Light and colour Main Index

Light and colour

The human eye is sensitive to a narrow band of electromagnetic radiation (emr) in the range 740 nm to 390 nm (nanometres. A nanometre is a thousand millionth of a metre.) We call emr in this range light. You need not know anything else about emr to understand this Page, but if you do want to find out more about emr please click here To page on electromagnetic radiation

Sunlight consists of a mixture of light of (almost) every wavelength from 740 to 390 nm. Different wavelengths are seen as different colours, but we can only actually see the individual colours if they are separated in some way.

The way our eyes work to enable us to see colours is so important that it has its own Page on this Web Site - to link to it please click here To colour vision

Refraction and the spectrum

Light travels through a vacuum, and also through air, water, glass and many other materials (media). When light passes from one medium to another the rays may be bent, or refracted. This is why a stick may appear to be bent where it breaks the surface of a lake and why a swimming pool appears much shallower than it really is.

Some of the light is also reflected off the surface: this is why you can see yourself in a shop window. If the glass is thick enough you may actually see yourself twice, once where the light is reflected off the outside of the glass and once where it is reflected off the inside. Birds may fly into windows if they see reflections of trees or the sky in them. You can reduce the danger to birds by putting stickers onto windows where this often happens, ideally a red sticker of a bird of prey silhouette: you can buy packs of suitable stickers from the RSPB. This reflection of a part of the light is important when we come to consider the rainbow, at the end of this Page, but is not considered further until then.


The different wavelengths are refracted by different amounts. If we pass sunlight through a rectangular glass block or a sheet of flat glass such as a window, although different colours are refracted by different amounts rays which start parallel end parallel, and so the colours are not separated.
Refraction in rectangular block

If however the opposite sides of the block are not parallel then rays of different colours are bent by different amounts, and so the colours are separated and we can see them. This effect is most marked if we use a triangular prism, but it happens to some extent whenever light passes through anything where the faces where the light enters and leaves are not parallel.
Triangular prism

The colours produced this way make up a full spectrum. The spectral colours are red, orange, yellow, green, blue and violet, although for reasons of clarity only the red, green and blue rays are shown on the diagram. Red has the longest wavelength and so is refracted the least. (For a discussion on indigo and the colours of the rainbow please click here Link to the colours of the rainbow)
If we just want to show how a spectrum is produced a simple prism is fine, but if we actually want to study the spectrum produced by a light source we need to use a spectrometer. This consists of an arrangement of lenses to produce a parallel beam, then two prisms to produce a greater separation of the colours, then another arrangement of lenses to focus the colours onto a screen (or camera or computer input device). Each wavelength shows as a line on the screen, hence the term spectral line.
Spectral lines

If the light source is producing light of many different wavelengths we cannot always see the individual lines as they may merge into each other. A light source producing light of only one frequency (just one spectral line) is called monochromatic.

Once we have separated the colours we can isolate any colour by using a moveable slit.

Monochromatic light

Interference and diffraction effects

We also often see colours in oil films or soap bubbles. This happens because the oil film is only a few nanometres thick. Some of the light is reflected off the top surface and some the bottom surface. Because the thickness of the oil film is about the same as the wavelength of the light the two reflected rays may interfere with each other. In a soap bubble, gravity is pulling the liquid downwards so the bubble is thinner at the top than the bottom, and also the water is evaporating, so the thickness of the soap film is constantly changing and so are the colours. The colours are not usually very clear, and may include browns and other non-spectral colours. Just before the bubble bursts part of the film has become so thin that it appears black: you can see this most clearly in big bubbles.

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This bubble is in the course of construction. You can make bubbles more than 2m in diameter which last for several minutes with this special wand and a bucket of special bubble mixture - search on giant bubble making to find out how to buy them. Enjoy! - and send me some pix of the bubbles.

Of course only a very little light is reflected off a bubble in this way, just enough for us to be able to see it.

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Most light goes straight through it which is of course why we can see through it. But just enough does not for the bubble to leave a very faint shadow on a sunny day.

