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


Looking for more news, inspiration, or advice? Try our Lighting Advice section.


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.

How Light Works: The Ultimate Guide

When you’re starting to choose lighting for your home, it can be tough to know where to begin, even if you’re certain of the style you’re looking to create in your home. With so many different lighting options available, and lots of variables between the different types of bulbs, it can be pretty mind blowing. Through this post, we’ll take you through the different categories of lighting that you need in a room, and the types of fixtures that you can use for each category. From there, we’ll cover the important issue of light bulbs, since they’re pretty complex, and understanding how they work makes buying them much simpler.

We’ve written this post with absolute beginners to lighting in mind – those who are setting out living on their own for the first time, new home owners, and those who have previously ‘made do’ but are now ready to create a stylish home that they can be proud of.


How light works a beginners guide light bulb sketch


What different types of lighting are there?

Where once we would have relied solely on the ceiling light to provide all the light in a room, today designers and lighting experts recommend having a range of different lighting available in a room – with at least one from each lighting category, to allow for optimal amounts of light for all the types of activities that may take place in the room. The main categories are ambient lighting, task lighting, and accent lighting.

As an example, in a bedroom:

  • Ambient lighting allows for getting dressed and moving around
  • Task lighting would allow for reading or for applying makeup
  • Accent lighting may be used to illuminate wall accessories, behind a TV or an aspect of the room’s design


Ambient lighting

Ambient light completely fills the room with light, and allows you to use the room as if it is daytime. It is often provided by the ceiling light in the centre of the room, but there are other types of lighting that can contribute to ambient indoor lighting.

It may be the case that producing the optimal amount of lighting requires a number of lights to be used at the same time.


Task lighting

Task lighting does exactly what it sounds like – it produces light for specific purposes. This could be for reading, cooking or for studying. Task lighting can be provided by a range of lights, including:

The type of task lighting that may be required is likely to differ from room to room and between the type of activity that it is needed for. For example, a desk lamp that is only used with a PC, or under-counter lights in the kitchen may be able to be a fixed type, whereas for craft activities, a lamp with a moveable head may be desirable.


Accent lighting

Accent lighting can take many forms, but is primarily there to highlight an accessory such as artwork or an architectural feature. This can be provided by lots of different types of lights:

Since accent lighting is there to highlight, then simpler light fixtures can help to keep the focus on the feature.


scene accent highlight lighting example in an art gallery


What type of lighting fixtures should I use?

The decision about which lighting fixtures you prefer is a personal one, and should be based on the type of look you are aiming to achieve in the room – but having at least one (if not more) of each type of lighting means that you’ll have the flexibility, and will be able to create the brightness that you need at any given time.


Ceiling lighting

Ceiling lighting is often the first lights that get chosen when moving into a new home. They tend to provide the general illumination in a room, but also add to the styling of the room.

While your personal preference, and the style and visual weight will ultimately guide your choice of ceiling lighting fixture, there are approximate calculations that you can use to narrow down the type of fixture that will work best in the space.

Working with the dimensions of your room in feet (rather than centimetres), add the number of feet in the width to the number in the length. The answer will give you the number of inches that your light fitting diameter should be. For example, if your room is 6 foot by 10 foot, the optimal size of light fitting would be 16 inches – which is about 40cm.

If you’re choosing statement lighting such as pendants that hang from the ceiling, the first thing to do is to get your tape measure out. It should go without saying, but you will need sufficient clearance underneath the fixture to avoid breakage and potential injuries. When hanging feature lighting from the ceiling, it is essential to have a minimum of two metres clearance between the floor and the bottom of the fixture (and if some members of the household are particularly tall, maybe a little more). If you’re hanging pendant lighting above dining tables, there should be at least 65cm between the table and the lighting fixture.


Wall lights

Wall lights can be used for all kinds of purposes – either to add to ambient lighting, as task lighting or as accent lighting – as well as providing a decorative accessory in a room with simpler styling. The function of the wall lights will influence the type of fixtures that you choose, while the style of the room will influence the type of wall lights that are chosen.

Where wall lights are used for effect, there may be a number of wall lights used – either paired, or in a series – or they may be positioned in an unusual spot. Using wall lights with colourful shades, and matching them with ceiling lighting can help to tie the overall look and feel of the room together, particularly where other accessories in the room coordinate.



Whether you are choosing floor lamps, table lamps or desk lamps, as with wall lights, you’ll make your decision based on the size of the room, what category of lighting it is providing, and what activity the lamp will be illuminating.

Depending on their size, table lamps may contribute to the ambient light in a room, or may be purely used as task lighting – for example, for reading, or as a portable lamp that can be used for sewing or other types of crafts. Since more of us are using electronic devices for reading today (and are attached to our phones too), many styles of lamp now include USB charging points as part of their design. Where lamps are used for reading, dimmer switches are desirable – since this allows for better lighting throughout the day and night, when different levels of lighting are appropriate.

When choosing a table lamp:

  • Be sure that the bottom of lamp shade is approximately at eye level when seated, so that the bulb doesn’t shine directly in your eyes
  • Situate the lamp close enough to illuminate the task sufficiently
  • If the room has people walking through it often, then ensure your lamp is stable, so to prevent it tipping over
  • Ensure that the relative proportion of the lamp is appropriate – it should be no more than one and a half times the height of the item furniture that it sits on

When choosing a floor lamp:

  • As with table lamps, ensure that the lower edge of the shade is at eye level when seated
  • Avoid obstructing the view of décor such as artwork, as well as between seating locations
  • The smaller the room, the smaller the lamp should be
  • Keep safety first – if the floor lamp is likely to become a trip hazard (especially for children or elderly residents) it isn’t the right one


table desk floor lamp lighting


What type of light bulb do I need?

