10 Question-answer on photometry | Learn Physics

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