Ray Optics and Optical Instruments · Question 32

Two identical glass ( g = 3/2) equiconvex lenses of focal length f each are kept in contact. The space between the two lenses is filled with water ( w = 4/3). The focal length of the combination is

1. f/3
2. f
3. 4f/3
4. 3f/4

An air bubble in a glass slab with refractive index 1.5 (near normal incidence) is 5 cm deep when viewed from one surface and 3 cm deep when viewed from the opposite face. The thickness (in cm) of the slab is

1. 8
2. 10
3. 12
4. 16

A person can see clearly objects only when they lie between 50 cm and 400 cm from his eyes. In order to increase the maximum distance of distinct vision to infinity, the type and power of the correcting lens, the person has to use, will be

1. convex, +2.25 diopter
2. concave, -0.25 diopter
3. concave, -0.2 diopter
4. convex, +0.15 diopter

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. [figure] The prism will

1. not separate the three colours at all
2. separate the red colour part from the green and blue colours
3. separate the blue colour part from the red and green colours
4. separate all the three colours from one another

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

1. L+IL-I
2. LI
3. LI+1
4. LI-1

Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of refractive index 1.7. The focal length of the combination is

1. -50 cm
2. 50 cm
3. -20 cm
4. -25 cm

The refracting angle of a prism is A, and refractive index of the material of the prism is (A/2). The angle of minimum deviation is

1. 90 - A
2. 180 + 2A
3. 180 - 3A
4. 180 - 2A

If the focal length of objective lens is increased then magnifying power of

1. microscope will increase but that of telescope decrease.
2. microscope and telescope both will increase.
3. microscope and telescope both will decrease.
4. microscope will decrease but that of telescope will increase.

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 return back through the same path after suffering reflection at the silvered surface. The refractive index , of the prism is

1. 2sinA
2. 2cosA
3. 12cosA
4. tanA

A plano 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 1 and 2 and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is

1. R( 1- 2)
2. 2R( 2- 1)
3. R2( 1+ 2)
4. R2( 1- 2)