Page 96 - Physics - XII

Q11. For a glass prism (µ= 3 , the angle of minimum deviation is equal to the angle of the prism.
)
The angle of the prism is
(a) 45° (b) 30° (c) 90° (d) 60°

Q12. A concave lens is placed in contact with a convex lens of focal length 25 cm. The combination
produces a real image at a distance of 80 cm. If an object is at a distance of 40 cm, the focal
length of concave lens is

(a) – 400 cm (b) – 200 cm (c) + 400 cm (d) + 200 cm
Answer key

1. (c), 2. (a), 3. (c), 4. (b), 5. (c), 6. (c), 7. (c), 8. (a), 9. (c), 10. (b), 11. (d), 12. (a)

Case Study-based Questions

Read the paragraph and answer the questions that follow.

Lens maker’s formula is the relation between the focal length of a lens to the refractive index of its
material and the radii of curvature of its two surfaces. Lens manufacturers use this formula to design
lenses with the necessary focal length from glass with a specifi ed refractive index.

If R and R are the radii of curvature of fi rst and second refracting surfaces of a thin lens of focal length
2
1
f, then lens maker’s formula is

1 n 1 1

f ( 12 1 ) R 1 R 2
where n is refractive index of material of lens with respect to surrounding medium.
1 2
Q1. A biconvex lens has a focal length 2/3 times the radius of curvature of either surface. The
refractive index of the lens material is
(a) 1.75 (b) 1.33 (c) 1.5 (d) 1.0

Q2. A convex lens of focal length 0.2 m and made of glass ( µ = 1.5) is immersed in water
a g
( µ = 1.33). Find the change in the focal length of the lens.
a w
(a) 5.8 m (b) 0.58 cm (c) 0.58 m (d) 5.8 cm
Q3. A thin lens of glass (µ = 1.5) of focal length 10 cm is immersed in water (µ = 1.33). The new
focal length is

(a) 20 cm (b) 40 cm (c) 48 cm (d) 12 cm

Q4. Double convex lenses are to be manufactured from a glass of refractive index 1.55, with
both faces of same radius of curvature. What is the radius of curvature required if the focal
length is to be 20 cm?
(a) 11 cm (b) 22 cm (c) 7 cm (d) 6 cm

Answer key

1. (a), 2. (c), 3. (b), 4. (b)

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