Once you’ve established the type of light fittings that you’re going to use in a room, and you’ve decided on the exact ones that you’re going to buy, you’ll need to choose your light bulb. Not all light fittings will have bulbs sent with them, and where they are, the bulb may not be to your preference. Choosing a light bulb isn’t always easy or straightforward either – since there are a number of variables that you’ll need to consider.


Choosing the right light bulb

If you’re replacing a bulb in an existing light fixture, the rule is to replace like with like – especially when it comes to the type of fitting. But the fitting isn’t the only thing you’ll need to consider. You’ll need to think about what the light is for (is it for ambient, accent or task lighting?), and what shape suits the fixture, as well as the cost both of the purchase, and in the longer term. Let’s take a look in more detail.



While there are plenty of different types of light bulb fitting, when we’re looking at domestic light fittings, there are three main types that prevail: bayonet, screw cap, and pin and push-fit base bulbs. Generally, with these types of light fixtures, it should be easy to find a bulb that will suit.

When changing a bulb, you’ll need to find the same type of fitting for the replacement. Although adaptors are available, they should rarely be required, since different types of bulbs are so easily found today, especially from lighting specialists.

But establishing the fitting of the bulb is the easy part, since you don’t have to opt for the same colour or brightness – which is what we’ll come to next.



As we’ve already determined, there are many different types of lighting fixtures, that can be used for different purposes. The function that the light fitting fulfils will determine the brightness, and the hue of the bulb that you choose.



Newer types of bulbs use different amounts of watts to produce the same amount of brightness – and there are differences between types. Historically, light bulb manufacturers used the number of watts that were required to produce the amount of light to illustrate the brightness of a bulb.

Today, bulbs are much more efficient, and to produce the same amount of light as an old style incandescent 60 watt bulb, an LED bulb needs just 10 watts, while a halogen bulb requires around 42 watts. The differences in watts required for the same amount of light is the reason that light bulb manufacturers no longer use the measurement of watts to describe the brightness of a light bulb. So rather than measure the amount of power required, manufacturers now use Lumens to describe the levels of brightness that the bulb will provide. The higher the number, the brighter the light provided by the bulb.



When choosing your light bulbs, you’ll also need to consider whether you want a cool toned, or a warm toned bulb. The decision will depend on several factors:

  • What type of fixture you’re using the bulb in
  • The space the light is being used in
  • What activities the light will illuminate
  • The time of day the room, and the light will be used

For rooms that require lighting levels that are near daylight, and that aren’t being used close to bedtime, then a cooler toned bulb can be used – bright white, or slightly blue-toned, will help to mimic natural daylight. Where being alert is not as desirable, such as for bedside table lamps, then warmer, yellow toned bulbs are preferable.

The measurement of bulb warmth is measured in Kelvins. The warmth of candle light is measured at approximately 1,500 Kelvin, while normal daylight (depending on whether it is a sunny or cloudy day) is measured at between 5,000 and 7,000 Kelvin. The higher the number on the Kelvin scale, the colder the light provided will appear, and cooler toned bulbs often appear to be brighter than warmer toned bulbs of the same Lumen measurement.


kelvin colour temperature scale infographic


Choosing the bulb

Although watts are no longer used as a measurement of light bulb brightness, most bulb manufacturers will display both the Lumen measurement, and the equivalent in watts in an older style incandescent bulb, as well as the Kelvin measurement on the packaging. Not only that, but there will also usually be a description in words to describe the type of light – such as ‘warm white’. That means that when you’re in a store picking a light bulb, if you’re used to choosing bulbs based on watts, you’ll be able to see the numbers that you need, and the description will help if you’re not sure.



Once you’ve decided on the colour, brightness and you know the fitting that you need, you can narrow down the shape that you want. There are an incredible array of different shaped bulbs, from traditional ‘bulb’ shapes, to candle, golf ball, pear shape, spiral and stick bulbs – and once you’ve established that you have the correct cap fitting (either screw or bayonet, for example) and the correct brightness, it is up to you. In many cases, if the bulb is hidden, the choice of bulb will be simply down to personal preference and it won’t matter. However, if your lighting fixture intentionally exposes the light bulb, then it may be better choose a style with a decorative filament, or that complements the design of the fixture.



While the general rule of buying the best quality you can afford definitely comes into play here – since a better quality bulb is likely to last longer too. The longer-term cost of running a bulb will also be relevant, especially if you are in your ‘forever home’. The initial cost of LED bulbs are higher than other types, but they are much more energy efficient, and can last for up to 25 years. Not only does this provide much more convenience – you won’t need to replace it as often – but the energy savings can add up dramatically. Savings can add up to more than £180 over the product’s lifetime, which makes it well worth the investment.  ­


Final thoughts

We’ve covered the basics of lighting in this post, and yet we’ve barely touched on design. Understanding the basics of lighting is important though, and can help to guide your decisions when you’re designing, and shopping for the room of your dreams. Our key takeaways:

  • Ensure you have a range of lighting in each room, with at least one light from each category of ambient, task and accent lighting
  • Be careful to measure before making a purchase – having lighting that is in proportion to the room, and that allows sufficient clearance is essential
  • Choosing the best bulb for the purpose should be done carefully, with the correct brightness and colour taken into consideration

Our range of lighting covers all the types of fixtures mentioned in this post, and we stock an extensive range of bulbs. If you are looking for a particular item, or need technical advice, please get in touch – our team of experts are happy to help.


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.