The illuminance of a small -2 area changes from 900 lumen m^-2 to 400 lumen m^-2 when it is shifted along its normal by 10 cm. Assuming that it is illuminated by a point source placed on the normal, find the distance between the source and the area in the original position.

The illuminance of a small -2 area changes from 900 lumen m^-2 to 400

1.	The illuminance of a small -2 area changes from 900 lumen m^-2 to 400 lumen m^-2 when it is shifted along its normal by 10 cm. Assuming that it is illuminated by a point source placed on the normal, find the distance between the source and the area in the original position.
Answer» Correct Answer - B::C Let, l=Luminous intensity of source E_A=900 lumne/m^2` `E_B=400 lumen/m^2` Now,` E_a=(costheta)/x^2` and `E_B=(lcostheta)/((x+10)^2)` so, `l=(E_Ax^2)/(costheta)=(E_A(x+10)^2)/(costheta)` `rarr 900x^2=400(x+10)^2` `rarr x/(x+10)=2/3` `rarr x=2x+20` `or x=20 cm` So the distance between the source and the origiN/Al position is 20 cm.

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The illuminance of a small -2 area changes from 900 lumen m^-2 to 400 lumen m^-2 when it is shifted along its normal by 10 cm. Assuming that it is illuminated by a point source placed on the normal, find the distance between the source and the area in the original position.

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