We all know what light is on a basic level, but its true nature has fascinated scientists from Ancient Greek times right up until the present day. Visible light (in other words, the light our eyes can perceive) is electromagnetic (EM) radiation – a type of energy which travels as a wave. Waves are disturbances which travel through space and, more specifically, EM waves are waves where the disturbances are changes in the electric and magnetic fields.
The light we see is just a tiny sliver of the EM spectrum, though, which encompasses the full range of wavelengths that electromagnetic waves can occupy. Other wavelengths of EM radiation include radio waves, ultraviolet, microwaves and X-rays, to name just a few.
But that’s just half the story. While light’s behaviour can often be understood by thinking of it as a wave, some of its properties only make sense when considering it to be a stream of particles – called photons. Confused yet? Physicists reconcile these conflicting observations by considering light to be both a particle and a wave – a concept which goes by the name of wave-particle duality. To avoid the headaches, it’s easiest to think of light as a wave for most purposes.
Light on our planet comes mostly from the Sun. The Sun’s blistering heat causes it to incandesce, or glow (just like the embers of a bonfire), emitting energy as light, which then travels some 149 million kilometres (93 million miles) to reach us. The same principle allows the filament in an old-fashioned incandescent light bulb to brighten up our homes.
Our eyes are sensitive to light of wavelengths between about 390 and 750 nanometres. Each ‘colour’ that we perceive corresponds to a band of wavelengths. Our brains interpret the shortest visible wavelengths as violet and the longest as red, thus giving rise to the terms ultraviolet (UV) and infrared (IR) for the invisible wavelengths lying just beyond the visible spectrum. While we tend to describe the rainbow as having seven colours, in reality it is a continuum of different shades.
Though colours may seem very real to us, they are just our brains’ way of interpreting this narrow band of EM radiation. Other animals see a completely different range of colours to us – and there are many variations in how humans see colour too.
Ever wondered why white isn’t part of the rainbow? As Isaac Newton demonstrated when he shone the Sun’s light through a prism, white light is actually made of the full spectrum of colours combined. When light waves hit an object they can be reflected, absorbed or transmitted. These interactions transform plain old white light to draw out the multitude of colours that we witness every day.
When you ‘see’ an object, what you are actually seeing is the light that it reflects. Reflection occurs when light hits a surface and the light waves bounce off it. Say you are looking at an apple. Light hits the apple and rebounds off it in all directions; this is called scattering. Some of this reflected light reaches your eyes, feeding your brain information on what the apple looks like.
If everything around us reflected the full spectrum of the Sun’s white light perfectly, we’d see the world in shades of black and white. Instead, almost everything transforms white light in some way, creating everything from brilliant blues to murky browns.
Chemical compounds called pigments are responsible for the majority of the colours we see in nature. Pigments absorb certain wavelengths of light and so reflect only a portion of the visible spectrum – these reflected wavelengths are what we detect with our eyes and perceive as colour. A red apple, for example, absorbs green and blue wavelengths of light, reflecting mainly red light.
Many pigments are present in rocks and minerals, but living things like animals, plants and insects also make pigments of their own. Humans, for instance, produce a type of pigment called melanin, which is responsible for the full range of skin tones, as well as eye and hair colours, found throughout the human race. And while a few hundred years ago artists had to rely on natural pigments to create the colours on their paint palettes, synthetic pigments mean we can now adorn our houses, clothes and fingernails with just about any colour under the Sun.
When you mix different pigments in paint, you are actually combining the wavelengths they absorb. So if you mix cyan (blue) paint, which absorbs red and green light, with yellow paint, which absorbs blue light, you get a colour which absorbs red and blue light and reflects green light – in other words, green.
Pigments are just one of the mechanisms splashing colour into our world though. Another is refraction, which allows spectacular colours to be separated out of plain old white light. Light travels at different speeds depending on the medium it is passing through. Glass or water, for example, enforce much lower speed limits on light than air. When two different materials are in contact, light travelling through is forced to slam on the brakes. The change in speed as it passes from one medium to the other causes the beam of light to bend. This, is refraction.
If you were to put a plastic straw into a glass of water and look at it from the side, it appears as though the straw is bent where the liquid meets the air. This is because light travels approximately 30 per cent more slowly through water than it does air. If you wear glasses or contact lenses you can thank refraction for helping you bring the world into focus.
