# Why can bright and dark fringes form with white light?

Here is my understanding:

Non laser light must be passed through a single slit before being directed at double slits. This is to:

1) make the light sufficiently monochromatic such that the double slits emit waves of the same frequency

2) to make the light into a point source such that the double slits emit waves with a constant phase difference

So white light must first be passed through a single slit before being directed at double slits.

If non monochromatic light is directed at the double slits, then the double slits emit light of changing frequencies and so aren't coherent - the interference points constantly change and no pattern is seen.

Therefore I do not understand how white light fringes can form. How do the fringes contain all wavelengths of the spectrum, when we are directing the white light through the single slit in order to make it sufficiently monochromatic. Surely the fringes should contain just one wavelength of light at different intensities instead of all the wavelengths.

Any help greatly appreciated.

The purpose of the double slits is to split the incoming wave front of a given frequency wave and so in effect you have two coherent sources, which are the slits, emitting light of that wavelength.

As the sources are coherent when the light from those sources overlap an interference pattern is formed.

Light of each wavelength coming through the slits produces its own interference pattern whose size depends on the wavelength of the light.

The zero order fringes (no path difference between the slits) are independent of the wavelength and so will occur at the same position.
If white light is incident on the double skits then that zero order fringe will be white.

The blue light fringes will have a spacing which is less than that for red light and so the first order fringes (one wavelength path difference) will occur in slightly different positions.
So the first order fringe will be relatively broad and bluish nearest the zero order fringe and reddish on the other side.
As one goes further away from the zero order fringe the displacement of the various colours becomes greater so that after a few fringes there is no clear fringe pattern.

• Thanks for your quick response! What is the need for a single slit? Non laser light must be directed through a single slit before a double slit, but for what purpose? For example light from a filament lamp has to be directed through a single slit before the double in order for an interference pattern to form, it can't just be directed through a double slit
– John
Apr 20, 2017 at 15:49
• The single slit is to produce light from a point source and it produces a diffraction pattern which covers the double slit and so both slits act a sources. If the single slit is made wider then the double slit fringes become less distinct. Apr 20, 2017 at 15:56
• So the single slit is necessary so that the light directed onto the double slits is from a point source. Am I right in thinking that, as we now have a white light point source, the phase difference between the two double slits is constant as a result.
– John
Apr 20, 2017 at 16:08
• And so the single slit does NOT make a light source monochromatic. All frequencies emitted from e.g. a white light bulb still make it through the single slit, and so the interference pattern is a result of all these difference frequencies.
– John
Apr 20, 2017 at 16:12
• Ok then. So is the reason why we can shine a laser directly at the double slits because it acts as a point source?
– John
Apr 20, 2017 at 16:58