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Physics Notes for Class 12 Chapter 9 Ray
Optics and optical Instruments
Light
Light is a form of energy eyes. which produces the Sources of light are of three types-
thermal sources and luminescent sources.
Photometry is a branch measurement of light energy.
Characteristics of Light
Light waves are electromagnetic waves, whose nature is transverse. The speed of light in
vacuum is 3 x 108 mls but it is different in different media.
The speed and wavelength of light change when it travels from one medium to another but its
frequency remains unchanged.
Important Terms
(i) Luminous Objects The objects which emits its own light, are called luminous objects, e.g.,
sun, other stars, an oil lamp etc.
(ii) Non-Luminous Objects The objects which do not emit its own light but become visible
due to the reflection of light falling on them, are called non-luminous objects, e.g., moon, table,
chair. trees etc.
(iii) Ray of Light A straight line drawn in the direction of propagation of light is called a ray of
light.
(iv) Beam of Light A bundle of the adjacent light rays is called a beam of light.
(v) Image If light ray coming from an object meets or appear to meet at a point after reflection
or refraction, then this point is called image of the object.
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(vi) Real Image The image obtained by the real meeting of light rays, is called a real image.
Real image can be obtained on a screen. Real image is inverted.
(vii) Virtual Image The image obtained when light rays are not really meeting but appears to
meet only, is called a virtual image.
Reflection of Light
The rebouncing back of light rays into the same medium on striking a highly polished surface
such as a mirror, is called reflection of light.
Laws of Reflection
There are two laws of reflection.
(i) The incident ray, the reflected ray and the normal at the point of incidence all three lie in the
same plane.
(ii) The angle of incidence (i) is always equal to the angle of reflection (r).
Types of Reflection
(i) Regular Reflection When a parallel beam of reflected light rays is obtained for a parallel
beam of incident light rays after reflection from a plane reflecting reflection is called regular
reflection.
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(ii) Irregular or Diffused Reflection When a non-parallel beam of reflected light rays is
obtained for a parallel beam of incident light rays after reflection from a surface, then such type
of reflection is called irregular or diffused reflection.
Mirror
A smooth and highly polished reflecting surface is called a mirror.
(i) Plane Mirror A highly polished plane surface is called a plane mirror.
Different properties of image formed by plane mirror
Size of image = Size of object
Magnification == Unity
Distance of image == Distance of object
A plane mirror may form a virtual as well as real image.
A man may see his full image in a mirror of half height of man.
When two plane mirror are held at an angle θ, the number of images of an object placed
between them is given as below
(a) n = [(360° / θ) – 1 ], where 360° / θ is an integer.
(b) n = integral part of 360° / θ, when 360° is not an integer.
[A plane mirror may form a real image, when the pencil of light incident on the mirror is
convergent. Children, during their play form an image of sun as wall by a strip of plane mirror.]
Kaleidoscope and periscope employ the principle of image formation by plane mirror.
If keeping an object fixed a plane mirror is rotated in its plane by an angle θ, then the reflected
ray rotates in the same direction by an angle 2 θ.
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Physics Notes for Class 12 Chapter 9 Ray
Optics and optical Instruments
Light
Light is a form of energy eyes. which produces the Sources of light are of three types-
thermal sources and luminescent sources.
Photometry is a branch measurement of light energy.
Characteristics of Light
Light waves are electromagnetic waves, whose nature is transverse. The speed of light in
vacuum is 3 x 108 mls but it is different in different media.
The speed and wavelength of light change when it travels from one medium to another but its
frequency remains unchanged.
Important Terms
(i) Luminous Objects The objects which emits its own light, are called luminous objects, e.g.,
sun, other stars, an oil lamp etc.
(ii) Non-Luminous Objects The objects which do not emit its own light but become visible
due to the reflection of light falling on them, are called non-luminous objects, e.g., moon, table,
chair. trees etc.
(iii) Ray of Light A straight line drawn in the direction of propagation of light is called a ray of
light.
(iv) Beam of Light A bundle of the adjacent light rays is called a beam of light.
(v) Image If light ray coming from an object meets or appear to meet at a point after reflection
or refraction, then this point is called image of the object.
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,2|Page
(vi) Real Image The image obtained by the real meeting of light rays, is called a real image.
Real image can be obtained on a screen. Real image is inverted.
(vii) Virtual Image The image obtained when light rays are not really meeting but appears to
meet only, is called a virtual image.
Reflection of Light
The rebouncing back of light rays into the same medium on striking a highly polished surface
such as a mirror, is called reflection of light.
Laws of Reflection
There are two laws of reflection.
(i) The incident ray, the reflected ray and the normal at the point of incidence all three lie in the
same plane.
(ii) The angle of incidence (i) is always equal to the angle of reflection (r).
Types of Reflection
(i) Regular Reflection When a parallel beam of reflected light rays is obtained for a parallel
beam of incident light rays after reflection from a plane reflecting reflection is called regular
reflection.
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, 3|Page
(ii) Irregular or Diffused Reflection When a non-parallel beam of reflected light rays is
obtained for a parallel beam of incident light rays after reflection from a surface, then such type
of reflection is called irregular or diffused reflection.
Mirror
A smooth and highly polished reflecting surface is called a mirror.
(i) Plane Mirror A highly polished plane surface is called a plane mirror.
Different properties of image formed by plane mirror
Size of image = Size of object
Magnification == Unity
Distance of image == Distance of object
A plane mirror may form a virtual as well as real image.
A man may see his full image in a mirror of half height of man.
When two plane mirror are held at an angle θ, the number of images of an object placed
between them is given as below
(a) n = [(360° / θ) – 1 ], where 360° / θ is an integer.
(b) n = integral part of 360° / θ, when 360° is not an integer.
[A plane mirror may form a real image, when the pencil of light incident on the mirror is
convergent. Children, during their play form an image of sun as wall by a strip of plane mirror.]
Kaleidoscope and periscope employ the principle of image formation by plane mirror.
If keeping an object fixed a plane mirror is rotated in its plane by an angle θ, then the reflected
ray rotates in the same direction by an angle 2 θ.
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