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Photometry MCQs for NEET

The science of measurement of light is known as photometry. The basic unit used in photometry is the lumen. Following are a few important terms in photometry:

  • Luminous Intensity – It is defined as the amount of luminous flux emitted by a source per unit solid angle Ω.
  • Luminance – The luminance of a surface is a measure of the brightness of the surface.

Following are the factors on which the illumination of the surface depends upon:

  1. The distance of the source from the surface
  2. The direction along which the light is incident on the surface
  3. The illumination power of the light source

Question and Answer

1. What is the power of the lamp if its luminous efficiency is 2 lumen/watt and its luminous intensity is 42 candela?

  1. 62 W
  2. 138 W
  3. 76 W
  4. 264 W

Answer: (d) 264 W

2. In a movie hall, if the distance between the screen and the projector is increased by 1% what happens to the illumination on the screen?

  1. Increases by 1%
  2. Decreases by 1%
  3. Increases by 2%
  4. Decreases by 2%

Answer: (d) Decreases by 2%

3. At a height of one meter above the centre of a circular table of diameter 4m, a bulb of 100 W hangs. If the intensity at a point on its rim is I0, then what is the intensity at the centre of the table?

  1. \(\begin{array}{l}2I_0\end{array} \)
  2. \(\begin{array}{l}I_0\end{array} \)
  3. \(\begin{array}{l}2\sqrt{5}I_0\end{array} \)
  4. \(\begin{array}{l}5\sqrt{5}I_0\end{array} \)

Answer: (d)

\(\begin{array}{l}5\sqrt{5}I_0\end{array} \)

4. Lux is a unit of which of the following?

  1. Illuminance on a surface
  2. Luminous intensity of a source
  3. Luminous efficiency of the source of light
  4. Transmission coefficient of a surface

Answer: (a) Illuminance on a surface

5. Lux is equivalent to which of the following?

  1. 1 candela/m2
  2. 1 lumen/cm2
  3. 1 lumen/m2
  4. 1 candela/cm2

Answer: (a) 1 candela/m2

6. If a lamp has a luminous efficiency of Five lumen/watt and a luminous intensity of 35 candela. What is the power of the lamp?

  1. 176 W
  2. 88 W
  3. 36 W
  4. 80 W

Answer: (b) 88 W

7. The distance between a point source of light and a screen is 60 cm and now this distance is increased to 180 cm. By how many fractions will the intensity on the screen change as compared to the original intensity?

  1. 9 times
  2. 3 times
  3. (1/9) times
  4. (1/3) times

Answer: (c) (1/9) times

8. Venus looks brighter than other stars because

  1. it has no atmosphere
  2. atomic fission occurs on its surface
  3. it has a higher density than other stars
  4. it is closer to earth than most stars

Answer: (d) it is closer to earth than most stars

9. Which of the following sources of light is the inverse square law valid for?

  1. Cylindrical source
  2. Searchlight
  3. Isotropic point source
  4. All types of light sources

Answer: (c) Isotropic point source

10. Which of the following has more luminous efficiency?

  1. 40 W fluorescent tube
  2. 40 W bulb
  3. Both have the same luminous efficiency
  4. Cannot say

Answer: (a) 40 W fluorescent tube

11. A lamp hangs at a height of 4 m above a table. The lamp is lowered by 1 m. By how much will the illuminance increase?

  1. 78%
  2. 64%
  3. 40%
  4. 92%

Answer: (a) 78%

12. A point source of 100 candela is held 5 m above a sheet of blotting paper which reflects 75% of the incident light. What is the illuminance of blotting paper?

  1. 4 lux
  2. 4 phot
  3. 3 lux
  4. 3 phot

Answer: (a) 4 lux

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