Very clear spectral colours are often produced by the recorded side of a compact disc, CD ROM or DVD - here the colours are produced by diffraction. But diffraction and interference are outside the scope of this web page.

Colour Temperature

If we heat a metal block it will start to radiate energy. The frequencies of the radiation it emits depend upon its temperature. Initially the frequencies of the radiation are less than that of light. This is infra-red - we cannot see infra-red radiation but we feel it as heat. As the block gets hotter it begins to emit visible light as well, first just red (red hot), then progressively the other spectral colours, until it appears to be white (white hot). Then it starts emitting even higher frequencies - these are ultra-violet. We cannot see ultra-violet radiation but it can damage our eyes and cause chemical changes in our skin.

We often refer to the colour of the light spectrum produced by a particular source in terms of its colour temperature: this is the temperature of a body that will produce a similar spectrum. For various reasons colour temperatures are usually given in Kelvin rather than degrees Celsius, but you can convert from Kelvin to degrees Celsius by subtracting 273, for example 273 K is 0oC. There is more about the Kelvin temperature scale on another Page of this Web Site - to link to it please click here html">To temperature scales As described on this Page remember that temperatures are given in Kelvin not degrees Kelvin.

A candle flame is very yellow, with a colour temperature of about 1900 K; we define white light as having a colour temperature of 6500 K.

Sunlight, daylight and the sky

Light from the Sun has to pass through the Earth's atmosphere to reach the surface of the Earth. The atmosphere contains water vapour, droplets of water and crystals of ice, and these scatter the Sun's rays. Different wavelengths are scattered by different amounts, the shorter wavelengths being scattered the most. This means that by day the sky appears very bright, and blue - the brightness of the sky is the reason why we cannot see the stars by day. If we go up in a high-flying aeroplane we are above most of the atmosphere and the sky is black, with the stars clearly visible even by day.

The atmosphere also absorbs some of the shorter wavelengths, so at midday the Sun itself appears slightly yellow, with a colour temperature of about 5000 K, while in the early morning or late evening it is much redder. Daylight is white light because it contains light from the whole sky not just the direct light from the Sun, and has a colour temperature of about 6500 K.

This is more fully discussed on the Blue Sky Page of my web site.

Atoms and Spectral Lines

When atoms are excited, for example by heat or electricity, they may give off radiation. The frequency of this radiation depends only upon the type of atom (element). Most types of atom give off radiation of only one (or possibly two) wavelengths, and the atoms of many metallic elements give off radiation in the visible range. Sodium atoms for example give off a bright monochromatic yellow light. That is why if we are cooking vegetables on a gas cooker and the water boils over the flame goes yellow: the water contains salt and salt (sodium chloride) contains sodium atoms. It is also of course why sodium street lights are yellow. Other metals give off light of different colours, for example lithium compounds produce a deep red, copper green and potassium lilac (two spectral lines).

If we pass light from any source, say a burning airship or a distant star, through a spectrometer we can identify many of the substances present, in the flames or the star, just by looking at the spectral lines. This is how we can now be certain about the cause of the Hindenburg airship disaster - to link to the Web Page on this please click here To Hindenburg disaster

Atoms which radiate light of a particular frequency also absorb light of that frequence. In 1868 an English astronomer was using a spectrometer to analyse light from the Sun and noticed that one spectral line was missing. He concluded that this was because an element present in the Sun was absorbing light of this frequency. This element had never been found on the Earth, so he called it helium after Helios the Greek Sun God, but with the -ium ending because the frequency of its spectral line made him think it was a metal, like sodium, calcium, lithium, potassium etc. It is in fact a gas, and is the second commonest element in the Universe, having been created in the immediate aftermath of the Big Bang. It does exist in minute quantities in the Earth's atmosphere but it was not identified on the Earth until 1895. Once it was realised it was a gas they tried to change its name to helon, to go with argon, neon, xenon etc, but the name stuck. There is more about helium on another Page of this Web Site which deals with its use in balloons and airships - to link to it please click here To helium and balloons

The Doppler Effect

When we are considering light, or any other radiation, sometimes it is most helpful to think about the rays of light, travelling in straight lines. However sometimes it is more helpful if we think about the light waves spreading out in a circular pattern from the light source, like the ripples spreading out in circles on the surface of a pond when we drop a small stone into it.

