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In this chapter:

  • Light travels in straight lines
  • Transparent objects let light pass through them
  • Translucent objects let some light pass through them
  • Opaque objects do not let any light pass through them
  • Shadows are formed when light shines on opaque objects

Introduction

Light can pass through some materials but not others. When light hits an object that it cannot pass through it forms a shadow. If light passes through an object easily it is a transparent object. A translucent object lets some light through but an opaque object lets no light through. This chapter looks at how light moves and what happens to it when it hits these types of objects.

How light moves

Light waves travel in straight lines which are often called rays. Sometimes you can actually see that light waves travel in straight lines. When the sunlight breaks through clouds you may be able to see beams of light. Or when the sun shines into a dusty room you should be able to see that the edges or the sunbeams are straight. If you put your hand in front of a torch you will see that your hand blocks the light. Light cannot bend around your hand. This shows that light waves travel in straight lines.

See image 1

Transparent objects

Transparent objects let light pass through without scattering the rays. This means that almost all of the light that falls on a transparent object passes through it so that we can see through the object.

Transparent substances or objects are clear. Air is transparent and glass can be transparent, too, if it does not reflect too much light. Objects that are transparent include plastic bottles, windows and clear glass. Some natural objects are also transparent; for example, see-through fish and transparent quartz crystals.

See image 2

Translucent objects

A translucent object lets some light through but it scatters the rest of the light so much that whatever is on the other side cannot be seen clearly. Frosted glass is an example of translucent material. This glass is often used in bathroom windows. Some light passes through the window but the rest of the light is scattered and people cannot see inside very clearly.

Translucency may also depend on the thickness of the object. Some thin sheets of paper are translucent but others are too thick to let any light through.

Some substances are more translucent when the temperature changes. Candle wax, for example, becomes see-through when it is melted. Butter also becomes clear when it is heated in a pan but it is opaque when cold.

Opaque objects

Materials that do not allow light to pass through are called opaque materials. Some of the light is absorbed by the material and some is reflected. Most metals, wood and animals are made of opaque material.

See image 3

Every opaque object casts a shadow. When light shines on an opaque object from above, a short shadow is cast. When light shines from the side, the object makes a longer shadow. Sometimes you may see a shadow with a grey border. This grey border is called the penumbra. The dark part of the shadow is called the umbra.

A shadow is further evidence that light travels in straight lines. If light travelled in curves or zigzags, it could bend around opaque objects and not make shadows.

Investigate the effect light has on materials

You can investigate what happens when light hits an object by shining a light from a torch or projector on to a wall or screen. Look at the shape of the beam. Explore ways of changing the beam. Make shadows on the wall with your hands. Try making a rabbit or a duck perhaps.

See image 4
 
Hold an object such as a tennis ball close to the torch and see what shadow it makes. Look for the penumbra and umbra. Investigate holding the torch further away from the object and see what happens to the shadow. Shine two different lights on an object and look for two shadows.
 
Collect a range of materials and predict whether they will be transparent, translucent or opaque. Test your predictions by shining a torch through each of the materials. Make a record of your observations. Materials that you could test include: clear plastic, glass, cotton wool, metal, coloured liquid, clothing, paper, leaves and so on.

By doing these investigations you will see that light can pass through some materials but not others. You will see also, that when light hits an opaque object it will form a shadow.


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Question 1/5

1. What is an example of translucent material?

An encyclopaedia

A steel pole

A brick

Frosted glass

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