Speed of light explained in-depth

light caught in skyrise city

The speed of light is one of the most important measurements used by scientists today. Although most of us will have been taught about it at school, it is frequently forgotten about, since the measurements and the numbers are so incredible, and honestly, it really is pretty difficult to get our heads around. In this post, we’ll try and make it simple! We’ll be looking at how fast the speed of light really is, whether it is the same everywhere, whether anything can travel faster than the speed of light and what light years have to do with the speed of light.


light caught in skyrise city


Who discovered the speed of light?

Scientists have been concerned with the speed of light for years, and in ancient times, some believed that the speed of light was infinite and could travel any distance instantaneously. Galileo Galilei attempted to measure the speed of light with an experiment in the early 17th century that involved two people and covered lanterns standing a known distance apart. One person uncovered their lantern, and as soon as the other person saw their light, they uncovered their lantern. During the experiment, Galileo tried to record the time between each lantern signal, but as we now know, light simply moves too quickly to be able to measure it over such small distances in this way.

Ole Roemer was the first person to determine that light is not infinite, in 1676. He did this by studying Jupiter’s moons, establishing that when there was an eclipse, they took place sooner when Jupiter was closer to Earth in their orbits. This observation meant that light was moving at a finite speed, because when Jupiter was further away, it took longer for the light to travel.

French physicist Hippolyte Fizeau was the first person to measure the speed of light that wasn’t an astronomer. Working in 1849, his method used light that passed through a rotating wheel, which was then reflected back from a mirror that was situated a significant distance away.

Albert Einstein’s work on the theory of special relativity was published in 1905, which was important for scientists to help uncover the speed of light – since it helped to determine that an object approaches the speed of light, its mass becomes infinite and it is unable to go any faster than light travels.


albert einstein


Albert Michelson was one of the first to make a precise calculation about the speed of light. He calculated how Earth’s motion through space affected how the speed of light is measured, and found that the speed of light is the same no matter what Earth’s motion is. Michelson’s most accurate research was carried out in the 1920s, using an eight-sided rotating mirror apparatus. Interestingly, this work helped Michelson to win a Nobel Prize, and he is the only person in history that won for a non-discovery of anything. He was attempting to detect the ‘luminiferous aether’ – the medium that it was hypothesised that light travelled through – and the experiment proved this did not exist.

Since Roemer’s work in 1676, there have been countless numbers of calculations to show the speed of light, with varying degrees of accuracy:

Roemer – 214,000 km/s

Fizeau – 315,000 km/s

Michelson (1879) – 299,910 km/s

Michelson (1926) – 299,796 km/s

The speed of light in a vacuum (which space is considered, because it is nearly empty) wasn’t determined until as late as 1983, when an international commission on weights and measures set it officially, using the calculation still in use today – which is 299,792 kilometres per second.


Why is the speed of light important?

The speed of light is important for scientists because it is a fundamental constant of nature – it is never changing. It is the measure that limits velocity in the universe (as far as we know) and is an important factor for everything.

Albert Einstein’s theory of relativity, which is known best by the famous E = mc2 equation shows us that energy equals mass times the speed of light squared. This suggests that nature sees energy and mass are different forms of the same thing, and so with the right conditions, energy can become mass, and mass can become energy.

Einstein’s work also showed that objects that travel at close to the speed of light can experience changes to length and time, and these changes depend on the relative motion of the observer and the object. Essentially, as we get closer to the speed of light, lengths look like they are getting shorter, and clocks seem to tick more slowly.




How fast is the speed of light?

The speed of light is simply how far light can travel in a straight line in a given amount of time. There are a number of different ways in which we can show the speed of light, depending on the unit of distance or time that we use. So, the speed of light can be shown using seconds, minutes, or hours, using metres, kilometres, or miles. It can look like:

299,792,458 metres per second

300,000 km per second

186,282 miles per second

1,080,000,000 kilometres per hour

671,000,000 miles per hour

If these numbers don’t mean anything to you because it is hard to visualise, then perhaps this might help. Assuming that there were no allowances required for the curve of the Earth, then a beam of light travels so fast that it could cover the distance around the equator (24,901 miles) seven and a half times in a single second.

