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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