What does this have to do with colours? Different wavelengths of light are refracted at slightly different angles, splitting white light into its component colours. Even minuscule droplets of water can refract light, painting rainbows in the sky. Refraction is also what gives diamonds their multicoloured sparkle.
If you’ve ever looked at soap bubbles up close, you’ll have seen the myriad colours on the surface. The technical term for this is iridescence, and it happens because light waves can interfere with one another when they cross paths. A bubble is, in essence, a very thin sheet of water sandwiched between two layers of soap molecules. When light hits the top surface of the bubble, some of it bounces off and the rest is transmitted down to the bottom surface, where it too is reflected, merging with the light reflected by the top surface.
Having travelled slightly farther, the light waves reflected from the bottom surface are now out of phase with those reflected from the top surface. When they meet, these two waves interfere with each other, amplifying certain wavelengths and dulling others. The result: vibrant colours that change depending on the angle from which you view them. The same effect can be seen on the underside of a CD or in some of nature’s shiniest creatures.
The speed of light
Travelling through a vacuum, light zips along at just under 300,000 kilometres (just over 186,000 miles) per second. Almost all particles in our universe contend with the Higgs field, which interacts with them to give them mass. Photons – the particles which make up light -are the exception. They don’t interact with the Higgs field and therefore possess no mass. This means that no energy is required to change their velocity and there is no limit to their speed. So why is 300,000 kilometres (186,000 miles) per second the cutoff? This is simply a fundamental property of our universe, a constant set in stone when the cosmos came into being.
How do we perceive colour?
The retinas of our eyes have three types of light receptors called cone cells. They respond to light in bands of wavelengths centred around red, green and blue. Each colour we see produces a different combination of responses from these cone cells, allowing our brains to tell millions of different colours apart.
Some people, however, have faulty cone cells, causing colour blindness. In people with red-green colour blindness, the green cone cells are mutated, making colours shift towards the red end of the spectrum. As a result, these people have trouble distinguishing shades of red, orange, yellow and green.
Other people’s brains are wired slightly differently, leading them to strongly associate colours with numbers or letters, or even see colours when they hear certain sounds. Play Beethoven’s symphony to someone with sound-colour synaesthesia and the music will trigger visual fireworks.
Beyond these extreme variations in our perceptions of colour, it’s quite possible that everybody experiences colour in subtly different ways.
Orange – In nature, yellow and orange are often produced by the carotenoid pigment. As their name suggests, carotenoids are found in large quantities in carrots, but also egg yolks and autumn leaves. While it’s not true that eating carrots will improve your vision, your body converts some carotenoids into vitamin A, which is a must for healthy eyes.
Yellow – Astronomers classify stars according to their colour, which matches their surface temperature. Our Sun is a yellow (or G-type) star, meaning that its surface temperature is around 5,500 degrees Celsius (10,000 degrees Fahrenheit). Stars remain in this class for around 10 billion years, so our Sun still has a good 4-5 billion years left.
Red – Red can’t be seen underwater. Water absorbs longer wavelengths of light, so once you get beyond about ten metres (33 feet), red light is almost entirely filtered out. The colour provides great camouflage for certain sealife and divers sometimes notice their blood looks dark green if they cut themselves!
What about pink? – There’s no pink in the spectrum of visible light. While the colours of the rainbow each correspond to a band of wavelengths of light, there is no ‘pink’ wavelength. What our brains interpret as pink is actually a mixture of red and blue light waves.
Violet – Mauveine was the first synthetic dye, discovered by British chemist William Henry Perkin in 1856. Until then, the colour purple had been laborious and expensive to create from natural sources, with its basic ingredient being mucus from certain molluscs. Perkin’s vivid violet dye made him very wealthy.
Blue – On a sunny day, the sky is a brilliant shade of blue. This colour comes from gas molecules in the atmosphere, which scatter mostly short blue wavelengths of light. The same effect can be observed by astronauts orbiting the Earth, who see a faint halo of blue around our planet.
Green – Why are plant leaves green? Plants use a pigment called chlorophyll to convert the Sun’s light into energy. Chlorophyll absorbs red and blue light (possibly due to some ancient evolutionary advantage), and therefore reflects mostly light in the yellow and green parts of the spectrum; this is what gives plants their lush, verdant tones.