Speed of Light: How Fast Is Light and How Was It Measured?

When you switch on a light, the room seems to brighten instantly. For most of human history, people thought light simply arrived everywhere at once. It was only in the 1600s that scientists showed light takes time to travel, and it took another three centuries to measure its speed precisely.

Today light speed is one of the most important numbers in science. It sets the scale of the universe, links mass to energy through E = mc², and keeps your phone’s GPS accurate. This guide explains its exact value, how it was measured, how it changes in different materials and why it acts as a cosmic speed limit.

What Is the Speed of Light?

Light speed is how fast light and all other electromagnetic waves travel through empty space. Its value is exactly 299,792,458 m/s. It is exact because, since 1983, the metre itself has been defined as the distance light travels in a vacuum in 1/299,792,458 of a second.

Unit	Value (approximate)
metres per second	3.00 × 10⁸ m/s
kilometres per second	299,792 km/s
kilometres per hour	1.08 × 10⁹ km/h
miles per second	186,282 mi/s

To picture how fast that is: light could travel around Earth’s equator about 7.5 times in a single second.

How Did Scientists First Measure c?

Measuring something that fast was a real challenge. Each new method gave a more accurate value.

Year	Scientist	Method	Result (approx.)
1638	Galileo Galilei	Two people flashing lanterns between hilltops	Too fast to measure
1676	Ole Rømer	Timing eclipses of Jupiter’s moon Io	About 220,000 km/s
1849	Hippolyte Fizeau	Light through a rapidly spinning toothed wheel	About 313,000 km/s
1862	Léon Foucault	Rotating mirror	About 298,000 km/s
1926	Albert Michelson	Improved rotating mirrors over 35 km	About 299,796 km/s
1983	International agreement	Speed fixed by definition	299,792.458 km/s exactly

Rømer noticed that Io’s eclipses came later than predicted when Earth was moving away from Jupiter. He realised the delay was the extra time light needed to cross the larger distance. It was the first proof that light has a finite speed.

How Fast Does Light Travel in Water and Glass?

Light slows down when it passes through matter, because it interacts with the atoms along the way. The amount it slows down is described by the refractive index, n.

Material	Refractive index (n)	Speed
Vacuum	1 (exactly)	299,792 km/s
Air	1.0003	about 299,700 km/s
Water	1.33	about 225,000 km/s
Glass	about 1.5	about 200,000 km/s
Diamond	2.42	about 124,000 km/s

This change of speed is what causes refraction, the bending of light at a boundary. It is why a straw looks bent in a glass of water and why diamonds sparkle so brightly.

Light Travel Times and the Light-Year

Because light takes time to travel, when you look at the sky you are seeing the past. You see the Moon as it was about 1.3 seconds ago and the Sun as it was over 8 minutes ago.

For distances between stars, astronomers use the light-year: the distance light travels in one year. It is about 9.46 × 10¹² km. The nearest star beyond the Sun, Proxima Centauri, is about 4.2 light-years away, so its light is over four years old when it reaches us.

Why Can Nothing Travel Faster Than Light?

Einstein’s theory of special relativity starts from the idea that light speed is the same for every observer, no matter how fast they are moving. One consequence is that as an object with mass speeds up, the energy needed to accelerate it further grows without limit. Reaching c would take infinite energy, so it can never happen.

Other strange effects follow from the same idea. Moving clocks tick more slowly (time dilation), moving objects are shorter in their direction of travel (length contraction), and mass and energy are linked by E = mc². The c in that famous equation is light speed.

Why Light Speed Matters in Everyday Life

• GPS navigation: your phone works out its position by timing radio signals from satellites. An error of one microsecond would put you about 300 m off.
• Fibre-optic internet: data travels as light pulses through glass at about 200,000 km/s.
• Radar and lidar: speed cameras and self-driving cars measure distance by timing reflected waves.
• Measuring the Moon: scientists bounce lasers off mirrors left by the Apollo astronauts. The round trip takes about 2.56 s and reveals the Moon’s distance to within millimetres.
• Thunderstorms: you see lightning almost instantly but hear thunder later, because sound is nearly a million times slower. Count the seconds and divide by 3 to estimate the distance in kilometres.

Light, Radio and X-Rays All Travel at c

Visible light is only a small part of the electromagnetic spectrum. Radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays are all the same kind of wave, made of changing electric and magnetic fields. In a vacuum they all travel at exactly the same speed, c. What makes them different is their frequency and wavelength.

The wave equation links these quantities: c = fλ. A radio station broadcasting at 100 MHz sends out waves 3 m long, while green light, with a frequency of about 6 × 10¹⁴ Hz, has a wavelength of only about 500 nm. You can try these numbers in the wave speed calculator.

Wave	Typical wavelength	Speed in a vacuum
Radio	1 m to 1 km	c
Microwave	1 mm to 30 cm	c
Visible light	400 to 700 nm	c
X-ray	about 0.01 to 10 nm	c

Exam Tips: Light Travel Time and Refractive Index Questions

Questions on light speed usually test one of three skills: using t = d ÷ c, using v = c ÷ n, or using c = fλ. A few habits make them straightforward.

1. Write c = 3.0 × 10⁸ m/s unless the question gives a more precise value.
2. Convert every distance to metres and every time to seconds before substituting.
3. Use standard form on your calculator to avoid typing mistakes with long strings of zeros.
4. Check that your answer makes sense: light in a material must be slower than c, and n must be 1 or more.
5. Round your final answer to the same number of significant figures as the data in the question.

Frequently Asked Questions