The following points highlight the two important laws of illumination. The laws are: 1. Inverse Square Law 2. Lambert’s Cosine Law.

**1. Inverse Square Law****:**

The illumination of a surface is inversely proportional to the square of distance between source and light surface provided that the distance between the surface and the source is sufficiently large so that source can be regarded as a point source.

This is known as inverse square law.

#### 2. Lambert’s Cosine Law:

According to this law illumination varies directly as the cosine of the angle between the normal to the surface and the direction of incident light.

Combining (i) and (ii),

where I is the luminous intensity of the source in candle power.

**Example:**

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A section of a road is being illuminated by two lamps, L_{1} of 500 C.P. and the other L_{2} of 400 C.P. both being horizontally 20 meters apart. The lamps are suspended at 6 meters above the road level.

**Calculate: **

(i) Illumination at a point A directly below the lamps L_{1}.

(ii) Illumination at a point B directly below the lamp L_{2}.

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(iii) Illumination at a point C on the centre line at midway between A and B.

**Solution:**

All the points viz. A, B and C are being illuminated by both lamps L_{1} and L_{2}.

**(i) At Point A: Illumination due to lamp:**

**(ii) At Point B: Illumination due to:**

**(iii) At Point C: Illumination due to:**