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Q.

A lamp hanging 44 meters above the table is lowered by 11 meter. The illumination on the table:

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a

Decreased by 25%

b

Increased by 25%

c

Decreased by 66.7%

d

Increased by 77.7%

answer is D.

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Detailed Solution

Concept: Here, we must locate the formula for illumination intensity at a specific location. The increase in illumination intensity's % can then be determined.
The term "luminous intensity" in photometry refers to a wavelength-weighted force produced by a light source in a certain direction per unit solid angle that is based on the luminosity function and standardized the sensitivity of the human eye.
The formula for luminous intensity in mathematics is

E = \frac{I_{o}}{d^{2}}

Given,
A lamp that was 4 meters above the table is now 1 meter lower.
Any point's illumination intensity is determined by

E = \frac{I_{o}}{d^{2}}

Distance

d_{1} = 4 m

E_{1} = 0.0625 I_{o}

d_{2} = 3 m

E_{2} = 0.1111 I_{o}

Percentage increase=

\frac{E_{2} - E_{1}}{E_{2}} \times 100

=77.7%
Hence, option 4 is the correct answer.

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