10 Question-answer on photometry

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

(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

(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

(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

(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

(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

(d) 3 lux

Ans: ( b)

7. Unit of luminance is:

(a) Lambert

(b) Lux

(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%

(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

(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

(d) 40 phot

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