Light from a point source falls on a screen. If the separation between the source and the screen is increased by 1%, the illuminance will decrease (nearly) by
Given:
The separation between the source and the screen is,
The angle between source and screen,
Calculation:
The illuminance is given as,
…(θ=00)
Differentiating above equation we get,
Negative sign describes the decrease in illuminance.
So, option C is correct.