3.2.1 Reflection of light (3)
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1.
The angle of incidence of a light ray striking a plane mirror is 30°.
(a) Calculate the angle of reflection. (2 marks)
(b) If the student moves closer to the mirror, describe how the position of the image changes. (2 marks)
(c) Explain, why the image formed by a plane mirror appears the same size as the object. (2 marks)
(a) The angle of reflection is equal to the angle of incidence. Therefore, the angle of reflection is 30°.
(b) As the student moves closer to the mirror, the image formed will appear to move further away from the mirror. The image will remain the same size, but its distance from the mirror will increase.
(c) The image formed by a plane mirror appears the same size as the object because the mirror reflects light rays in a way that preserves the relative distances of the object and the image from the mirror. The image is formed by the convergence of reflected rays, and the size of this convergence is the same as the size of the object.
2.
Describe the formation of an optical image by a plane mirror. Your answer should include a clear explanation of how rays of light interact with the mirror surface.
When light travels towards a plane mirror, it reflects according to the law of reflection: the angle of incidence equals the angle of reflection. This means that the angle at which the light ray strikes the mirror is the same as the angle at which it bounces off.
Consider a ray of light originating from a point on an object. This ray travels to the mirror and reflects. The reflected ray then appears to originate from a point behind the mirror. This apparent origin is the location of the image. The image is formed by the apparent intersection of the reflected rays.
Specifically, the image appears to be the same distance behind the mirror as the object is in front of the mirror. The rays of light do not actually converge; they only appear to do so when traced backwards.
3.
A student stands 1.5 m from a plane mirror. They move 0.5 m further away from the mirror. Calculate the change in the distance of the image from the mirror. Show your working.
Initial distance of student from mirror: 1.5 m
Final distance of student from mirror: 1.5 m + 0.5 m = 2.0 m
Distance of image from mirror when student is 1.5m away: 1.5 m
Distance of image from mirror when student is 2.0m away: 1.5 m
Change in distance of image from mirror: 1.5 m - 1.5 m = 0 m
Working: The distance of the image from the mirror remains the same as the distance of the object from the mirror for a plane mirror. Therefore, even though the student moves further away, the image remains at the same distance from the mirror.