# Optics

### The angular width of the central maximum in a single slit diffraction pattern is 60°

The angular width of the central maximum in a single slit diffraction pattern is 60°. The width of the slit is 1 µm. The slit is illuminated by monochromatic plane waves. If another slit of same width is made near it, Young's fringes can be observed on a screen placed at a distance 50 cm from the slits. If the observed fringe width is 1 cm, what is slit separation distance ? (i.e. distance between the centres of each slit.)

1. 50 µm
2. 75 µm
3. 100 µm
4. 25 µm

### Diameter of a plano-convex lens is 6 cm and thickness

Diameter of a plano-convex lens is 6 cm and thickness at the centre is 3 mm. If speed of light in material of lens is 2 × 108 m/s, the focal length of the lens is:

1. 10 cm
2. 15 cm
3. 20 cm
4. 30 cm

### Two beams, A and B, of plane polarized light with mutually

Two beams, A and B, of plane polarized light with mutually perpendicular planes of polarization are seen through a polaroid. From the position when the beam A has maximum intensity (and beam B has zero intensity), a rotation of Polaroid through 30° makes the two beams appear equally bright. If the initial intensities of the two beams are IA and IB respectively, then IA/IB equals:

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

### A car is fitted with a convex side-view mirror of focal length 20 cm

A car is fitted with a convex side-view mirror of focal length 20 cm. A second car 2.8 m behind the first car is overtaking the first car at a relative speed of 15 m/s. The speed of the image of the second car as seen in the mirror of the first one is

1. (1/10) m/s
2. (1/15) m/s
3. 10 m/s
4. 15 m/s

### The maximum number of possible interference maxima

The maximum number of possible interference maxima for slit-separation equal to twice the wavelength in young’s double-slit experiment is

1. infinite
2. five
3. three
4. zero

### If two mirrors are kept at 60° to each other

If two mirrors are kept at 60° to each other, then the number of images formed by them is

1. 5
2. 6
3. 7
4. 8

### Let the x - z plane be the boundary between two transparent media

Let the x - z plane be the boundary between two transparent media. Medium 1 in z ≥ 0 has refractive index of √2 and medium 2 with z < 0 has a refractive index of √3. A ray of light in medium 1 given by the vector A = 6√3i + 8√3j - 10k in incident on the plane of separation. The angle of refraction in medium 2 is

1. 75°
2. 60°
3. 45°
4. 30°

### An astronomical telescope has a large aperture to

An astronomical telescope has a large aperture to

1. reduce spherical aberration
2. have low dispersion
3. increase span of observation
4. Have high resolution

### An object 2.4 m in front of a lens forms a sharp image on a film 12 cm behind the lens

An object 2.4 m in front of a lens forms a sharp image on a film 12 cm behind the lens. A glass plate 1 cm thick, of refractive index 1.50 is interposed between lens and film with its plane faces parallel to film. At what distance (from lens) should object be shifted to be in sharp focus on film?

1. 5.6 m
2. 7.2 m
3. 3.2 m
4. 2.4 m

### An observer looks at a distant tree of height 10 m with a telescope of magnifying power of 20

An observer looks at a distant tree of height 10 m with a telescope of magnifying power of 20. To the observer the tree appears:

1. 10 times nearer
2. 20 times taller
3. 20 times nearer
4. 10 times taller