Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 12

# An Electric Lamp and a Candle Produce Equal Illuminance at a Photometer Screen When They Are Placed at 80 Cm and 20 Cm from the Screen Respectively. - Physics

Sum

An electric lamp and a candle produce equal illuminance at a photometer screen when they are placed at 80 cm and 20 cm from the screen respectively. The lamp is now covered with a thin paper which transmits 49% of the luminous flux. By what distance should the lamp be moved to balance the intensities at the screen again?

#### Solution

Let I1 be the intensity when placed at a distance 80 cm and I2 be the intensity when placed at a distance 20 cm apart from the screen.

Now,

$\frac{I_1}{I_2} = \left( \frac{80}{20} \right)^2 = 16$

According to the question, let the new distance between the lamp and the screen be xsuch that even after covering the lamp with a thin paper the intensities at the screen is balanced.

So,

$\frac{0 . 49 I_1}{I_2} = \left( \frac{x}{20} \right)^2$

$\Rightarrow 0 . 49 \times 16 \times 400 = x^2$

$\Rightarrow x = 56 cm$

Thus, the lamp has to be moved by 80 cm - 56 cm = 24 cm.

Concept: Light Process and Photometry
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#### APPEARS IN

HC Verma Class 11, Class 12 Concepts of Physics Vol. 1
Chapter 22 Photometry
Exercise | Q 15 | Page 455
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