Physics - Optics
Exam Duration: 45 Mins Total Questions : 30
Two lamps of liminous intensity of 8 Cd and 32 Cd respectively are lying at a distance of 1.2 m from each other. Where should a screen be placed between two lamps such that its two faces are equally illuminated due to two sources?
- (a)
10 cm from 8 Cd lamp
- (b)
10 cm from the 32 Cd lamp
- (c)
40 cm from 8 Cd lamp
- (d)
40 cm from 32 Cd lamp
A lamp is hanging at a height of 40 cm from the center of a table. If its height is increased by 10 cm the illuminance on the table will decrease by
- (a)
10%
- (b)
20%
- (c)
27%
- (d)
36%
A small lamp is hung at a height of 8 m above the center of a round table of diameter 16 m. The ratio of intensities of illumination of the center and at points on the circumference of the table will be
- (a)
1:1
- (b)
2:1
- (c)
\(2\sqrt { 2 } :1\)
- (d)
3:2
Time of exposure for a photographic print is 10 second, when a lamp of 50 Cd is placed at 1m from it. Then another lamp of luminous intensity I is used, and is kept at 2m from it. If the time of exposure now is 20 s, the value of I (in Cd) is
- (a)
100
- (b)
25
- (c)
200
- (d)
20
A double convex air bubble in water will behave as
- (a)
convergent lens
- (b)
divergent lens
- (c)
plane glass slab
- (d)
concave mirror
A thin lens has focal length f and its aperture has radius r. It forms an image of intensity I. Now the central part of the aperture upto radius r/2 is blocked by an opaque material. Then the focal length and the intensity of the image will be
- (a)
\(\frac { f }{ 2 } and\frac { I }{ 2 } \)
- (b)
\( { f }{ } and\frac { I }{ 4} \)
- (c)
\(\frac {3 f }{ 4 } and\frac { I }{ 2 } \)
- (d)
\( { f }{ } and\frac { 3I }{ 2 } \)
How does refractive index \(\mu _{ }\) of a material vary with respect to wavelength \(\lambda \) ? A and B are constants
- (a)
\(\mu =A+\frac { B }{ { \lambda }^{ 2 } } \)
- (b)
\(\mu =A+B{ \lambda }^{ 2 }\)
- (c)
\(\mu =A+\frac { B }{ { \lambda }^{ } } \)
- (d)
\(\mu =A+B{ \lambda }^{ }\)
In displacement method if D is the distance of the object and screen and 'd' is the seperation between the two position of the lens, then the ratio of magnified image to that of the object is
- (a)
\(\frac { D }{ d } \)
- (b)
\({ \left( { \frac { D }{ d } } \right) }^{ 2 }\)
- (c)
\(\frac { D+d }{ D-d } \)
- (d)
\(\left( \frac { D+d }{ D-d } \right) \)
A ray of light incident on a 600 angled prism of refractive index \(\sqrt { 2 } \) suffers minimum deviation. The angle of incidence is
- (a)
700
- (b)
00
- (c)
450
- (d)
600
A spectometer set up with a prism on its prism table is immersed in water. This will result in
- (a)
increase in the angle of minimum deviation
- (b)
no change in the angle of minimum deviation
- (c)
decrease in the angle of minimum deviation
- (d)
no change in the angle of dispersion
An achromatic combination of lenses produces
- (a)
images in black and white
- (b)
coloured images
- (c)
images unaffected by variation of refractive index with wavelength
- (d)
highly enlarged images
The dispersive power of material of a lens of focal length 25 cm is 0.05. The longitudinal chromatic aberration of the lens is
- (a)
0.05 cm
- (b)
0.2X10-2 cm
- (c)
1.25 cm
- (d)
12.5 cm
The focal lengths of the objectives and eye lenses of a telescope are respectively 200 cm and 5 cm. The maximum magnifying power of the telescope will be
- (a)
-40
- (b)
-48
- (c)
-60
- (d)
-100
The minimum magnifying power of a telescope is M. If the focal length of its eye-lens is halved, the magnifying power will become
- (a)
M/2
- (b)
2M
- (c)
3M
- (d)
4M
The objective of a compound microscope is essentially
- (a)
a concave lens of small focal length and small aperture
- (b)
convex lens of small focal length and large aperture
- (c)
convex lens of large focal length and large aperture
- (d)
convex lens of small focal length and small aperture
The resolving limit of healthy eye is about
- (a)
1'
- (b)
1''
- (c)
10
- (d)
\(\frac { 1 }{ 60 } \)''
In order to increase the magnifying powr of a compound microscope
- (a)
the focal lengths of objective and eyepiece should be small
- (b)
objective should have small focal length and eyepiece should have large focal length
- (c)
both should have large focal lengths
- (d)
both should have same focal length
A person of 6 feet in height can see his full size erect image in a mirror of 2 feet in height. This mirror has to be
- (a)
plane or convex
- (b)
plane or concave
- (c)
necessarily convex
- (d)
necessarily concave
The critical angle of prism is 30°. The velocity of light in the medium is
- (a)
1.5 x 108 m/s
- (b)
3 x 108 m/s
- (c)
4.5 x 108 m/s
- (d)
None of the above
The refractive index of a prism for a monochromatic wave is \(\sqrt{2}\) and its refractive angle is 60°. For minimum deviation, the angle of incidence will be,
- (a)
30°
- (b)
45°
- (c)
60°
- (d)
75°
For a prism, its refractive index is \(\cot { \frac { A }{ 2 } } \) . Its minimum angle of deviation is
- (a)
180° - A
- (b)
180° - 2A
- (c)
90° - A
- (d)
\(\frac{A}{2}\)
The focal length of the objective and the eyepiece of a microscope are 4 mm and 25 mm respectively. If the final image is formed at infinity and the length of the tube is 16 cm, then the magnifying power of microscope will be (object is at to)
- (a)
- 337.5
- (b)
- 3.75
- (c)
3.375
- (d)
33.75
The apparent depth of water in cylindrical water tank of diameter 2R cm is reducing at the rate of X cm/min when water is being drained out at a constant rate. The amount of water drained in cc/min is (n1 = refractive index of air, n2 = refractive index of water)
- (a)
\(\frac { x\pi { R }^{ 2 }{ n }_{ 1 } }{ { n }_{ 2 } } \quad \)
- (b)
\(\frac { x\pi { R }^{ 2 }{ n }_{ 2 } }{ { n }_{ 1 } } \quad \)
- (c)
\(\frac { 2\pi { R }{ n }_{ 1 } }{ { n }_{ 2 } } \quad \)
- (d)
\(\pi { R }^{ 2 }x\)
An equilateral prism deviates a ray through 45° for the two angles of incidence differing by 20°. The angle of incidence, if the angle of prism is 60°, is
- (a)
62.5°
- (b)
42.5°
- (c)
Both (a) and (b) are correct
- (d)
Both (a) and (b) are wrong
A jar of height h is filled with a transparent liquid of refractive index \(\mu \) (figure). At the centre of the jar on the bottom surface is a dot. Find the minimum diameter of a disc, such that when placed on the top surface symmetrically about the centre, the dot is invisible.
- (a)
\(d=\frac { h }{ \sqrt { { u }^{ 2 }-1 } } \)
- (b)
\(d=\frac { 2h }{ \left( { \mu }^{ 2 }-1 \right) } \)
- (c)
\(d=\frac { 2h }{ \sqrt { \left( { \mu }^{ 2 }-1 \right) } } \)
- (d)
None of the above