The diagram shows the reflector behind the bulb in a hand torch. The bulb is placed at the focus of a concave reflecting surface. Light that would otherwise...
Question 1 Report
The diagram shows the reflector behind the bulb in a hand torch. The bulb is placed at the focus of a concave reflecting surface. Light that would otherwise travel backwards reflects from the surface and leaves the torch in an approximately parallel beam. This concentrated beam helps a walker see a path at night without using a high current from the battery.
(a) Name the reflector shape. [1]
(b) Describe the direction of reflected rays leaving the torch. [2]
(c) Explain why placing the bulb away from F makes the torch beam less useful. [2]
Fig. 1 shows part of a reflecting telescope at an observatory. Light from a distant star enters the open end as nearly parallel waves. A large concave primary mirror reflects the light towards a smaller plane mirror. The plane mirror sends the light sideways into an eyepiece. The telescope avoids a glass lens at the front, so there is no colour separation caused by refraction.
(a) What does the primary mirror do to the incoming parallel light? [1]
(b) Describe the role of the small plane mirror. [2]
(c) Give one reason why the primary mirror must have a very smooth surface. [2]
Answer Details
Torch reflector
- (a) The reflector is a concave mirror, or concave reflector. [1]
- (b) The rays are reflected forwards [1] and leave approximately parallel to one another. [1]
- (c) If the bulb is not at F, the reflected rays are not parallel. [1] The beam spreads out and is less concentrated. [1]
Reflecting telescope
- (a) The primary mirror converges or focuses the incoming parallel light. [1]
- (b) The small plane mirror reflects the converging light sideways [1] and directs it into the eyepiece. [1]
- (c) The primary mirror must be very smooth to give regular or specular reflection. [1] This produces a sharp image rather than scattered light. [1]
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