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(C) Illuminance $(E)$ is defined as the luminous flux per unit area,given by $E = \frac{\Phi}{A} \cos 	heta$,where $	heta$ is the angle between the

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It will not change

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(C) Illuminance $(E)$ is defined as the luminous flux per unit area,given by $E = \frac{\Phi}{A} \cos 	heta$,where $	heta$ is the angle between the normal to the area and the direction of the incident light.Initially,the area is perpendicular to the incident light,so the normal to the area is parallel to the light rays,meaning $	heta = 0^o$ and $\cos 0^o = 1$.When the area is rotated about the axis of the incident light,the orientation of the normal vector relative to the incident light rays remains unchanged.Since $	heta$ remains $0^o$,the value of $\cos 	heta$ remains $1$.Therefore,the illuminance does not change.

Light from a point source falls on a small area placed perpendicular to the incident light. If the area is rotated about the incident light by an angle of $60^o$,by what fraction will the illuminance change?

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