10 Question-answer on photometry
1. Inverse square law for illuminance is valid for
(a) Isotropic point-source
(b) Cylindrical source
(c) Search light
(d) All type of sources
Ans: (a)
2. The luminous intensity of 100 W unidirectional bulb is 100 candela. The total luminous flux emitted from bulb will be:
(a) 100 π lumen
(b) 200 π lumen
(c) 300 π lumen
(d) 400 π lumen
Ans: (d)
3. A lamp of 250 candle power is hanging at a distance of 6 m from a wall. The luminance at a point on the wall. The illuminance at a point on the wall at a minimum distance from lamp will be:
(a) 9.64 lux
(b) 9.69 lux
(c) 6.94 lux
(d) None of these
Ans: (c)
4. Two light sources with equal luminous intensity are lying at a distance of 1.2 m from each other. Where a screen should should be placed between them such that illuminance on one of its faces is four times that on the other face?
(a) 0.2 m
(b) 0.4 m
(c) 0.8 m
(d) 1.6 m
Ans: (c)
5. Two lamps of luminous intensity 8 Cd and 32 Cd respectively are lying at a distance of 1.2 m from each other. Where a screen should be placed between two lamps such that its two faces are equally illuminated by the two sources?
(a) 10 cm from 8 Cd lamp
(b) 10 cm from 32 Cd lamp
(c) 40 cm from 8 Cd lamp
(d) 40 cm from 32Cd lamp
Ans: ( c)
6. A point source of 100 candela is held 5 m above a sheet of blotting paper which reflects 75% of light incident upon it. The illuminance of blotting paper is:
(a) 4 phot
(b) 4 lux
(c) 3 phot
(d) 3 lux
Ans: ( b)
7. Unit of luminance is:
(a) Lambert
(b) Lux
(c) Candela
(d) Watt
Ans: (a)
8. 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%
Ans: (d)
9. The distance of a point from a point source of light is changed from 1.5 m to 0.5 m. How is the intensity of illumination affected?
(a) Becomes 9 time
(b) Becomes 3 time
(c) Becomes 1/9 time
(d) Becomes 1/3 time.
Ans: (a)
10. A fluorescent tube which is equivalent to a line source of 100 candela per metre is hung horizontally 5 meters above the table. The illuminance at a point vertically below the tube will be:
(a) 8 lux
(b) 8 phot
(c) 40 lux
(d) 40 phot