Unit 10, lesson 49. About 12 minutes.

Speed of light

Could you ever see the Sun as it looks right now?

Play

Getting the experiment ready…

Explain

Light is the fastest thing there is.

It takes 8 minutes from the Sun.

Light travels incredibly fast: about 300,000 km every second. It could go around the Earth over seven times in one second.

Space is so huge that even light takes time. Light from the Sun takes about 8 minutes to reach us.

So you always see the Sun as it was 8 minutes ago!

c ≈ 300,000 km/s, or 3 × 10⁸ m/s. Nothing with mass can reach this speed.

A light year is the distance that light travels in a year, about 9.5 trillion km.

Worked example: the Sun is 150 million km away. So t = 150 000 000 / 300 000 = 500 s, or about 8.3 minutes.

Light leaves the Sun
It crosses space
8 minutes later: you see it

Want the full story?

The exact value of c is 299,792,458 m/s, and the sim uses this. Light goes more slowly in materials. Distances between planets change as they orbit, so the figures here are typical values. Radio signals to Mars can take over 20 minutes in each direction.

The Formula

time = distance ÷ speed of light

t = \frac{d}{c}

$t$: the travel time, measured in seconds (s)
$d$: the distance, measured in kilometres (km)
$c$: the speed of light, about 300 000, measured in kilometres per second (km/s)

Try the numbers

Distance: 150 million km

Time for light to get there: 500.35 s

Try it at home

You need a calculator, a torch.

You need: a calculator, a torch.

Turn on a torch. The light is there at once!

Shine a torch at a far wall.
See how the light seems to arrive at once.
Work out how long light takes for 30 m: 30 ÷ 300 000 000.

What's happening: light takes only a tenth of a millionth of a second for 30 m, which is too short to notice.

Physics is life

Your body

You see things almost at once.

Light reaches your eyes so fast that you see things almost as they happen.

Daily life

The internet uses light.

Fibre optic cables carry the internet at about two-thirds of the speed of light.

Nature

Lightning comes before thunder.

You see lightning before you hear thunder, as light is so much faster.

Space

Starlight is very old.

The light from some stars that you see set off hundreds of years ago.

Quick check

Question 1 of 3

How fast is light?

Words to know

Light year: The distance light travels in one year.

Light year: The distance light travels in one year.
Light: Energy that travels as waves you can see.
Speed: How far something goes in each second.

Next up

Static electricity