Question

Figure shows a small diffused plane source S placed over a horizontal table-top at a distance of 2.4 m with its plane parallel to the table-top. The illuminance at the point Adirectly below the source is 25 lux. Find the illuminance at a point B of the table at a distance of 1.8 m from A.

Answer 1

Let x be the distance moved.

Let the illuminance of the candle and the lamp at a distance 1 m be I_C and I_Lrespectively. According to question,

`E=I_L/(0.8)^2=I_c/(0.2)^2`

`rArr I_L/I_c=16`

Again after the wrapping the paper around

`49/100I_L/(0.8+x)^2=I_c/(0.2)^2`

Either,

x = -0.24 

Or,

x = -1.36 this value is not acceptable because it is more than 0.8

so, x = 0.24m (-ve sign denotes distance is to be decreased)

x = 24cm