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

# Two Light Sources of Intensities 8 Cd and 12 Cd Are Placed on the Same Side of a Photometer Screen at a Distance of 40 Cm from It. Where Should a 80 Cd Source Be Placed to Balance the Illuminance? - Physics

Sum

Two light sources of intensities 8 cd and 12 cd are placed on the same side of a photometer screen at a distance of 40 cm from it. Where should a 80 cd source be placed to balance the illuminance?

#### Solution

Total intensity of the 8 cd and the 12 cd light source is 20 cd.

Let d be the distance of the 80 cd source.

∴ Illuminance due to the 20 cd source (E1) is

$E_1 = \frac{20}{\left( 0 . 4 \right)^2}..........(1)$

∴ Illuminance due to the 80 cd source (E2) is

$E_2 = \frac{80}{d^2}............(2)$

As, E1 = E2

$\therefore\frac{20}{\left( 0 . 4 \right)^2}=\frac{80}{d^2}$

⇒ d = 0.8 m = 80 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 16 | Page 455
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