Why are the colours visible only at the fringes of the light refracted through a rectangular prism?
[Physics] Dispersion in a rectangular prism
refractionvisible-light
Related Solutions
Light is refracted on the way in and on the way out. The refractive index varies with the wavelength of the light, red being refracted less than blue.
If the "in" and "out" faces of the prism were parallel then the difference in refraction effectively cancels out - look through a window and light directions are not changed. But in a triangular prism the "cancelling " doesn't happen. You can see this if you draw a ray diagram, consult an elementary level book on optics, or just look at the above diagram.
Best Answer
White light is not light of a specific colour but the superposition of light rays of a range of different colours. The white light beam is comprised of a reasonably uniform distribution of frequencies across the visible spectrum that are also reasonably uniformly distributed spatially across the width of the beam. As the white beam is refracted the comprising rays are also refracted at varying angles depending on their frequency. This creates a blue and red fringe at the edges of the refracted beam. If the refracted beam was projected onto a screen sufficiently far enough away, then the white light in the centre of the refracted beam would resolve into regions of all the colours of the rainbow between red and blue. Since this distance would have to be very great for incident beams of visible diameter, the other colours of the rainbow simply "mix" to form white light.