Ray Optics and Optical Instruments · Question 11

Two thin lenses are of same focal lengths (f), but one is convex and the other one is concave. When they are placed in contact with each other, the equivalent focal length of the combination will be :

1. f/4
2. f/2
3. Infinite
4. Zero

A biconvex lens has radii of curvature, 20 cm each. If the refractive index of the material of the lens is 1.5, the power of the lens is

1. +2 D
2. +20 D
3. +5 D
4. Infinity

Two transparent media A and B are separated by a plane boundary. The speed of light in those media are 1.5 108 m/s and 2.0 108 m/s, respectively. The critical angle for a ray of light for these two media is

1. -1(0.500)
2. -1(0.750)
3. -1(0.500)
4. -1(0.750)

During a cloudy day, a primary and a secondary rainbow may be created, then the :

1. secondary rainbow is due to single internal reflection and is formed above the primary one.
2. primary rainbow is due to double internal reflection and is formed above the secondary one.
3. primary rainbow is due to double internal reflection and is formed below the secondary one.
4. secondary rainbow is due to double internal reflection and is formed above the primary one.

An astronomical refracting telescope is being used by an observer to observe planets in normal adjustment. The focal lengths of the objective and eye piece used in the construction of the telescope are 20 m and 2 cm respectively. Consider the following statements about the telescope : The correct statements are :

1. A, B and D only
2. A, B and C only
3. B, C and D only
4. C, D and A only

Find the value of the angle of emergence from the prism. Refractive index of the glass is 3 [figure]

1. 90
2. 60
3. 30
4. 45

A convex lens 'A' of focal length 20 cm and a concave lens 'B' of focal length 5 cm are kept along the same axis with a distance 'd' between them. If a parallel beam of light falling on 'A' leaves 'B' as a parallel beam, then the distance 'd' in cm will be :

1. 30
2. 25
3. 15
4. 50

A lens of large focal length and large aperture is best suited as an objective of an astronomical telescope since :

1. a large aperture contributes to the quality and visibility of the images.
2. a large area of the objective ensures better light gathering power.
3. a large aperture provides a better resolution.
4. All the above.

A point object is placed at a distance of 60 cm from a convex lens of focal length 30 cm. If a plane mirror were put perpendicular to the principal axis of the lens and at a distance of 40 cm from it, the final image would be formed at a distance of - [figure]

1. 20 cm from the plane mirror, it would be a virtual image.
2. 20 cm from the lens, it would be a real image.
3. 30 cm from the lens, it would be a real image.
4. 30 cm from the plane mirror, it would be a virtual image.

A ray is incident at an angle of incidence i on one surface of a small angle prism (with angle of prism A) and emerges normally from the opposite surface. If the refractive index of the material of the prism is , then the angle of incidence is nearly equal to :

1. 2A
2. A
3. A2
4. A2