In an astronomical telescope in normal adjustment a straight black line of length L is drawn on inside part of the objective lens. The eye-piece forms a real image of this line. The length of this image is l. The magnification of the telescope is:

(a)L/l+1

(b)L/l-1

(c)L+1/L-1

(d)L/l

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A beam of light consisting of red, green and blue colours is incident on a right angled prism. The refractive index of the material of the prism for the above red, green and blue wavelengths are 1.39, 1.44 and 1.47, respectively.


The prism will

(a) separate the blue colour part from the red and green colours

(b) separate all the three colours from one another

(c) not separate the three colours at all

(d) separate the red colour part from the green and blue colours

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Total internal reflection
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If the focal length of the objective lens is increased, then magnifying power of :

(a) microscope will increase but that of telescope decrease

(b) microscope and telescope both will increase

(c) microscope and telescope both will decrease

(d) microscope will decrease but that of telescope will increase

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The angle of a prism is A. One of its refracting surfaces is silvered. Light rays falling at an angle of incidence 2A on the first surface returns back through the same path after suffering reflection at the silvered surface. The refractive index μ of the prism is

(a)2sinA

(b)2cosA

(c)1/2cosA

(d)tanA

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A plane-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices in  μ1 and μ2 and R is the radius of curvature of the curved surface of the lenses. then the focal length of the combination is

(a) R/2(μ1+μ2)

(b) R/2(μ1-μ2)

(c) R/(μ1-μ2)

(d) 2R/(μ2-μ1)

Concept Questions :-

Lensmakers' formula
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For a normal eye, the cornea of eye provides a converging power of 40 D and the least converging power of the eye lens behind the cornea is 20 D. Using this information, the distance between the retina and the cornea-eye lens can be estimated to be

(a) 5 cm

(b) 25 cm

(c) 1.67 cm

(d) 1.5 cm

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When a biconvex lens of glass having refractive index 1.47 is dipped in a liquid, it acts as a plane sheet of glass. This implies that the liquid must have refractive index 

(a)equal to that of glass

(b)less than one

(c)greater than that of glass 

(d)less than that of glass

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A ray of light is incident at an angle of incidence, i, on one face of a prism of angle A (assumed to be small) and emerges normally from the opposite face.If the refractive index of the prism is μ, the angle of incidence i, is nearly equal to

(a)μA                                    (b)μA2

(c)A/μ                                  (d)A/2μ

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A concave mirror of focal length f1 is placed at a distance of d from a convex lens of focal length f2. A beam of light coming from infinity and falling on this convex lens concave mirror combination returns to infinity. The distance d must be equal

(a)f1+f2                                 (b)-f1+f2

(c)2f1+f2                                (d)-2f1+f2

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The magnifying power of a telescope is 9. When it is adjusted for parallel rays the distance between the objective and eyepiece is 20cm. The focal length of lenses are 

(a) 10cm,10cm

(b) 15cm,5cm

(c) 18cm,2cm

(d) 11cm,9cm

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