5.111 Lecture Summary #3 Monday, September 8, 2014
Reading for today: Section 1.2 and Section 1.4 with a focus on pgs 10-12 (4th ed or 5th ed).
Read for Lecture 4: Section 1.5 – The Wave-Particle Duality of Matter, and Section
1.6 – The Uncertainty Principle. (Same sections in 4th ed. or 5th ed.)
________________________________________________________________________________
Topics: The wave-particle duality of light
I. Light as a wave, characteristics of waves
II. Light as a particle, the photoelectric effect
________________________________________________________________________________
With the discovery of subatomic particles, the need for a new type of mechanics
(Quantum mechanics) began to emerge. To explain the observations that scientists were
making, two tenets were required: 1. Radiation and matter display both wavelike and
particle-like properties; and 2. Energy is quantized into discrete packets (called
photons).
THE WAVE-PARTICLE DUALITY OF LIGHT
LIGHT AS A WAVE; CHARACTERISTICS OF WAVES
Waves have a periodic variation of some quantity.
Water Wave Sound Wave
+ High + High
level density
Average Average
level density
_ Low _ Low
level density
Light ( radiation) is the periodic variation of an electric field.
We can characterize electromagnetic radiation (or any wave) in terms of:
Amplitude (a): the deviation from an average level. Can have (+) or (-) value.
Wavelength (λ): the between successive m axima or minima
Frequency (ν): the num ber of per unit time
λ
+ positive
amplitude
negative
_ amplitude
1
, 1/ν = = the tim e for one cycle to occur
Units of frequency (ν) : cycles per second =
I ntensity of a wave =
We can calculate the speed of a wave:
Speed = distance traveled / time elapsed = =
Electromagnetic radiation has a constant speed, c (the “speed of light”).
c = λν = ms -1
For any wavelength of light, the product of λ * ν is always c. λ and ν are NOT
independent of each other. If you know λ, you can calculate ν. If you know ν, you can
calculate λ.
The color of visible light waves is determined by their wavelength:
RED has longest λ ~700 nm (7.0 x 10-7 m) and ν ~4.3 x 1014 Hz
ORANGE ~620 nm (6.2 x 10-7 m)
YELLOW ~580 nm (5.8 x 10-7 m)
GREEN ~530 nm (5.3 x 10-7 m)
BLUE ~470 nm (4.7 x 10-7 m)
VIOLET has shortest λ ~420 nm (4.2 x 10-7 m) and highest ν ~7.1 x 10 14 Hz
Visible light is only a small part of the entire electromagnetic spectrum:
radio waves λ = 1 m to 105 m
microwaves λ = 1 mm to 1 m
infrared λ = 750 nm to 1 mm
visible λ = 390 nm to 750 nm
ultraviolet λ = 10 nm to 400 nm
X-rays λ = 0.01 nm 10 nm
gamma-rays λ < 0.02 nm
(You are not responsible for knowing specific wavelength or frequency ranges, but you
should know the relative order of colors and types of waves.)
Waves have the property of superposition
in phase constructive interference out-of-phase destructive interference
2
Reading for today: Section 1.2 and Section 1.4 with a focus on pgs 10-12 (4th ed or 5th ed).
Read for Lecture 4: Section 1.5 – The Wave-Particle Duality of Matter, and Section
1.6 – The Uncertainty Principle. (Same sections in 4th ed. or 5th ed.)
________________________________________________________________________________
Topics: The wave-particle duality of light
I. Light as a wave, characteristics of waves
II. Light as a particle, the photoelectric effect
________________________________________________________________________________
With the discovery of subatomic particles, the need for a new type of mechanics
(Quantum mechanics) began to emerge. To explain the observations that scientists were
making, two tenets were required: 1. Radiation and matter display both wavelike and
particle-like properties; and 2. Energy is quantized into discrete packets (called
photons).
THE WAVE-PARTICLE DUALITY OF LIGHT
LIGHT AS A WAVE; CHARACTERISTICS OF WAVES
Waves have a periodic variation of some quantity.
Water Wave Sound Wave
+ High + High
level density
Average Average
level density
_ Low _ Low
level density
Light ( radiation) is the periodic variation of an electric field.
We can characterize electromagnetic radiation (or any wave) in terms of:
Amplitude (a): the deviation from an average level. Can have (+) or (-) value.
Wavelength (λ): the between successive m axima or minima
Frequency (ν): the num ber of per unit time
λ
+ positive
amplitude
negative
_ amplitude
1
, 1/ν = = the tim e for one cycle to occur
Units of frequency (ν) : cycles per second =
I ntensity of a wave =
We can calculate the speed of a wave:
Speed = distance traveled / time elapsed = =
Electromagnetic radiation has a constant speed, c (the “speed of light”).
c = λν = ms -1
For any wavelength of light, the product of λ * ν is always c. λ and ν are NOT
independent of each other. If you know λ, you can calculate ν. If you know ν, you can
calculate λ.
The color of visible light waves is determined by their wavelength:
RED has longest λ ~700 nm (7.0 x 10-7 m) and ν ~4.3 x 1014 Hz
ORANGE ~620 nm (6.2 x 10-7 m)
YELLOW ~580 nm (5.8 x 10-7 m)
GREEN ~530 nm (5.3 x 10-7 m)
BLUE ~470 nm (4.7 x 10-7 m)
VIOLET has shortest λ ~420 nm (4.2 x 10-7 m) and highest ν ~7.1 x 10 14 Hz
Visible light is only a small part of the entire electromagnetic spectrum:
radio waves λ = 1 m to 105 m
microwaves λ = 1 mm to 1 m
infrared λ = 750 nm to 1 mm
visible λ = 390 nm to 750 nm
ultraviolet λ = 10 nm to 400 nm
X-rays λ = 0.01 nm 10 nm
gamma-rays λ < 0.02 nm
(You are not responsible for knowing specific wavelength or frequency ranges, but you
should know the relative order of colors and types of waves.)
Waves have the property of superposition
in phase constructive interference out-of-phase destructive interference
2
