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Light is all around us. It not only lets us see in the dark, but the properties of light are important to many aspects of our lives. Reflections in rear-view mirrors of cars help to keep us safe. Refraction through lenses of eyeglasses or contact lens’ helps some people see better. More generally, electromagnetic waves (of which visible light is one example) are transmitted as a signal that our radios pick up so we can listen to music. Pulses of infrared light are transmitted as signals so we can communicate with our TVs. This backgrounder is all about visible light and how we interact with it.
Light and its Properties
In a vacuum (a container with no air), light travels at the speed of approximately 299 792 458 metres per second (m/s). This is known as the speed of light. It is the fastest that anything in the universe is able to move! For comparison, the speed of sound is only approximately 300 m/s. This is why during a storm you always see lightning before hearing thunder.
An important thing to know about light is that it travels in a straight line through a material.
Waves and the Spectrum of Light
Light has the properties of waves. Like ocean waves, light waves have crests and troughs. The distance between one crest and the next, which is the same as the distance between one trough and the next, is called the wavelength. The frequency of a wave is the number of crests (or troughs) that pass a point in one second. The wavelength multiplied by the frequency equals the speed at which the wave travels.
The colours of visible light are red, orange, yellow, green, blue, indigo, and violet. These different colours of light have different wavelengths and frequencies. Red light has the longest wavelength, and the lowest frequency of the visible spectrum. Violet has the shortest wavelength, and the highest frequency of the visible spectrum. Look at the two waves in the picture below. You can imagine how, if they were both moving to the right at the same speed, the number of violet crests passing the edge of the box in one second would be higher than the number of red crests.
There is also light that is not visible to humans. Ultraviolet light and x-rays are also light, but have too small a wavelength and too high a frequency to be visible to us. Infrared light which can be detected by night-vision goggles, and radio waves, which are picked up by your radio so you can hear music, have wavelengths which are too long and frequencies which are too low to be seen by the human eye.
Visible light, together with these invisible types of lights, make up what is known as the electromagnetic spectrum (EMS).
Primary Colours of Light
You will remember from art class that the primary colours are red, yellow and blue. These can mix to form the secondary colours orange, green and purple. Light has primary colours as well. But these are different colours than the colours we use in paint and markers. The primary colours of light are red, green, and blue. The secondary colours of light are cyan (made by combining blue and green), magenta (made by combining blue and red) and yellow (made by combining green and red). Computer screens use various amounts of red, blue, and green light to make all the colours that you see. When the primary colours of light are combined, they make white light (see below).
The human eye perceives colour using three types of photoreceptor cells which are sensitive to long, medium, and short wavelengths of visible light. Yellow wavelengths of light, for example, are perceived the same as a combination of red and green light, as in the image above. This is because they stimulate the cells in the eye in the same way. In other words, pure yellow light is physically different from a combination of red and green light, but they are both perceived by us as yellow. Did you know that two shades of green are easier for the human eye to differentiate, than the other colours? If you go into a paint shop and lay down all the red and all the green paint options, you will be able to more easily differentiate between the green shades than red. This is due to the fact that green is in the middle of the visible spectrum.