Optical Fibres
...requencies. Light waves travel through space in a straight line as long as nothing disturbs it. If the Earth had no atmosphere, the sun’s light would travel directly from the Sun in a straight line towards our eyes and we would see the Sun as a very bright star in sea of blackness. However, the Earth does have an atmosphere, and as light moves through this, it continues to go in a straight line until it meets a dust or a gas molecule. Then what happens to the light depends on its wavelength and the size of the molecule it meets. If the light waves meet a molecule larger than its own wavelength, for example a dust particle, it gets reflected – “bounces off”- in different directions. The different colours of light are all reflected by the dust particle in the same way and so the reflected light appears white because it still contains all of the same colours as when it met the particle. However, if the light waves meet a molecule smaller than its own wavelength, for example an oxygen molecule, it acts differently. When the light waves meet a gas molecule such as oxygen, some of the light may be absorbed. After a while, the gas molecule radiates the light that it has absorbed in a different direction. The colour that is radiated is the same colour that was absorbed and the different colours of light are affected differently. All of the colours can be absorbed, but the shorter wavelengths (blues) are absorbed more often than the longer wavelengths (reds). This process is called Rayleigh scattering. (It is named after Lord John Rayleigh, an English physicist, who first described it in the 1870's.) WHY IS THE SKY BLUE? The process of Rayleigh scattering is responsible for the sky appearing blue. As light moves through the atmosphere, most of the longer wavelengths (the red, orange and yellow light) pass straight through and is unaffected by the air. However, the gas molecules in the atmosphere absorb much of the shorter wavelength light (i.e. the blues). The absorbed blue light is then radiated in different directions and scattered all around the sky. This means that whichever direction you look, some of this scattered blue light reaches your eyes. Since you see the blue light from everywhere overhead, the sky appears blue (as shown in figure 1). As you look closer to the horizon, the sky appears much paler in colour. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions therefore less blue light reaches your eyes (as shown in figure 2). The colour of the sky near the horizon therefore appears paler or white. WHY IS THE SKY NOT VIOLET? If shorter wavelengths are scattered most strongly, then why is it that the sky does not appear violet, the colour with the shortest visible wavelength? Part of the answer is that our eyes are less sensitive to violet, yet a rainbow shows that there remains a significant amount...