1.
Figure 1
A student uses a metre ruler with 1 mm divisions to take readings. He uses these readings to
obtain measurements a, b and c, the distances between centres of the spots as shown in
Figure 1.
Table 1 shows his measurements and his estimated uncertainties.
Table 1
Measurement Distance / mm Uncertainty / mm
a 289 2
b 255 2
c 544 2
(a) Explain why the student’s estimated uncertainty in measurement a is greater than the
smallest division on the metre ruler.
You should refer to the readings taken by the student in obtaining this measurement.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(b) The distance between the centres of spots A and C and the distance between the centres
of spots C and E are equal.
That is:
a+b=c
Calculate the percentage uncertainty in the sum of a and b.
percentage uncertainty = ____________________
(2)
(c) Discuss why the experimental measurements lead to a different percentage uncertainty in
c compared to that in a + b.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(d) Eye protection should be used to prevent eye damage when using a laser.
Describe one other safety measure to minimise the risk of eye damage when using a
laser in the laboratory.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(1)
(e) Figure 2 shows the experimental arrangement with y, the perpendicular distance
between the diffraction grating and the screen, equal to 1.280 m.
Table 2 shows some of the data from Table 1.
Table 2
Measurement Distance / mm
a 289
b 255
c 544
Figure 2
Calculate the angle θ shown on Figure 2.
θ = ____________________ degrees
(1)
(f) Spot E is the second-order maximum.
The diffraction grating has 3.00 × 105 lines per metre.
Calculate the wavelength of the laser light.
wavelength = ____________________ m
(1)
(g) The student plans to repeat the experiment using the same diffraction grating and laser.
State and explain one way the student can change the experimental arrangement to
reduce the percentage uncertainty in the measurement of the wavelength.
Assume the percentage uncertainty in sin θ is the sum of the percentage uncertainties in
y and c.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(Total 12 marks)
Q2.
A student removes the reflective layer from a DVD. She uses the DVD as a transmission
diffraction grating.
Figure 1 shows light from a laser pointer incident normally on a small section of this diffraction
grating. The grooves on this section act as adjacent slits of the transmission diffraction grating.
A vertical pattern of bright spots (maxima) is observed on a circular screen behind the disc.
Figure 1
(a) Light of wavelength λ travels from each illuminated slit, producing maxima on the screen.
State the path difference between light from adjacent slits when this light produces a first-
order maximum on the screen.
___________________________________________________________________
(1)
(b) Explain how light from the diffraction grating forms a maximum on the screen.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(3)
The student has three discs: a Blu-ray disc, a DVD and a CD. She removes the reflective
coating from the discs so that they act as transmission diffraction gratings. These diffraction
gratings have different slit spacings.
The student also has two laser pointers A and B that emit different colours of visible light.
Table 1 and Table 2 show information about the discs and the laser pointers.
Table 1
Disc Slit spacing / µm
Blu-ray disc 0.32
DVD 0.74
CD 1.60
Table 2
Laser pointer Wavelength of light emitted / 10−7 m
A 4.45
B 6.36
(c) Deduce the combination of disc and laser pointer that will produce the greatest possible
number of interference maxima.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(d) The student uses the CD and laser pointer B as shown in Figure 2. A diffraction pattern
is produced on the screen. Laser pointer B and the CD are in fixed positions. The laser beam is
horizontal and incident normally on the CD. The height of the screen can be adjusted.
Figure 2
The screen has a diameter of 30 cm and is positioned behind the CD at a fixed horizontal
distance of 15 cm.
The student plans to adjust the height of the screen until she observes the greatest
number of spots.
The student predicts that, using this arrangement, the greatest number of spots on the
screen will be 3.
Determine whether the student’s prediction is correct.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(3)
(Total 9 marks)
Q3.
The diagram shows Young’s double-slit experiment performed with a tungsten filament lamp as
the light source.
