Fig. 1 shows the optical path in a compact periscope fitted to an inspection mast on a harbour wall. A worker uses it to look over a barrier at a moored boa...
Question 1 Report
Fig. 1 shows the optical path in a compact periscope fitted to an inspection mast on a harbour wall. A worker uses it to look over a barrier at a moored boat without climbing the wall. Two parallel plane mirrors are fixed inside the box. Light from the boat enters the top opening, changes direction at mirror A, then reaches mirror B. The worker sees the outgoing ray at the lower opening. Both mirrors are at 45° to the vertical sides of the box. This is light radiation, not a sound wave, and the electrical fan used to cool the box has no effect on the ray path.
(a) Describe the changes in direction of the light ray between the boat and the worker. [3]
(b) Calculate the angle of incidence at mirror A. [2]
(c) Draw a solid line over each dashed section to show the path of the reflected light. [2]
(d) What are two properties of the image of the boat seen through this periscope? [2]
(e) Give two reasons why the mirrors should be smooth and shiny. [2]
Answer Details
(a) Light travels horizontally into the top of the periscope. [1] Mirror A reflects it downwards. [1] Mirror B then reflects it horizontally towards the worker. [1]
(b) The angle of incidence at mirror A is 45°. [1] This angle is measured between the incident ray and the normal, not between the ray and the mirror surface. [1]
(c) The reflected path is shown by the solid downward line from A to B and the solid horizontal line from B towards the worker. [2]
(d) The image is virtual [1] and upright. [1] It is also the same size as the object.
(e) The mirrors should be smooth so that they give regular reflection. [1] They should be shiny so that they reflect a high proportion of the light, giving a brighter, clearer image with less scattering. [1]
Download The App On Google Playstore
Everything you need to excel in your exams
