3.4 The colour of light

Light has always fascinated people: artists, philosophers, scientists and ordinary folk alike. In medieval times, it was thought to be an emanation from a kind of fire in our eyes. Newton saw it as a stream of ‘corpuscles’ or particles flowing from luminous objects. Experiments by his eighteenth century contemporary Thomas Young suggested it was more like a rippling wave, apparently contradicting Newton’s theory. The following century saw these waves defined as incredibly rapid oscillations of tiny electrical and magnetic fields.

Newton’s corpuscular view regained credibility however, when in 1905 Einstein used the concept of discrete particles (called photons) to explain findings from a more recent experiment. Today we simply have to alternate between these apparently incompatible models to explain different aspects of the behaviour of light.

To explain the spectrum of sunlight we use the wave model. It conceives of light as an immaterial vibration. What vibrates is not a physical thing, but a tiny, tiny electrical influence, a micro voltage, rising and falling, then reversing direction. Accompanying this, a similarly oscillating magnetic influence, rises and falls at right angles to the electrical one.

Like ripples on a pond these oscillations move outwards from their source.  Press ‘Ctrl and click’ on the diagram  or here to show just how (courtesy of And1mu).

The strength of these oscillations is miniscule, too small for us to feel it, but strong enough to activate the molecules in our retina that give us vision. Unlike waves on a pond, however, these electromagnetic waves don’t need water, glass, air or any other medium to support them. They have the remarkable capacity to travel unsupported through a vacuum. That is how we manage to see light from the Sun and stars after it has travelled through empty space.

Any vibrations can, of course, be rapid or leisurely – more precisely: high frequency or low. As you might expect, the more rapid the vibrations the shorter the distance between peaks, known as the wavelength

The Sun, or any other hot source, gives out waves of a huge range of frequencies. Our eyes respond to some of these frequencies and these are what our bodies recognise as visible light. Waves of frequencies outside the visible range are not picked up by our eyes but may be detected by other means. For example, we experience one type of lower frequency waves (known as infrared) as heat; still lower frequency radio waves are not detectable by us humans, but are picked up by radio aerials or antenna (click here for an animation showing this). Higher frequency waves such as ultraviolet or X rays can be detected by their effects on, for example, photographic plates. We humans aren’t able to sense these but they can damage our tissues; that’s why we need to avoid exposing ourselves too long to strong sunlight or X rays in hospital.

Within the visible range, our eyes are capable of not only detecting light waves but also of differentiating its various frequencies. Different molecules in our retina respond to low, medium and high frequency light waves. Signals are sent from the eye to the brain via the optic nerve, giving detailed information about light entering the eye, including the various frequencies included in it. The most extraordinary transformation happens next: our brain associates the various frequencies of the light impinging on our retina with what we sense as colour. The human sensation of colour is a creation of our brains.

The beautiful rainbow colours cast by sunlight catching the edge of a glass door arise from a spreading out of the light of differing frequencies. It’s a reminder that light from the Sun is not simple and monochromatic but a composite of waves of differing frequencies. As you see in any spectrum or rainbow, the spread of colours is continuous; there’s no border defining the end of red and beginning of orange. It’s a continuum of gradually shifting hues, as this image confirms. Our quest is to understand what it is about the Sun that makes it radiate light over this continuous range of wavelengths.

Light originates in atoms; more precisely in changes that occur in atoms. In normal conditions, here on Earth, gases emit a strictly limited range of frequencies of light. A good example, is the orange street lamps, which contain sodium gas which only emits one colour of light – orange. In contrast, light from the Sun come from its very hot surface layers, where atoms and freely moving particles (called electrons) are buzzing around at very high speeds and interacting with one another frequently. This causes them to emit light which, under these extreme conditions, is of any frequency (including infrared, ultraviolet and X rays).

Light of all the frequencies of visible light – i.e. all the colours of the rainbow – is the result. As mentioned above, there are also some frequencies that lie outside the range of visible light: ultraviolet, which has a higher frequency than violet light and infrared which is lower than red light.  

The jagged white line in the diagram shows just how much energy the Sun radiates over a range of different wavelengths. The amount of energy is depicted along the vertical axis and the wavelength along the horizontal one. You can see that the peak of the white line (i.e. the greatest energy) in the graph occurs for visible light, where the spread of colours from violet to red is shown. So the main type of radiation from the Sun is what we see as light, in all its colours. A considerable fraction, however,  is also radiated that is not visible to us; it lies beyond the red zone,  in the infrared range (which we experience as heat) and below the violet in the ultra (UV) range

So, light of all visible frequencies is contained within sunlight. Our brain represents this mixture of all colours as what we call white, like all colour impressions, an artefact of the brain. (The Sun look closest to white when it is overhead; in the morning and evening it appears pink or orange or yellow, due to effects when it hits the atmosphere).

The final step in our query is to explain why the great mixture of frequencies in the light from the Sun should get split up into the colours of the rainbow by a chance encounter with a glass door. In essence, we are asking why light gets split into its constituent colours when it passes through a prism. The edge of the glass door, held ajar, with the sunlight playing upon it at a glancing angle, was acting just as a prism.

It’s refraction, the bending of a wave front when it hits a boundary that is in play here. The diagram below uses an analogy to explain why this happens. The runners to the left present a straight-line front travelling at an angle to the water that lies ahead (mauve). The front wave moves more slowly in the mauve area, the pace of swimming rather than running. The effect is that the front of the wave (dotted line) changes direction. Light waves travel more slowly in glass or water than in air. As result the front of a light wave changes direction when it hits a different medium, like glass, at an angle.

Refraction at a boundary

Image courtesy of Cdang

The case of sunlight is slightly more complicated because it contains light of many different frequencies or colours. Each of these frequencies travels at a different speed in a slower medium such as glass. Red light travels fastest and violet slowest. So, when sunlight hits glass at an angle, it not only bends but in addition, each frequency bends (or refracts) a slightly different amount. The image below (or better still the animated version) shows how this happens.

   (press ctrl and click to see animated version)

The arresting sight of sunlight on a bright day spreading its colours across a shop floor has led us to explore some of the most fundamental ideas in physics: the nature of light, the meaning of colour, the interaction of light with solid matter. A beautiful sight with beautiful theories to explain it.  

© Andrew Morris 08.08.17