# Photometry Short Question Solution | Physics Grade XI

### Photometry Short Question Solution

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#### Kinematics

Introduction to Kinematics

#### Viva Voice Questions

1. Differentiate between luminous and illuminous objects.

• Luminous objects emit light whereas illuminous objects do not emit light
• Luminous objects are visible whereas illuminous objects are invisible.
2. What is the unit of illumination?
The unit of illumination is lux or lm/ m2.

3. What are the factors on which illumination of a surface depends?
The illumination of a surface depends upon following factors.
1. The illuminating power of the light source.
2. Distance of the surface from the source.
3. The direction along which light is incident along the surface.
4. What is photometry? What is its importance?
The branch of optics that deals with the quantitative study of light energy are called photometry. Its importance’s are:
1. The knowledge of photometry helps us to design the lighting schemes of houses, factories, etc.
2. It helps us to make a quantitative measurement of light.
5. What do you mean by one lumen?
One lumen is the amount of luminous flux emitted per unit solid angle by a source of light of luminous intensity on candela.

6. What do you mean by one candela?
One candela is the luminous intensity in a given direction due to a source, which emits monochromatic radiation of frequency 540 * 1012 Hz and of which the radiant intensity in that direction is 1/683 watt per steradian.

7. What do you mean by one phot?
It is the amount of light energy falling normally on a square centimeter spherical surface of radius one centimeter by a source on candle power.

8. What is meant by luminous efficiency?
The efficiency with which power input to the source is used up to produce brightness is called luminous efficiency.
i.e. Luminous efficiency = luminous flux emitted/ power input to the source

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