Interference

In 1801, English physicist Thomas Young undertook an experiment that led to a change in the understanding of light. Young had studied the eye and the human voice and started to investigate the similarities between light and sound. Like the Italian scientist Francesco Grinmaldi in 1665 who discovered diffraction when he noted that rays of light bent or were spread out when they pased through a small slit, Young looked at what happened when sunlight passed through two slits side by side and then fell on a screen. He noticed that if the slits were large and far apart he saw two overlapping patches of light, but if the slits were very narrow and close together the light produced bands of colour called interference fringes. Young realised that these coloured bands could only be produced by waves. As the light waves from each slit spread out, they meet each other. If the waves are in step, they will add together and this effect is known as constructive interference. If they are out of step, they cancel each other out and create destructive inteference. The effect of the two can be seen becauce they make bright and dark 'finges' where the light hits the screen. Interference is procuded by anything that splits lights into waves that can be added together or cancelled out. Diffraction gratings, compact discs, bubbles, butterfly wings or even peacock feathers all create inteference patterns. Peacock feathers are an example of interference colours being created. The colours that can be seen depend upon the angle at which the feather is viewed. The colours will change if viewed at another angle and may disappear altogether. This happpens because irridescent colours are produced by the shape of seperate surfaces that are a tiny distant apart. The surfaces reflect light in different ways, making the light rays interfere with each other.