Light and colour: mirrors, rainbows and a blue sky
How light travels, how mirrors reflect, how white light splits into colours, what gives objects their colour, and why the sky is blue. With a make-a-rainbow experiment.
Read
Light travels in straight lines called rays. It can change direction when it hits a mirror or passes from one material into another, such as from air into water.[3]
When light hits a mirror, it bounces off at the same angle it arrived: the angle of reflection equals the angle of incidence. A smooth mirror reflects neatly, so you see an image. Rough paper scatters light in all directions, which is why you can read a page from any angle.[4]
White light is a mixture of all the visible colours. A glass prism or a raindrop bends each colour by a slightly different amount, so the colours spread out: red, orange, yellow, green, blue and violet. Violet bends most, red least. A rainbow forms when sunlight is bent and reflected inside raindrops.[5]
Why is a strawberry red? An object's colour is the light it reflects; the other colours are absorbed. A strawberry reflects mainly red light and absorbs most of the rest.[2]
Why is the sky blue? Sunlight contains all colours. The gases in the air scatter blue light, which travels as shorter waves, much more than red light, so blue light reaches our eyes from all over the sky. At sunset the light passes through much more air, most of the blue is scattered away, and the reds and yellows are left.[1]
Light is the fastest thing there is: about 300,000 km every second. Even so, sunlight takes about eight minutes to travel the 150 million km from the Sun to us.[6][7]
Try it: Make a rainbow
You need
- A clear glass of water
- A sunny window (or a strong torch)
- A sheet of white paper
- Optional: an old CD
Safety
Never look directly at the Sun. Keep the water away from electrical sockets.
What to do
- Fill the glass almost to the top with water.
- Put it on a table or windowsill in bright sunlight, near the edge.
- Hold the white paper on the floor or a chair below, and move it until you see a band of colours.
- Write down the order of the colours you see.
- Optional: tilt a CD in the light. Do you see the colours again?
What happens, and why?
Sunlight is a mixture of colours. As it passes into and out of the water, it bends, and each colour bends by a slightly different amount, so the colours spread out into a small rainbow. Violet bends most and red least, so they appear at opposite edges.[5]
Play: Light detective
What is light doing here? Reflecting, splitting into colours, scattering or being absorbed?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16