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## Introduction

When there is a big movie premiere searchlights sometimes reach up into the night sky as long parallel beams of light. Can you use your knowledge of how light travels through lenses and from reflectors to create your own parallel beams?

Focal length of a curved reflector.

1. Open the Yenka file Model 1.
2. Drag the curved reflector into the parallel beam of light. Make sure it is fully in the beam. Describe what happens to the beam.
It is brought to one point, called the focus, and then spreads out again afterwards.
3. Use the orange ruler to find the distance from the centre of the lens to the point where the rays cross over. How far is it?
10 cm
4. The reflector is made of a part of a circle. The radius of the circle is shown by the yellow line. How does your answer to question 3 compare with the radius?
It is half the length of the radius.
5. Can you suggest how this arrangement could be used to help you survive if you crashed your plane on a desert island? Where could you get the reflector?
It could concentrate the sun's rays onto one spot to help you start a cooking fire. The reflector could come from wreckage like the skin of your crashed plane or from tinfoil if you dug a bowl in the soil. It could also come from the reflectors at the back of any lamps that survived.

### Summary

Satellite television signals are radio waves that can be reflected like light. The TV dishes on the side of many houses are curved reflectors. They concentrate the signal from a satellite onto one spot above the dish. The aerial is held there on a support rod.

1. Keep Yenka file Model 1 open. Drag the reflector from simulation A down into simulation B. Has the focal length of the reflector changed?
No
2. Drag the reflector until its centre is 1 focal length away from the bulb in simulation B. Use the ruler to help you do this. Describe what you see happen to the light coming from the bulb.