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A ray of light passes…what angle is the ray reflected?

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PHYSICS

A ray of light passes through the center of curvature of a concave mirror and strikes the mirror. At what angle is the ray reflected?

  • A. 180 degrees
  • B. 90 degrees
  • C. 0 degrees
  • D. 60 degrees

QUICK ANSWER…

A

DETAILS…

If the ray of light passes through the center of curvature, then it will follow exactly the middle horizontal line passing through the principal focus to strike the mirror at exactly the middle where it will be reflected back in the same line but exactly opposite in direction. The angle of reflection is thus 180 degrees.

Now for the right answer to the above question:

  1. Option A is correct. the ray will be turned 180 degrees back, which is exactly in the same line of incidence, but opposite in direction.
  2. Option B is incorrect. the ray will not start travelling 90 degrees upwards or downwards.
  3. C is incorrect. 0 degrees will mean it will not be reflected at all.
  4. D is not the correct answer.

KEY-POINTS…

You may please note these/this:

  • Please do not mistake the right answer for 0 degrees.
  • This is because 0 degrees will mean there is no reflection at all.
  • If an object starts travelling exactly in opposite direction to the initial direction, the angle difference is not 0 but 180 degrees.
  • It is 180 degrees because the direction has changed to exactly opposite.
  • 0 degrees will then mean unchanged direction.

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/culled from 2019 JAMB-UTME physics question 17/

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