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Science of Coloured Light

Physics | 7-14 yrs | Interactive, Learning Pod

Have you ever found yourself standing in front of a beautiful picture, with all its beautiful colours, areas of light and shadow, and wondered to yourself- ‘How do I see that?’ Many of us take the act of seeing for granted, but if you stop to examine the ability to see colour, you will surely be fascinated.

What is light?

Light is a form of energy that is present everywhere in our world. The reason we see colours and shapes as second nature to us, is because of the presence of light. It is a wonderful phenomenon because it is at the same time obvious and mysterious. There are many questions about the nature of light that scientists have dedicated their entire careers to researching- its chemical, physical, biological and mathematical properties.

What is a wave?

The first thing you must understand before we move on to the relation between light and colour is waves. A wave is a disturbance in space where this motion transfers energy from one point to the next. A wave has a high point (peak) and a low point (trough). The distance between the peak and the trough is known as the wavelength. The amount of energy present in the wave determines how long the wave is. A long wave has a low frequency and low energy whereas a short wave has a high frequency and a high amount of energy.

Electromagnetic spectrum

Light from the sun is essentially a form of energy known as electromagnetic radiation. The phenomenon we call light is the portion of electromagnetic radiation that is visible to the human eye and is called the visible spectrum. The different colours of light on the visible spectrum is defined by their respective wavelengths.

Rainbow – The visible spectrum

When you look at a rainbow, you will always see the colour red at the top and violet at the bottom. That is because red has the highest wavelength and violet has the least. The colours of the rainbow in increasing order of wavelength are Violet, Indigo, Blue, Green, Yellow, Orange, Red.

Waves such as infra-red, microwaves, and radio waves have a higher frequency than red light. Ultra Violet, X-ray, and gamma rays have a lower frequency than violet. These cannot be perceived by the human eye and therefore fall on the invisible spectrum.

Together the visible and invisible spectra of light make up the electromagnetic spectrum.

Colour from light

Colour would not exist without light. When light hits an object it is either transmitted, reflected, or absorbed.

When you say a leaf is green, you are actually saying that the leaf reflects green wavelengths of light and absorbs all the others on the visible spectrum. The swan in the picture is white because it reflects all wavelengths of light equally. His beak is yellow because it reflects the yellow wavelength of light and absorbs the rest.

On the other hand, a black stone is black because it absorbs all wavelengths of light. Therefore black is the absence of light, just like a shadow.

Transmission is when an objects allows light to pass through it. The escaped light will be the colour of the material it passes through.

How we perceive colour?

So we know that a leaf is green because it reflects only green light. We can see this because we have colour and light sensitive receptors in our eyes. These are called rods (receptive to light) and cones (receptive to colour).

Being able to see colour is a sensation just like the sense of smell or taste. And just like things taste different to different people, no two people see colour in the same way because the rods and cones in their eyes vary.

Try this yourself –

Take a glass of water and add some red food colour into it. If you put the glass in the sunlight, you will see a ray of red light on the other side. This is because the red food colour only allows red light to pass through it. You can do this with any colour, not just red.