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Photometry — Physics STD 12 Science — Question

Gujarat BoardEnglish MediumSTD 12 SciencePhysicsPhotometry1 Mark
Question
Figure shows a small diffused plane source S placed over a horizontal table-top at a distance of 2.4m with its plane parallel to the table-top. The illuminance at the point A directly below the source is 25 lux. Find the illuminance at a point B of the table at a distance of 1.8m from A.
 
✓

Answer

Let x be the distance moved.Let the illuminance of the candle and the lamp at a distance 1m be $I_C$ and
$I_L$ respectively. According to question
$\text{E}=\frac{\text{I}_\text{L}}{(8.0)^2}=\frac{\text{I}_\text{c}}{(0.2)^2}$
$\Rightarrow\frac{\text{I}_\text{L}}{\text{I}_\text{c}}=16$
Again after the wrapping the paper around
$\frac{49}{100}\frac{\text{I}_\text{L}}{(0.8+\text{x})^2}=\frac{\text{I}_\text{c}}{(0.2)^2}$
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).
$x = 24cm$

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