**Inverse Square Law**

_{1}and r

_{2}. Then the intensity of illumination for area 4πr

_{1}

^{2}and 4πr

_{2}

^{2}.

_{1}= Q/ 4πr

_{1}

^{2}...…... (i)

_{2}= Q/ 4πr

_{2}

^{2}.………. (ii)

_{1}/I

_{2}= Q/ 4πr

_{1}

^{2}* 4πr

_{2}

^{2}/ Q

_{1}/I

_{2}= r

_{2}

^{2}/ r

_{1}

^{2}

^{2}

Thus, the intensity of illumination of the point is inversely proportional to the square of the distance from the source.

**Lambert Cosine Law**

It states that, "when light falls obliquely on a surface, the illumination of the surface is directly proportional to the cosine of the angle of incidence of light on the surface". It is used to find the illumination of a surface, when light falls on the surface along an oblique direction.

Suppose a light beam from a source S falling on a surface area A as shown in the figure. The normal to the surface makes an angle θ with the direction of light. Then, component of A normal to the direction of light ray is A cosθ.

^{2}

^{2}

^{2}

^{2}

_{0}cosθ …… (i)

_{0}= L/ r

^{2}, maximum illumination of the surface. That is I α cosθ ……….. (ii)

Which is Lambert’s cosine Law, illumination of a surface is directly proportional to the cosine of the angle. The maximum illumination of a surface is obtained when light falls normally on the surface.

Hence illumination at a point due to a source is

- Directly proportional to luminous intensity of the source
- Inversely proportional to the square of the distance of the point from the source
- Directly proportional to the cosine of the angle of incidence of luminous flux.

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