Two lens of focal length f₁ and f₂ are kept in contact coaxially.The resultant power of combination will be

\(\frac{f_{1}f_{2}}{f_{1}-f_{2}}\)
\(\frac{f_{1}+f_{2}}{f_{1}f_{2}}\)
f₁ + f₂
\(\frac{f_{1}}{f_{2}}+\frac{f_{2}}{f_{1}}\)

Solution

An astronomical telescope has an angular magnification of magnitude 5 for distant objects. The separation between the objective and the eye-piece is 36 cms and the final image is formed at infinity. The focal length f₀ of the objective and f_e of the eye piece are

f₀ = 45 cm and f₂= –9 cm
f₀= 50 cm and f_e= 10 cm
f₀ = 7.2 cm and f_e = 5 cm
f₀ = 30 cm and f_e= 6 cm

Solution

A simple telescope, consisting of an objective of focal length 60 cm and a single eye lens of focal length 5cm is focussed on a distant object in such a way that parallel rays emerge from the eye lens. If the object subtends an angle of 2º at the objective, the angular width of the image is

10º
24º
50º
(1/6)º

Solution

An achromatic convergent lens of focal length 20 cms is made of two lenses(in contact) of materials having dispersive powers in the ratio of 1: 2 and having focal lengths f₁ and f₂
.Which of the following is true ?

f₁ = 10 cms,f₂= –20 cms
f₁ = 20 cms,f₂= 10cms
f₁ = –10 cms,f₂= –20 cms
f₁ = 20 cms,f₂= –20 cms

Solution

A combination is made of two lenses of focal length f and f’in contact, the dispersive powers of the materials of the lenses are wand w’. The combination is achromatic, when

ω = ω₀, ω' = 2ω₀ f'= 2f
ω = ω₀, ω' = 2ω₀ f'= f/2
ω = ω₀, ω' = 2ω₀ f'= –f/2
ω = ω₀, ω' = 2ω₀ f'= –2f

Solution

Two lenses in contact form an achromatic lens. Their focal lengths are in the ratio 2 : 3. Their dispersive powers must be in the ratio of

1 : 3
2 : 3
3 : 2
3 : 1

Solution

A ray of light passes through an equilateral prism(μ= 1.5).The angle of minimum deviation is

45º
37º 12'
20º
30º

Solution

A thin convergent glass lens (μ_g = 1.5) has a power of+ 5.0 D. When this lens is immersed in a liquid of refractive index m, it acts as a divergent lens of focal length 100 cm.The value of m must be

Solution

Two thin lenses are in contact and the focal length of the combination is 80 cm. If the focal length of one lens is 20 cm,then the power of the other lens will be

1.66 D
4.00 D
– 100 D
– 3.75 D

Solution

It is desired to photograph the image of an object placed at a distance of 3 m from plane mirror. The camera,which is at a distance of 4.5 m from mirror should be focussed for a distance of

3 m
4.5 m
6 m
7.5 m

Solution

Distance of image from plane mirror = 3m at the back.To photograph the image, camera must be focussed for a distance of 4.5 + 3 = 7.5 m.

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