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The Diffraction Grating.
The Diffraction Grating:
It is difficult to determine wavelengths accurately when using the double-slit method.
The interference pattern produced is blurred, which makes measuring the fringe separation
This problem can be solved by using multiple slits. Gratings can be made in which
there can be 500 or more slits per millimeter.
The same interference pattern is produced, but the regions are much brighter,
narrower and sharper, so more accurate measurements can be taken.
This diagram shows the light being diffracted at an angle where the path difference
between each ray is exactly one wavelength. This means that each wave will constructively
interfere and a bright fringe will be seen.
However, for angles that are slightly different and produce a path difference of 0.99
wavelengths, will mean that the path difference of the next ray will be 0.98 wavelengths and
the next one 0.97 wavelengths etc.
When there are thousands of slits, this would mean each one of these waves will
always have a wave that is in anti-phase. So where the path difference is not a whole number
of wavelengths, no light will be seen. This results in an interference pattern that produces
very sharp fringes.