Despite the journey being 150 million km, it takes light from our Sun just 8 minutes and 20 seconds to travel to Earth. For light to travel from the nearest star to our Sun (Proxima Centauri), it takes 4.2 years, which helps us to understand exactly how far away that star is. It also means that when we see light from other stars, we’re seeing light that started travelling a long time ago.


What is a light year?

A light year is a measure that is linked with the speed of light and is the distance that a beam of light travels in a straight line during one Earth year – that is, 365 Earth days. Rather than being a measure of time as the name might imply, it measures distance. To calculate the actual distance of a light year, you simply need to multiply the speed of light by the number of seconds in a year. If you want to do the calculations, you can follow along here. If maths isn’t your thing, then:

A light year is 6 trillion miles (that’s 6,000,000,000,000 miles), or 9 trillion kilometres.

To try and picture how far a light year is, you can use the circumference of the Earth (24,900 miles). Imagine this distance, if you can – and then lay out 7.5 more (this distance is a single light second – how far light travels in a straight line in a single second) and then add 31.6 million more of those lines end to end. It is a huge distance to try and imagine, and even with this visual, it is incredibly hard!

This measurement can help us to understand just how huge space is – the Milky Way galaxy is one hundred thousand light years across, and Earth is twenty six light years away from the centre of the Milky Way galaxy. The space shuttle Discovery travels at around five miles per second, which means it takes around 37,200 years to cover just one light-year – so the reality is that with current technology, humans are unlikely to cover this distance any time soon.


speed of light explained galaxy


Is the speed of light the same everywhere?

The speed of light is measured in a vacuum. Through the vacuum of space, no matter how much energy a photon has, it always travels at the speed of light – 300,000 km per second. The highest-energy photon and the lowest-energy photon ever observed both travel at exactly the same speed in a vacuum.

When outside of a vacuum, or when travelling through a different material, light can be slowed down. Materials that are transparent to light allows photons to travel through it, including water, acrylic, crystals, glass, and air. But electrons in those materials interact with photons and slow them down. So, if light is in a material, it is being slowed down. Light travels through Earth’s atmosphere almost as fast as light travels in space (it is affected by anything that is in the atmosphere, but it is almost the same speed), while light passing through a diamond travels at around half the speed of light. But even half the speed of light is incredibly fast, around 124,000 km per second!

You’ve probably seen adverts from internet service providers that suggest that signals are travelling at the speed of light through their optical fibres. They’re implying that their service is the fastest possible, to encourage customers to sign up, but the reality is that as light travels through those cables, there is interference from other photons being released from the glass atoms. Since the speed of light is measured through a vacuum, the actual speed is 40% faster in a vacuum than it is through fibre optic cables.

That isn’t all though. Since different photons have different energies and their electric and magnetic fields oscillate at different rates, photons with different energies will slow at different rates. When shining white light (which is made up of all the colours of the spectrum) through a prism or water, the more energetic photons slow down more than less energetic ones, which causes the colours to separate – causing that rainbow effect that we all know so well.

However, as long as atoms remain the same, our definitions of time, length, and the speed of light will never change.


Can anything travel faster than the speed of light?

Science fiction has had humans imagining that we may be able to travel faster than the speed of light for decades – and it makes sense, since in order to get to, and explore other planets, we would need to travel that fast. Star Trek has facilitated this – with ‘warp speed’ being multiples of the speed of light.

However, despite the imaginations of science fiction writers, so far nothing built by humans has been able to go faster than the speed of light. One of the fastest objects made by humans is the New Horizons space probe, which launched in January 2006 and passed Pluto and Charon in July 2015. It has been travelling at a speed similar to that of the Earth, just over 16 km per second – which is nowhere near the speed of light (300,000 km per second).

