Q5 of 38 Page 454

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.

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