white light fringes-a level physics

I'm not quite sure how to describe the pattern of white light fringes

-First of all, they have a white central maximum then coloured fringes and then smaller white fringes on either side, these smaller white fringes don't repeat so why are they only seen near the middle?

-And in the red picture, you can see bright and dark fringes clearly but what are these dark and bright areas on the coloured picture

sorry if this is a dumb question :biggrin:

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Reply 1

ClayJensen817

This is supposed to be a three mark question in most cases.

So when monochromatic light passes through doubke slits, it diffracts forming an interference pattern. Where the waves formed are in phase they construct to give bright fringes and when waves destruct because they are out of phase they give dark fringes. The spacing between fringes depends on the distance from slits to screen, wavelength of light and distance between slits. Okay?

Reply 2

papie

Original post by student144

in first pic white light is used, so dispersion occurs. white light separates according to different wavelengths that constitute it. red light has greatest wavelength so angle of diffraction for red light is greatest.
so you ask why the bright white spot is seen only in the middle? this is because all waves at this point (in the center) have phase difference of 0,so constructive interference happens, so intensity of light here is the strongest.

Reply 3

jamiecjx

White light is made of all the colours, which all have different wavelengths.

Red light has the longest wavelength so its fringes are bigger, whilst the blue/violet light has the shortest wavelength and has smaller fringes.

The reason you see coloured fringes is just because at that point it is only a maximum for that particular colour, it is a minimum for all other wavelengths so you only see that colour. At the edge of the white central maximum you see red, because the fringes for red are the largest and at that point it is a minimum for all other colours.

You only see white in the middle because all the colours have different diffraction patterns and so never overlap perfectly again to create white light (All the colours have to overlap at the same time)