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Photometry — Physics STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 SciencePhysicsPhotometry1 Mark
Question
Light is incident normally on a small plane surface. If the surface is rotated by an angle of $30^\circ$ about the incident light, does the illuminance of the surface increase, decrease or remain same? Does your answer change if the light did not fall normally on the surface?
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Answer

If the surface is rotated by $30^\circ ,$ the illuminance will decrease.
This is because illuminance depends upon the cosine of the normal angle.
Yes, if the light does not fall normally on the surface initially, it may increase or decrease depending upon the former's angle. If the $30^\circ$ rotation brings the table closer to the normal of the surface, the illuminance will increase; otherwise, it will decrease

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