While we haven’t yet been able to create anything that is able to travel as fast as light, it is possible to make some things travel at speeds close to the speed of light. As early as the 1960s, William Bertozzi established that it was possible to make electrons travel at increasing velocities. The more energy that was applied to repel the electrons, the faster the electrons were accelerated. While Bertozzi’s team were able to get close to the speed of light, they weren’t able to reach it.

To date, the only particles that have been observed travelling at the speed of light are photons – which are the particles that light is made up of. The reason that they can travel this fast is that they have no mass, meaning that – unlike electrons – they don’t get heavier as they accelerate. The second that photons are released; they are already travelling at the top speed they are able to achieve.


speed of light atom explained


Since the amount of energy that it would take to propel a small (10m x10m x 10m) spaceship to travel at speeds faster than light (and manipulating space-time in order to do so, using ‘warp bubbles’ – also known as the Alcubierre Warp Drive) would be similar to the entire mass of Jupiter – and that energy would need to be provided constantly – at present, there isn’t a solution to how travelling that fast will be possible.

Research is ongoing though, and visible light is just the start of it – because photons make up plenty of other types of waves. Radio waves, microwaves, ultraviolet radiation, X-rays, and gamma rays from decaying atoms are all made up of photons – which make up the electromagnetic spectrum.

There is one thing that we know is travelling faster than the speed of light: the universe. The universe is expanding at a rate of roughly 68 kilometres per second per megaparsec, where a megaparsec is 3.26 million light-years. This is a whole other discussion though! You can read more about that here.


Why is light so important?

Light is important to humans (and many other mammals) for a number of reasons. Natural light sets the course of our days, ruling our circadian rhythms and determining when we fall asleep and when we rise. Of course, humans have been creating light using fire, and in ever brighter amounts since the discovery of electricity, and the invention of the electric motor.

Lighting doesn’t just make our modern life possible – allowing us to work around the clock, or create optimal ambience for the activities that we have planned – it is essential for us to thrive. Our bodies don’t just need light to see, we need it to keep us healthy – both physically and mentally. Particularly in the Northern Hemisphere, throughout the winter months when there is less daylight, doctors report a huge increase in the number of people with symptoms of depression, which tends to lift as the days get longer in the spring.

It isn’t just our health that light can have an effect on. Light can change our mood dramatically, making us more likely to make rational decisions, to concentrate or negotiate better, or even to make us more receptive to making a purchase decision in a shopping centre.


speed of light cityscape


Key takeaways

The speed of light is an essential concept for scientists to understand, since it is one of the physical constants – a fundamental invariant quantity observed in nature. For most of us outside of the world of science, it is also just that – a physical constant that we take for granted as never changing. Whether you’re studying the speed of light for the first time, or refreshing your memory, there’s a lot to get your head around. The main points to remember though are:

  • All types of light – whether high energy or low energy – always travel at the speed of light if it is travelling in a vacuum
  • You can’t change the speed of light with your own motion or the light’s motion
  • The speed of light can be changed while it is in a non-vacuum material (including air), for as long as it is in that material
  • When in a material, light of different energies will change speed by different amounts, depending on the properties of the material
  • When the light leaves the material and re-enters a vacuum, it continues at the speed of light
  • As far as we understand to date, the speed of light is exactly the same at all times throughout the Universe


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charles author bio

Charles Barnett Managing Director

Charles started Lyco in 1995 with just 4 enthusiastic employees and has grown it considerably over the past 25 years. Charles is also the Managing Director of Lighting Direct and newly acquired Online Lighting. He now has a team of 50 lighting experts working on growing Lyco Group to be the UK leader in lighting for both businesses and homes. Away from the office he is a keen cyclist and is proud to have cycled 1017 miles from Lands End to John O’Groats to raise money for a new residential centre for adults with multiple learning difficulties.