If a light source is producing light of a certain frequency and is a certain distance from us and is not moving then the wave fronts it is emitting will reach us at the frequency of the light. If however the light source is moving towards us, in the time it takes one wave front to reach us the light source will have moved closer to us so the next wave front will have less far to travel and will arrive more quickly. This means that the time interval between successive wave fronts reaching us will be less so the light will appear to be a higher frequency. Similarly if the light source is moving away from us the light will appear to be a lower frequency.

Doppler Effect

This is called the Doppler Effect, and is why when a train goes through a station sounding its whistle the pitch of the sound changes as it passes us.

In astronomy the distant galaxies are moving away from us at very great speeds. This means that the light they are emitting appears to be a lower frequency, shifted towards the red end of the spectrum. By measuring the red shift we can work out the speed at which the galaxies are moving away from us.

Sir Isaac Newton and the Colours of the Rainbow

The first European to publish an account of the rainbow was Sir Isaac Newton (1642 - 1727). Most people give him the credit for this explanation, but in fact almost everything Newton wrote in his Theory of Optics had been discovered six hundred years earlier, by the 11th century Arab mathematician al-Haytham. (Newton was familiar with his work.)

We see a rainbow when rays of sunlight that have been both reflected and refracted by raindrops enter the eye. This diagram shows only the rays which produce the rainbow.

Some books say that the rays are totally internally reflected inside the rainbow, but this is not true, and Newton himself did not say it. Only a part of the light is reflected: the angle at which the rays must strike the inside of the raindrop in order to reach the eye is not shallow enough for total internal reflection to take place.

Rainbow 1

Only raindrops which are in exactly the right position produce coloured light rays which enter the eye: this means the Sun must usually be quite low in the sky. The raindrops which produce a rainbow lie in an arc of a circle, and the raindrops which produce the red are in a slightly different position to those which produce the violet - and of course are constantly changing.

Rainbow 2

If we move our position the raindrops producing “our” rainbow will be different, so we can never reach the End of the Rainbow (and find the Leprauchaun’s Crock of Gold which is buried there).

A primary rainbow has the red on the outside. Sometimes we may see a second, much fainter, rainbow with the violet on the outside: this is produced when rays of light that have been reflected inside each raindrop a second time enter the eye.

It is extremely difficult to take a good photograph of a rainbow, but to visit a site with some beautiful rainbow pictures please click here To photos of rainbows

Sir Isaac Newton was one of the greatest scientists and mathematicians of all time, but there was also a very dark side to him. He was what is called a Magus and dabbled in alchemy and other strange beliefs. He believed that the rainbow had mystical properties and so had to have seven, the mystical number, of colours.

“Everyone knows” the seven colours of the rainbow: red, orange, yellow green, blue, indigo and violet. There is just one problem: if challenged to actually count the colours in a real rainbow the majority of people can only see six. Sir Isaac inserted indigo to bring the number up to seven. There is a colour called indigo, as any artist will tell you, but it is not present in the rainbow.

At one time Newton’s Discs were very popular, and if you are at school your school might have one, although they are not often used today. A Newton’s Disc consists of a disc painted with sectors of “all the seven colours of the rainbow.” When you spin it all the colours should combine to form white.

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There are only three problems: most people see a muddy brown rather than white; you get a much better result by using a disc with just the three primary colours of red, green and blue (see Page on Colour Vision); and, be honest, have you ever seen these colours in a rainbow?

You are welcome to look for a distinct colour between blue and violet in a rainbow (or spectrum), and if you do find one please e-mail me to say you have done so. The only Rule is that you must be looking at a real rainbow or spectrum: because of the way coloured images are produced (this is discussed on the Page on Colour Vision referred to above) a photograph, drawing, television or computer image, computer print, poster or coloured picture in a book or anything similar will not do.

© Barry Gray August 2019

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