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An Electric Bulb is Hanging Over a Table at a Height of 1 M Above It. the Illuminance on the Table Directly Below the Bulb is 40 Lux - Physics

MCQ
Fill in the Blanks

An electric bulb is hanging over a table at a height of 1 m above it. The illuminance on the table directly below the bulb is 40 lux. The illuminance at a point on the table 1 m away from the first point will be about ___________ .

Options

  • 10 lux

  • 14 lux

  • 20 lux

  • 28 lux

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Solution

14 lux

 

Here,

\[r   =   \sqrt{2}\]

\[\tan\theta   =   \frac{BC}{AB} = 1\]

\[ I_o  =   40\text{ lux}\]

\[\theta =  \tan^{- 1} \left( 1 \right) =  {45}^0 \]

The illuminance is given by

\[E   =   \frac{I_o \cos\theta}{r^2}\]

\[         = \frac{40 \times \cos( {45}^0 )}{\left( \sqrt{2} \right)^2}\]

\[           = 14\text{ lux}\]

Concept: Light Process and Photometry
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APPEARS IN

HC Verma Class 11, 12 Concepts of Physics 1
Chapter 22 Photometry
MCQ | Q 4 | Page 454
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