June 88 P2 Q3
The diagram below shows the structure of a simple periscope used at sporting events to see over the heads of the crowd.
(a) When the periscope is in use, total internal reflection occurs at one surface of each prism. Indicate this surface for one of the prisms by a letter "S" on the diagram.
(b) Complete the paths of the two rays through the periscope to the eye.
(c) Indicate why these paths are not affected by the colour of the light.
(d) Is the image the right way up or upside down? Justify your answer.
(e) In choosing a material for the prism, the critical angle of the material must be less than 45
°. What is the smallest possible refractive index for the material of the prism?*************************
Answer:
(a) & (b)
(c) As total internal reflection takes place, the angle of incidence = angle of reflection for any colour of light. Hence, these paths are not affected by the colour of the light.
(d) The image is the right way up. The first prism inverts and the second prism reinverts the image. (We can see from the colour of the light rays)
(e) n = 1/sin c
n = 1/sin 45°
n = 1.414
Thus, smallest possible refractive index = 1.414
