(a) State the laws of refraction of light. Explain the term absolute refractive index of a medium and write an expression to relate it with the speed of light in vacuum.
(b) The absolute refractive indices of two media 'A' and 'B' are 2.0 and 1.5 respectively. If the speed of light in medium 'B' is , calculate the speed of light in:
(i) vacuum,
(ii) medium 'A'.
Laws of refraction states that:
(1) The incident ray, the refracted ray and the normal to the interface of two media at the point of incidence all lie in the same plane.
(2) For the light of a given color and for given pair of media, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant.
This is also known as Snell's Law.
Mathematically it can be written as:
Here, 
is the refractive index of medium B with respect to medium A.
Refractive index of a medium with respect to the vacuum is known as the absolute refractive index.
c is the speed of light in vacuum and c is the speed of light in medium B.
b) Absolute refractive of medium A, 
=2
Absolute refractive index of medium B, 
=1.5
(a) If the image formed by a lens is diminished in size and erect, for all positions of the object, what type of lens is it?
(b) Name the point on the lens through which a ray of light passes undeviated.
(c) An object is placed perpendicular to the principal axis of a convex lens of focal length 20 cm. The distance of the object from the lens is 30 cm. Find (i) the position (ii) the magnification and (iii) the nature of the image formed.
(a) If the image formed by a lens is diminished in size and erect, for all positions of the object, then the lens is a concave lens.
(b) The point on the lens through which a ray of light passes undeviating is known as Pole.
(c) Given,
Object distance, u = -30 cm
Focal length, f =20 cm
i) Now, using the len’s formula,
(iii) Image formed is inverted.
(a) State the laws of refraction of light. Give an expression to relate the absolute refractive index of a medium with speed of light in vacuum.
(b) The refractive indices of water and glass with respect to air are 4/3 and 3/2 respectively. If the speed of light in glass is 2 × 108 ms−1, find the speed of light in (i) air, (ii) water.(a) Laws of refraction states that:
First law of refraction: The ratio of the sine of the angle of incidence to the sine of the angle of refraction is constant. This is known as Snell’s law.
Mathematically, we have 
Here, nab is the relative refractive index of medium a with respect to medium b.
Second law of refraction: The incident ray, the refracted ray, and the normal to the interface of two media at the point of incidence; all lie in the same plane.
If the light ray goes from medium 1 to 2 then the refractive index of medium 1 with respect to medium 2 is,
where v1 and v2 are the speeds of light in medium 1 and 2 respectively.
Given,
Refractive index of water, nw = 4/3, and
Refractive index of glass, ng= 3/2
Speed of light in glass, vg = 2×108 m/s
i. 
where, na is the refractive index of light in air and va is the speed of light in air. 
is the required speed of light in air.
ii. Speed of light in water is given by,
One-half of a convex lens is covered with a black paper. Will such a lens produce an image of the complete object? Support your answer with a ray diagram.
(b) An object 5 cm high is held 25 cm away from a converging lens of focal length 10 cm.
(i) Draw the ray diagram and
(ii) Calculate the position and size of the image formed.
(iii) What is the nature of the image?
(a) Image is formed by a large number of rays from the object. If one part of the lens is blackened, image will be formed. But, intensity of the image will be reduced.
(b) i)
Object distance, u = -25 cm
Focal length, f = 10 cm
Height of the image, h = 5 cm
ii) Now, using the lens formula,
(iii) Negative sign indicates that the image is real & inverted.
A convex lens can form a magnified erect as well as a magnified inverted image of an object placed in from of it. Draw ray diagram to justify this statement stating the position of the object with respect to the lens in each case.
An object of height 4 cm is placed at a distance of 20 cm from a concave lens of focal length 10 cm. Use lens formula to determine the position of the image formed.
a) When the object is placed between O and F1, magnified erect image is formed. 
b) When the object is placed between F1 and 2F1 , magnified inverted image is formed.
Object distance, u = 20 cm
Image distance, v=?
Focal length, f = 10 cm
According to the sign convention, f = -10 cm and u=-20 cm
Now, using the lens formula,
The image is formed at a distance of 6.6 cm from the lens at the same side where the object is placed.
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