The Speed of Light and How We Measured It

The fastest thing there is

Flick a light switch and the room fills with light so fast it feels instant. For most of history people assumed it was instant. They were wrong — but only just. Light travels at about 300,000 kilometres every second, fast enough to circle the Earth seven and a half times in the time it takes to say "one". Measuring that staggering speed took some of the cleverest experiments in science.

Physicists call the speed of light c. Its exact value is 299,792,458 metres per second, usually rounded to 3 × 10⁸ m/s. Light is part of the electromagnetic spectrum, and every member of that spectrum travels at this same speed in a vacuum.

Rømer: light is not instant (1670s)

The first proof that light takes time came from the sky. The Danish astronomer Ole Rømer was timing the eclipses of Io, one of Jupiter's moons, as it ducked behind the planet. He noticed the eclipses ran late when Earth was far from Jupiter and early when Earth was close.

The reason: when Earth is farther away, Io's light has extra distance to cross, so it arrives later. From the size of the delay Rømer concluded that light travels at a finite, measurable speed — a revolutionary idea in 1676. His value was a bit low by today's standards, but he had proved the key point.

Fizeau: catching light with a wheel (1849)

Nearly two centuries later, the French physicist Hippolyte Fizeau measured c on Earth. He shone a beam of light through the gap between the teeth of a rapidly spinning wheel, sent it to a mirror about 8 km away, and let it bounce back.

If the wheel spun at just the right speed, the returning light would be blocked by the next tooth instead of passing back through the gap. Knowing the wheel's spin rate and the round-trip distance, Fizeau calculated the speed of light to within a few percent of the modern value. Later, Léon Foucault refined the method with a spinning mirror, and in the 20th century lasers and atomic clocks pinned c down so precisely that the metre is now defined from it.

Worked example: how far in one minute?

How far does light travel in one minute?

distance = speed × time distance = (3 × 10⁸ m/s) × 60 s distance = 1.8 × 10¹⁰ m = 18 million km

In a single minute, light covers more than the distance from Earth to Mars at its closest. Over a whole year it travels about 9.5 trillion kilometres — that distance is called a light-year, and astronomers use it to measure the gulf between stars.

Worked example: signal delay to the Moon

The Moon is about 384,000 km away. How long does a radio signal (which travels at c) take to reach it?

time = distance / speed time = 384,000 km ÷ 300,000 km/s time ≈ 1.3 seconds

This is why there is an awkward pause when mission controllers talk to astronauts near the Moon — the conversation has a built-in 1.3-second one-way lag.

The cosmic speed limit

Einstein's theory of relativity revealed something deeper: c is not just the speed of light, it is the maximum speed in the universe. No matter, energy or information can travel faster. As an object approaches c it gets harder and harder to push, and reaching the limit would take infinite energy. So the speed of light is woven into the basic rules of reality — the cap on how fast cause can ever reach effect.

Try it yourself! 🧪 (safe version)

You can estimate the speed of light at home with a microwave oven and a chocolate bar.

1. Remove the turntable from a microwave so the food cannot rotate (ask an adult first).
2. Lay a long bar of chocolate or a plate of marshmallows across the floor of the oven.
3. Heat for just 15–20 seconds, until you see the first melted spots appear, then stop. Let it cool a moment before touching.
4. Measure the distance between two melted spots. That distance is half a wavelength of the microwaves, so double it to get the full wavelength λ.
5. Find the oven's frequency f on its label (usually 2450 MHz = 2.45 × 10⁹ Hz). Multiply: speed = f × λ.

You should land close to 3 × 10⁸ m/s. You have just measured the speed of light with a snack, using the same wave equation physicists use for every wave.