allow for greater dispersion of white light, and make the stone appear to ‘sparkle’. Dispersion in rainbows and gemstones is due to refraction and total internal reflection.
Sun Red
Violet Red Violet Red Violet
Refraction of light in a rainbow Petrol film and soap bubbles
Often it is possible to see all the colours of the rainbow in a soap bubble (see photo). This is because of the combination of refraction, total internal reflection and interference of the light hitting the bubble. The variety of colours in a soap bubble comes from simultaneous reflection of light from both the inside and outside surfaces of the thin soap film. Some of the light is reflected off the outer surface of the bubble; the rest undergoes refraction as it enters the bubble and is reflected off the inner surface by total internal reflection.
When the waves reflected from the inner and outer surfaces of the soap film recombine, they will interfere with each other to either remove or reinforce some wavelengths of white light by destructive or constructive interference (Fig 14.26).The same phenomenon is observed on a film of petrol on water; light is reflected off the top of petrol, and also off the water.
Dispersion of light in a soap bubble
Constructive interference (wavefronts in step)
Incident light path
Reflected light paths
Destructive interference (wavefronts out of step)
Incident light path
Outer
surface of bubble
Thin part of bubble
Reflected light paths
Fig 14.25: Illustration showing why an observer sees the rainbow with red at the top and violet at the bottom, with all colours in between
Inner
surface of bubble
Fig 14.26: Refraction, total internal reflection and interference can all be observed in the coloured pattern of a soap bubble
