Question

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°, by what fraction will the illuminance change?

Answer 1

No change in illuminance.

Explanation:

Given that, rotation angle $\theta = 60\°$

The equation of the illuminance $(E)$ is given by  

$E = \frac{P}{ 4\pi \times d^{2}}$

Where,

$P = \textrm{luminous flux of the source}$

$d = \textrm{distance from the surface}$

So we can say that the illuminance does not depend on the angle of rotation hence no change in illuminance.

Answer 2

No Change in illuminance

Explanation:

Photometry says, at any surface,  

illuminance = $\frac {Total\ Luminous\ Flux}{Unit\ Area}$

• The measurement of the incident light being illuminated over the surface is called as Illuminance.  

Described as -

Luminous Function associated with The Perception of Brightness by Human.

Luminous Emittance = $\frac {Luminous\ Flux}{Unit\ Area}$, it can also be called as Luminous Exitance.

Illuminance Equation says-

$E = \frac {P}{4 \pi d^2}$

where P = Luminous Flux in Lumens

d = distance from the surface

Keeping the above formula, there is no angular representation, so angle change does not effect the illuminance.