(a) On the axes in the diagram above, sketch a graph to show how the intensity varies with
position for a monochromatic light source.
(2)
(b) (i) For an interference pattern to be observed the light has to be emitted by two
coherent sources.
Explain what is meant by coherent sources.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(1)
(ii) Explain how the use of the single slit in the arrangement above makes the light from
the two slits sufficiently coherent for fringes to be observed.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(1)
(iii) In this experiment light behaves as a wave.
Explain how the bright fringes are formed.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(3)
(c) (i) A scientist carries out the Young double-slit experiment using a laser that emits
violet light of wavelength 405 nm. The separation of the slits is 5.00 × 10–5 m.
Using a metre ruler the scientist measures the separation of two adjacent bright
fringes in the central region of the pattern to be 4 mm.
Calculate the distance between the double slits and the screen.
distance = ____________________ m
(2)
(ii) Describe the change to the pattern seen on the screen when the violet laser is
replaced by a green laser. Assume the brightness of the central maximum is the same for
both lasers.
______________________________________________________________
______________________________________________________________
______________________________________________________________
(1)
(iii) The scientist uses the same apparatus to measure the wavelength of visible
electromagnetic radiation emitted by another laser.
Describe how he should change the way the apparatus is arranged and used in order to
obtain an accurate value for the wavelength.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(3)
(Total 13 marks)
Q4.
(a) State what is meant by coherent sources of light.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
(b)
Figure 1
Young’s fringes are produced on the screen from the monochromatic source by the
arrangement shown in Figure 1.
You may be awarded marks for the quality of written communication in your answers.
(i) Explain why slit S should be narrow.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(ii) Why do slits S1 and S2 act as coherent sources?
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(4)
(c) The pattern on the screen may be represented as a graph of intensity against position on
the screen. The central fringe is shown on the graph in Figure 2. Complete this graph to
represent the rest of the pattern by drawing on Figure 2.
Figure 2
(2)
(Total 8 marks)
Q5.
Figure 1 shows two prisms A and B of different refractive indices joined to make a block.
A ray of monochromatic light is shown entering and then leaving the block.
Figure 1
(a) Complete, on Figure 1, the path of the ray of light inside the block.
(1)
(b) Deduce which prism, A or B, has the greater refractive index.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(2)
The block is used with a telescope to investigate stars.
The block can be replaced with a diffraction grating.
(c) Describe one non-astronomical application of a diffraction grating.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(1)
(d) Figure 2 shows a spectrum of light. Two lines in the spectrum are labelled X and Y.
Figure 2
The light passes at normal incidence through a diffraction grating. The number of lines
per metre for the grating is G.
The first-order diffraction angle of X is at 28.2° to the normal.
Calculate G.
G = ____________________ m−1
(3)
(e) A scientist wants to obtain an accurate value for the difference in wavelength between
line X and line Y.
She has two options:
• option 1: to analyse the second-order spectrum from the original grating
• option 2: to analyse the first-order spectrum from a grating with 2G lines per metre.
Discuss which option she should choose.
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
___________________________________________________________________
(3)
(Total 10 marks)
Q6.
In 1870 John Tyndall sent a beam of light along a stream of water.
Figure 1 shows a modern version of Tyndall’s experiment using a laser beam.
Water has a refractive index of 1.33
Figure 1
(a) Explain why the laser beam stays inside the stream of water.
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(2)
(b) Calculate the speed of the laser light in the water.
Give your answer to an appropriate number of significant figures.
speed = ____________________ m s–1
(3)
(c) Calculate the critical angle for the water−air boundary.
critical angle = ____________________ degrees
(1)
(d) Tyndall’s experiment led to the development of optical fibres.
Figure 2 shows a step-index optical fibre.
Figure 2
Discuss the properties of a step-index optical fibre.
Your answer should include:
• the names of part X and part Y
• a description of the functions of X and Y
• a discussion of the problems caused by material dispersion and modal dispersion
and how these problems can be overcome.
