3.1 Light
Summary: As you go deeper and deeper into an ocean, it gets darker and darker until you can hardly see a thing. Some fish that live there make their own light. Why is it so dark? Tags: science oxford-science student-book-7 physics Created: 2026-05-19 Last Updated: 2026-05-19
After this topic, you will be able to:
Think back
- Name three things that give out light.
- Name three things that you can see through.
- Name three things that you cannot see through.
Key idea Rays are straight lines that we draw to show how light travels from a source or luminous object. The light is reflected off a non-luminous object into the eye.
Key words source, emit, reflect, absorb, ray, luminous, non-luminous, transmit, transparent, translucent, opaque, light-time
As you go deeper and deeper into an ocean, it gets darker and darker until you can hardly see a thing. Some fish that live there make their own light. Why is it so dark?
What happens to light as it travels?
What happens when someone looks at a book? A source of light, like a light bulb, emits (gives out) light. This light reflects off the book and into their eye. They see the book when the light is absorbed in their eye.
We draw lines called rays to model what beams (lines) of light do. You can see this in Figure 1.
Figure 1You can see objects because light reflects off them.
Objects that give out light are luminous. Most objects that you look at are non-luminous. They do not emit light. You can see them because they reflect light into your eyes. Light spreads out, just like Sound.
When someone looks through a window, light travels through the glass and into their eye. The glass transmits the light. When light travels through glass, Perspex, or surface water, most of the light goes through. These materials are transparent – you can see through them. Transparent materials still absorb a small amount of light. In very deep water, like in Figure 2, most of the light is absorbed.
A Describe the difference between the words ‘emit’ and ‘transmit’.
Figure 2Water absorbs light, so it is dark at the bottom of the ocean.
Materials like tissue paper or frosted glass (see Figure 3) are translucent. Light can travel through them but it is scattered (spread far apart), so you cannot see through them clearly.
Figure 3Frosted glass is translucent.
Materials that do not transmit light are opaque. Opaque materials produce shadows. You can predict the size and shape of shadows because light travels in straight lines.
B Describe the difference between a translucent material and a transparent material.
What can light travel through?
Light can travel through gases, like the air. It can travel through some liquids, like water; and some solids, like glass. It can even travel through completely empty space, which is called a vacuum. It travels as a wave, but does not need a medium to travel in.
Literacy skills Make up three sentences about a light bulb and a flower in a vase of water. Use the words below. Try to use them only once if you can.
- emit
- transmit
- reflect
- absorb
- luminous
- non-luminous
- transparent
- opaque
How fast does light travel?
It takes light about eight minutes to reach Earth from the Sun, a distance of 150 000 000 (150 million) km (see Figure 4). The speed of light is about 300 000 km/s. Sound travels about a million times slower than light (0.34 km/s in air). Astronomers use ‘light-time’ to measure distances in space.
- A light-minute is the distance that light travels in one minute.
- A light-year is how far light travels in a year.
Light-time is a measure of distance, not time.
Figure 4There are two ways of showing the distance to the Sun.
C Define a light-second.
Stretch zone
- Copy and complete the sentences. The Sun is luminous/non-luminous because it emits/transmits light. The light reflects/transmits off an object that is luminous/non-luminous into the eye. Most objects do not transmit light. They are translucent/opaque. A ‘light-year’ is the distance/time light travels in a year.
- Explain why it is so dark at the bottom of the ocean even though water is transparent.
Stretch zone
- Use key words to describe the journey that light takes from the Sun to the eye when someone looks at fish in a lake.