Ray Optics and Optical Instruments · Question 18

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

In total internal reflection when the angle of incidence is equal to the critical angle for the pair of media in contact, what will be angle of refraction?

1. equal to angle of incidence
2. 90
3. 180
4. 0

Pick the wrong answer in the context with rainbow.

1. An observer can see a rainbow when his front is towards the sun.
2. Rainbow is a combined effect of dispersion, refraction and reflection of sunlight.
3. When the light rays undergo two internal reflections in a water drop, a secondary rainbow is formed.
4. The order of colours is reversed in the secondary rainbow.

Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is F1. When the space between the two lenses is filled with glycerin (which has the same refractive index ( = 1.5) as that of glass) then the equivalent focal length is F2. The ratio F1 : F2 will be :

1. 1 : 2
2. 2 : 3
3. 3 : 4
4. 2 : 1

The refractive index of the material of a prism is 2 and the angle of the prism is 30 . One of the two refracting surfaces of the prism is made a mirror inwards, by silver coating. A beam of monochromatic light entering the prism from the other face will retrace its path (after reflection from the silvered surface) if its angle of incidence on the prism is

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

An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm. If the object is displaced through a distance of 20 cm towards the mirror, the displacement of the image will be

1. 30 cm away from the mirror
2. 36 cm away from the mirror
3. 30 cm towards the mirror
4. 36 cm towards the mirror

An astronomical refracting telescope will have large angular magnification and high angular resolution, when it has an objective lens of

1. small focal length and large diameter
2. large focal length and small diameter
3. large focal length and large diameter
4. small focal length and small diameter

A beam of light from a source L is incident normally on a plane mirror fixed at a certain distance x from the source. The beam is reflected back as a spot on a scale placed just above the source L. When the mirror is rotated through a small angle , the spot of the light is found to move through a distance y on the scale. The angle is given by

1. yx
2. x2y
3. xy
4. y2x