# Explain How Lenses Form Images

In this worksheet, students will describe how lenses work, and also look at focal points and periscopes.

Key stage:  KS 3

Curriculum topic:   Physics: Waves

Curriculum subtopic:   Light Waves

Difficulty level:

#### Worksheet Overview

What is refraction and what uses it?

When light shines from one transparent material to another, it refracts. This means the light changes speed and changes direction.

Lenses use the principle of refraction of light when it enters a different medium, for example, from air to glass. Magnifying glasses are lenses and they make things look bigger. They are convex lenses.

Convex lenses gather the rays of light at a certain point: the focal point. This way, they make the object look bigger, sharper and brighter. They are useful for long-sighted people, who cannot focus on objects close to them. They are also used in cameras and telescopes. The diagram above shows a convex lens.

Concave lenses (see diagram below), on the other hand, scatter the light rays (spread them apart), creating a clearer image of objects that are far. They are good for short-sighted people.

What uses reflection?

As you know, light reflecting means bouncing off at the same angle that it struck the surface of a material. Mirrors are the most obvious example of something that uses reflection.

A periscope (shown above) is made using the principle of double reflection of light. They are used in submarines to look above the surface of water by using two mirrors, off which light is reflected. The diagram below shows how this works.

Light enters the periscope through the gap in the top. This is the part that would be above the surface of the water. By having both mirrors positioned at 45 degree angles, it allows the reflected light to travel in a perfect straight line downwards at the first mirror, and then horizontally when it strikes the second mirror. This means the incident ray of light and reflected ray end up being parallel, so the image looks the right way up instead of being upside down. The eyepiece is then at the bottom of the persicope, where the light leaves, and the person can see the reflected image.

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