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

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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?

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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
  Is there an error in this question or solution?


HC Verma Class 11, Class 12 Concepts of Physics Vol. 1
Chapter 22 Photometry
Exercise | Q 16 | Page 455

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