Light | The Schools' Observatory

When we talk about light we usually mean the light we can see with our eyes.

This is also called visible or optical light.

Light is one way energy can be transferred - moved from one place to another.

Most of the light we have on Earth comes from the Sun.

The energy it brings means that life can survive on Earth.

A bright sun at the top of image, slightly off-centre, shines down over tall, thin trees with green foliage. The sky is bright blue with some white clouds visible.

This work by Emmanuel Huybrechts is licensed under Creative Commons Attribution 4.0 International

Luminous and Non-Luminous Objects

There are other sources of light on Earth, like fire and lightbulbs. Lightbulbs use electricity or solar power, and we see some of the emitted energy as visible light.

Objects that create their own light are known as luminous objects.

Other objects do not create their own light but reflect the light landing on them, like a book or clothing. The Moon reflects light from the Sun.

We call these non-luminous objects.

How does light travel?

A graphic of a light bulb at the bottom-centre of the image. There are five red arrows pointing away from it, all at different angles. — A ray diagram showing light (represented by the red arrows) emitted from a bulb

For us to see, light must travel from a light source to our eye. The light we see on Earth travels in straight lines.

A ray diagram shows the path light takes as it travels.

It includes what happens when it reaches a surface, such as a lens, mirror or other object.

Light is represented with straight lines, and the arrows point in the direction the light travels.

A cartoon sun is on the left with two horizontal red arrows leaving it. The arrows travel through a graphic of a window in the centre, labelled "Window (Transparent Object) and continue until the end of the image on the right. — Light (represented by the red arrows) passing through a transparent object.

Anything in the path of light has an effect on what we see.

For example, a window is transparent – this means that the light can pass straight through it.

An opaque object, like a ball, will block the light travelling towards it. This is what creates a shadow.

There is a cartoon sun on the left of the image wit four red arrows leaving it. The two arrows in the middle extend up to a blue circle labelled "Ball (Opaque Object) and stop at the edge. The arrow leaving the sun from the top and bottom are angled to extend past the ball without hitting it, and they continue until the end of the image on the right. In the space between them, a larger grey circle is labelled "Shadow". — Light (represented by the red arrows) is blocked by an opaque object. Only light not in the objects path can travel past it. This creates an area with no light, which is the objects shadow.

A cartoon sun is in the top left corner. A red line with an arrowhead pointing downwards extends to a graphic of an open book with a green cover. Another red line, seemingly mirroring the other one but with an upwards arrowheads this time, extends up to a graphic of a human eye in the top right corner. — Light (represented by the red arrows) reflecting off the surface of the book to reach our eyes.

Some of the blocked light is absorbed by the object, making it hotter, and the rest of the light is reflected.

This is how we see things.

Light hits the surface of an object and bounces off the surface, sending the light to our eyes.

Some objects reflect more light than others. We can change the direction of light by placing a very reflective object, like a mirror, in its path. This is how most telescopes work.

Colours of Light

The light we see looks white, but it is made up of every colour of the rainbow.

You can split white light into all of its colours by placing a glass prism in its path. We see this effect when the Sun shines through rain in the air to make a rainbow.

On the left, an angled black arrow travels upwards and hits the large triangle in the centre of the image which is labelled as "Glass Prism". Where the arrow touches the edge of the triangle, it seemingly splits apart to show rows of red, orange, yellow, green, blue, indigo, and violet stacked on top of each other. These angled rows of color reach the other edge of the triangle and leave at a slightly different angle, causing the rows to become slightly taller. Each colour is labelled on the right.

This work by Jibin 1840404 is licensed under Creative Commons Attribution Share Alike 4.0 International

Three overlapping circles of red (labelled "R"), green (labelled "G"), and blue (labelled "B"). Between the red and green is a yellow section. Between the red and blue is a pink section. Between the green and blue is a cyan (light blue) section. In the centre, where all 3 circles overlap, is a white section. — How the primary colours of light combine

The 3 primary colours are red, green and blue.

Our eyes have special cones inside of them that detect colours. We have 3 types of cones - one for each of the primary colours.

When more than one colour is detected, they combine together.

These different mixtures create all the other colours we can see.

An object's colour is the result of the light which is reflected. White objects reflect all colours of light landing on them, perfectly black objects reflect no light.

On the bottom left there is a white rectangle labelled "White Surface" and a black rectangle labelled "Black surface" on the bottom right. Each have arrows of red, orange, yellow, blue, green, indigo, and violet coming from above and hitting the surface. Each group of arrows is labelled "White light". The arrows are reflected upwards from the white surface, but not on the black surface. — White light (made of all the colours of the rainbow) hitting a white surface and a black surface

If an object looks green it’s because it is reflecting the green light landing on it, but absorbing the other colours.

From the top left of the image, arrows colored red, orange, yellow, blue, green, indigo, and violet are collectively labelled as "White light". The arrows traveling downwards and hit the surface of a green circle in the centre of the image at the bottom. The circle is labelled "Object". Another green arrows mirrors from the first one and travels upwards, labelled as "Green light". It then reaches a graphic of an eye towards the top right of the image.

Light is actually much more complicated this. As well as the light we can see with our eyes there are lots of other types of light. The full range of light is called the electromagnetic spectrum.

Part of

Physics

Light

Electromagnetic Spectrum

Colour

Energy

Reflection, Refraction and Diffraction

Shadows

Shadow

Waves

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