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(6)
(e) Scientists use optical fibres to monitor earthquakes. Light travelling through an optical
fibre can be reflected by impurities in the fibre, as shown in Figure 3.
Figure 3
Earthquakes bend the optical fibre slightly, as shown in Figure 4. This changes the
amount of reflected light.
Figure 4
Suggest why the amount of reflected light changes as the fibre bends.
You may draw on Figure 4 as part of your answer.
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(2)
(f) The waves caused by earthquakes can be longitudinal or transverse.
Describe the difference between longitudinal waves and transverse waves.
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(2)
(Total 16 marks)
Q7.
Figure 1 shows a type of refractometer.
A semi-circular glass block is arranged so that its semi-circular faces are vertical. A drop of
liquid is placed at the centre of the flat horizontal surface of the block.
Figure 1
Light enters the block through the curved surface and is incident on the midpoint of the
horizontal surface at angle of incidence θ.
Light that reflects at the glass–liquid boundary is detected on a screen that lies parallel to the
horizontal surface.
(a) Explain why the light ray in Figure 1 does not change direction as it enters the block.
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(1)
(b) The refractometer is calibrated using a drop of liquid.
When θ = 15°, light is partially refracted at the glass–liquid boundary.
Calculate the angle of refraction at this boundary.
refractive index of glass block = 1.84
refractive index of liquid = 1.33
angle of refraction = ____________________ °
(2)
The refractometer is used to determine the critical angle θc at the glass–liquid boundary.
Figure 2 shows dimensions of the arrangement.
Figure 2
The intensity of the light ray on the screen is observed as θ is increased from 15°. When θ = θc
the intensity of the light ray is seen to increase sharply at a point T on the screen.
The distance between the left-hand edge of the screen and T is x.
(c) Explain why the intensity of the light ray on the screen increases at T.
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(2)
(d) The liquid is replaced with a drop of sugar solution.
The refractive index of the sugar solution is greater than 1.33
Deduce how this change affects the position at which the sharp increase in intensity is
observed on the screen.
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(2)
(e) The refractometer in Figure 2 is used to determine the concentration of a sugar solution.
Figure 3 shows the variation of refractive index with concentration of sugar solution.
Figure 3
For a drop of a particular sugar solution, x = 69 mm.
Determine the percentage concentration of the sugar solution.
refractive index of glass block = 1.84
percentage concentration = ____________________
(3)
(Total 10 marks)
Q8.
Porro prisms are used in binoculars to reverse the path of the light. The prism is in the shape of
a right-angled isosceles triangle.
Figure 1 shows a ray of light, at normal incidence on the longest side, passing through a glass
Porro prism.
Figure 1
The critical angle for light in the prism is 41.5°.
(a) Show that the glass used to make the prism has a refractive index of about 1.5
(1)
(b) Explain why the ray emerges parallel to the incident ray.
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(2)
Figure 2 shows a ray of light entering the prism at an angle of incidence θ and reflecting off
one of the shorter sides.
Figure 2
θ is the largest angle of incidence for which all of the light leaves through the longest side.
(c) Draw on Figure 2 the path of the ray of light as it continues inside the prism and emerges
from the longest side.
(3)
(d) When the angle of incidence is greater than θ, some of the light escapes the prism
through one of the shorter sides.
Assume that the refractive index is 1.5 and the critical angle is 41.5°.
Show that θ is about 5°.
You can use Figure 2 in your answer.
(4)
(e) A manufacturer wants to make a prism with a larger value of θ.
Two alternative changes to the original design of the prism are suggested:
1. use a prism of the original glass in the shape of an equilateral triangle, as shown in
Figure 3
2. use a prism of the original shape made from glass with a smaller refractive index,
as shown in Figure 4.
Figure 3 Figure 4
Discuss whether either of the two suggestions would work.
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(4)
(Total 14 marks)
