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Light- Reflection and Refraction
Light is a form of energy that produces in us the sensation of sight.

• Reflection of light is the phenomenon of bouncing back of light in the same medium on striking
the surface of any object.
• The two laws of reflection are:
(i) the incident ray, the reflected ray and the normal (at the point of incidence), all lie in the
same plane.
(ii) the angle of reflection (r) is always equal to the angle of incidence (i)
∠r = ∠i
• In a plane mirror, the image of a real object is always
(i) virtual,
(ii) erect
(iii) of same size as the object,
(iv) as far behind the mirror as the object is in front of the mirror.
(v) laterally inverted.
• Absolute refractive index(n) of a medium is the ratio of speed of light in vacuum or air(c) to the
speed of light in the medium(v) i.e.
• Refraction of light is the phenomenon of change in the path of light in going from one medium to
another.
• In going from a rarer to a denser medium, the ray of light bends towards normal and in going from
a denser to a rarer medium, the ray of light bends away from normal.
• Snell’s law of refraction,
• No refraction occurs, when

(i) light is incident normally on a boundary,
(ii) refractive indices of the two media in contact are equal.
• New Cartesian Sign Convention for spherical lenses:

(i) All distances are measured from optical centre C of the lens.
(ii) The distances measured in the direction of incidence of light are taken as positive and vice–
versa.
(iii) All heights above the principal axis of the lens are taken as positive and vice versa.
• The linear magnification produced by a lens is
• Power of the combination of lenses

P = p1 + p2 + p3 ...
• Linear magnification produced by a spherical mirror is

• For a convex mirror, m is +ve and less than one, as the image formed is virtual, erect and shorter
than the object.
• For a concave mirror, m is +ve when image formed is virtual and m is –ve, when image formed is
real.
• According to New Cartesian Sign Convention, for spherical mirror.
(i) All distances are measured from the pole of the spherical mirror.
(ii) The distances measured in the direction of incidence of light are taken as positive and vice–
versa.
(iii) The heights above the principal axis of the mirror are taken as positive and vice–versa.
•
In spherical mirror, focal length
Test Your Knowledge
1. Focal length of plane mirror is

a. At infinity b. Zero c. Negative d. None of these
2. Image formed by plane mirror is
a. Real and erect b. Real and inverted c. Virtual and erect d. Virtual and inverted
3. A concave mirror gives real, inverted and same size image if the object is placed
a. At F b. At infinity c. At C d. Beyond C
4. Power of the lens is -40, its focal length is

a. 4m b. -40m c. -0.25m d. -25m
5. A concave mirror gives virtual, refract and enlarged image of the object but image of smaller size
than the size of the object is
a. At infinity b. Between F and C c. Between P and F d. At E
6. In optics an object which has higher refractive index is called

a. Optically rarer b. Optically denser c. Optical density d. Refractive index
7. The optical phenomena, twinkling of stars, is due to

a. Atmospheric reflection b. Total reflection c. Atmospheric refraction d. Total refraction
8. Convex lens focus a real, point sized image at focus, the object is placed
a. At focus b. Between F and 2F c. At infinity d. At 2F
9. The unit of power of lens is

a. Metre b. Centimeter c. Diopter d. M-1
10. The radius of curvature of a mirror is 20cm the focal length is

a. 20cm b. 10cm c. 40cm d. 5cm
Answers
1.- A , 2. - C, 3-. C , 4-. C, 5. - C, 6.- B, 7.- C , 8.- C, 9.- C, 10.- B
Refraction of Light through Prism
(i) The refraction of light takes place at two surfaces firstly when light enters from air to prism and j
secondly when light emerges from prism.
(ii) Angle of prism: The angle between the two lateral faces of the prism is called angle of prism.
(iii) Angle of deviation: The angle between incident ray (produced forward) and emergent ray I
(produced backward).
• Twinkling of stars
• Since the stars are very distant,
they approximate point-sized
sources of light so the light
coming from them is
considered to be coming
through fairly narrow line. As
Test Your Knowledge
1. Which of the following colours is least scattered by fog, dust or smoke :
A. Violet B. Blue C. Red D. Yellow
2. The coloured light that reflects most while passing through a prism is :
A. yellow B. Violet C. Blue D. Red
3. The effect of glass prism is only to separate the seven colours of

A. White light B. light from bulb C. Sunlight D. All
4. The __ colour is at the top and ____ colour is at the bottom of spectrum.
A. Red, Violet B. Red, Blue C. Violet, red D. None
5. Who discovered the experiments with glass prism that white light consists of seven
colours
A. Newton B. Faraday C. Maxwell D. Young
6. The light that refracts most while passing through a prism

A. Red B. Violet C. Indigo D. Yellow
7. At noon the sun appears white as

A. light is least scattered.
B. all the colours of the white light are scattered away.
C. blue colour is scattered the most.
D. red colour is scattered the most.
8. Which of the following colours is the least deviated on passing through a prism :
A. Red B. Yellow C. Violet D. Indigo
9. The ratio of real depth to apparent depth is called :
A. Refractive index B. Critical angle C. Lateral displacement D. None
10. Which of the following statements is correct regarding the propagation of light
of different colours of white light in air?
A. Red light moves fastest.
B. Blue light moves faster than green light.
C. All the colours of the white light move with the same speed.
D. Yellow light moves with the mean speed as that of the red and the violet light.
Answers
1.- C , 2 -. B, 3-. D , 4.- C, 5 - A, 6-. B, 7.- A , 8.A, 9. - A, 10.- C

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Light- Reflection and Refraction

Light is a form of energy that produces in us the sensation of sight.

• No refraction occurs, when

• New Cartesian Sign Convention for spherical lenses:

• Power of the combination of lenses

• Linear magnification produced by a spherical mirror is

In spherical mirror, focal length

Test Your Knowledge

1. Focal length of plane mirror is

4. Power of the lens is -40, its focal length is

6. In optics an object which has higher refractive index is called

7. The optical phenomena, twinkling of stars, is due to

9. The unit of power of lens is

10. The radius of curvature of a mirror is 20cm the focal length is

Refraction of Light through Prism

3. The effect of glass prism is only to separate the seven colours of

6. The light that refracts most while passing through a prism

7. At noon the sun appears white as

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