# Parallel Beams 1 - Activity

## 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?

## Task 1

Finding a focal length.

- Open Yenka file Model 1.
- Drag the lens into the parallel beam of light. Make sure it is fully in the beam. Describe what happens to the beam.
AnswerIt is brought to one point, called the focus, and then spreads out again afterwards.
- 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?
Answer10 cm
- The distance described in question 3 is called the focal length of the lens. Carefully drag the lens left and right in the middle of the beam. Is the focal length of the lens affected?
AnswerNo.
- Can you suggest how this arrangement could be used to help you survive on a desert island? Where could you get the lens?
AnswerIt could concentrate the sun's rays onto one spot to help you start a cooking fire. The lens could come from a magnifying glass, a telescope, binoculars or someone's reading glasses. It could even be made from water in a glass container.

### Summary

A convex lens can be used to concentrate parallel light on one spot. A concave lens would just spread the light beam apart.

## Task 2

- Keep Yenka file Model 1 open. Drag the lens from simulation A down into simulation B. Has the focal length of the lens changed?
AnswerNo.
- Drag the lens 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.
AnswerThe light passes through the lens to form a parallel beam.
- Describe the difference between what happens in question 2 in Task 2 with your answer to question 2 in Task 1.
AnswerIn Task 1 the light comes from the right and is focused to a point on the left of the lens after passing through it. In Task 2 the light travels in the opposite direction. It leaves the focal point, passes through the lens and is bent into a parallel beam traveling to the right.

### Summary

If you have managed to complete Task 2 you will have some idea of how to produce a parallel beam of light. The lens in a lighthouse works in this way. In the next activity you will use your knowledge of reflectors to improve on these designs.

### Teacher Summary

- Parallel Beams 2 will use curved reflectors to deal with parallel beams of light.