Unit: Waves
Year: 1992 Question#: 42
Question: The Diagram below represents a light ray being reflected from a plane mirror. The angle between the incident ray and the reflected ray is 70 degrees.
(1 ) 20 degrees
(2 ) 35 degrees
(3 ) 55 degrees
(4 ) 70 degrees
What is this question really asking?
Explanation by: Dustin C
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Answer 1
20 degrees
INCORRECT
This answer is incorrect because when you draw the normal line perpendicular to the plane mirror it cuts the 70 degree angle in half. Therefore it could not be 20 degrees because that is not half of 70. return to top
Answer 2
35 degrees
CORRECT
This answer is correct because from either of the rays to the perpendicular line is 35 degrees. This is found because it is exactly half of the original angle. Also, all angles get measured from the normal. return to top
Answer 3
55 degrees
INCORRECT
This answer is incorrect because it was not measured from the normal to the rays. It was measured from the plane to the rays. To find the real answer you take half of 70. return to top
Answer 4
70 degrees
INCORRECT
This is incorrect because the measure of the angle between the two rays is 70 degrees, so it can't be the angle of incidence. To get this you divide that angle,70, in half to get 35 degrees. return to top
What's this question really asking?
What is the angle of incidence for this ray when it is reflected off a plane mirror? return to top
