SPM Physics Question
The diagram below shows an incident ray of light directed to a plane mirror, PQ with \mathrm{i}=50^{\circ}
What is the reflected angle of light ray when the plane mirror \mathrm{PQ} is rotated 30^{\circ} to \mathrm{P}^{\prime \prime} \mathrm{Q}^{\prime \prime} ?
A)70°
B)30°
C)50°
D)20°
SPM Physics Solution
Before rotation, angle of incidence = 50°, angle of reflection = 50° as shown above
When a 30° clockwise rotation is performed, you have to identify the “new normal to the plane” (as indicated by red line), and the new angle of incident will now be 20°.
Since angle of incidence is 20°, for a perfectly smooth plane, the angle of reflection will also be 20°.
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Thank you very much 
