Multiple choice questions on Photometry:
Five lumen/watt is the luminous efficiency of a lamp and its luminous intensity is 35 candela. The power of the lamp is
(a) 80 W
(b) 176 W
(c) 88 W
(d) 36 W
A point source of 3000 lumen is located at the center of a cube of side length 2m. The flux along one side is
(a) 500 lumen
(b) 600 lumen
(c) 750 lumen
(d) 1500 lumen
In the above problem, the luminous flux emitted by the sun will be
(a) 4.43 x 1025Â lm
(b) 4.43 x 1026Â lm
(c) 4.43 x 1027Â lm
(d) 4.43 x 1028Â lm
A lamp is hanging at a height of 4m above a table. The lamp is lowered by 1m. The percentage increase in illumination will be
(a) 40 %
(b) 64%
(c) 78%
(d) 92%
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Two stars situated at distances of 1 and 10 light years respectively from the earth appear to possess the same brightness. The ratio of their real brightness is
(a) 1Â : 10
(b) 10Â : 1
(c) 1Â : 100
(d) 100 : 1
If the luminous intensity of a 100 W unidirectional bulb is 100 candela, then the total luminous flux emitted from the bulb is
(a) 861 lumen
(b) 986 lumen
(c) 1256 lumen
(d) 1561 lumen
“Lux” is a unit of
(a) Luminous intensity of a source
(b) Illuminance on a surface
(c) Transmission coefficient of a surface
(d) Luminous efficiency of source of light
The distance between a pointed source of light and a screen, which is 60 cm, is increased to 180 cm. The intensity on the screen as compared with the original intensity will be
(a) (1 / 9) times
(b) (1 / 3) times
(c) 3 times
(d) 9 times
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In a movie hall, the distance between the projector and the screen is increased by 1% illumination on the screen is
(a) Increased by 1%
(b) Decreased by 1%
(c) Increased by 2%
(d) Decreased by 2%
Light from a point source falls on a small area placed perpendicular to the incident light. If the area is rotated about the incident light by an angle of 60o, by what fraction will the illuminance change?
(a) It will be doubled
(b) It will be halved
(c) It will not change
(d) It will become one-fourth
Two lamps of luminous 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 32Cd lamp
(c) 40 cm from 8 Cd lamp
(d) 40 cm from 32 Cd lamp
A movie projector forms an image 3.5m long of an object 35 mm. Supposing there is negligible absorption of light by aperture, then illuminance on slide and screen will be in the ratio of
(a) 100 : 1
(b) 104 : 1
(c) 1 : 100
(d) 1 : 104
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Correct exposure for a photographic print is 10 seconds at a distance of one metre from a point source of 20 candela. For an equal fogging of the print placed at a distance of 2 m from a 16 candela source, the necessary time for exposure is
(a) 100 sec
(b) 25 sec
(c) 50 sec
(d) 75 sec
1% of the light of a source with luminous intensity 50 candela is of incident on a circular surface of radius 10 cm. The average illuminance of the surface is
(a) 100 lux
(b) 200 lux
(c) 300 lux
(d) 400 lux
A source of light emits a continuous stream of light energy which falls on a given area. Luminous intensity is defined as
(a) Luminous energy emitted by the source per second
(b) Luminous flux emitted by source per unit solid angle
(c) Luminous flux falling per unit area of a given surface
(d) Luminous flux coming per unit area of an illuminated surface
A lamp is hanging at a height 40 cm from the center of the 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%
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Inverse square law for illuminance is valid for
(a) Isotropic point source
(b) Cylindrical source
(c) Search light
(d) All types of sources
Venus looks brighter than other stars because
(a) It has a higher density than other stars.
(b) It is closer to the earth than other stars
(c) It has no atmosphere
(d) Atomic fission takes place on its surface
If the luminous efficiency of a lamp is 2 lumen/watt and its luminous intensity is 42 candela, then the power of the lamp is
(a) 62 W
(b) 76 W
(c) 138 W
(d) 264 W
