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# Diffraction gratings and equations

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1. What is path difference?
2. A diffraction grating is simply a plate with many, incredibly small and very closely spaced parallel slits ruled into it.
Based upon the close proximity of said slits a very sharp interference pattern is produced (much better and clearer than with Young's Double slits) when a source of monochromatic and coherent light is passed through it.
This is due to the fact that there's such a large proportion of diffraction gratings interfering (experiencing superposition).
Waves that meet 180 degrees out of phase experience destructive interference and so produce a sharp minimum (dark point), and light rays that meet in phase (or 360 degrees out of phase) experience constructive interference and so produce a sharp maxima (bright point).

The diffraction grating equation is (for my spec - AQA):

n = d sin( )

where:

n = the order of the beam produced
d = distance between two slits in the diffraction grating (often extremely small)
= the wavelength of the incident light being diffracted by the grating.
= the angle between the order beam in question (a maxima) and the zero order (normal).
3. What do you mean by the order bit for the angle?
4. (Original post by Jennifer50)
What do you mean by the order bit for the angle?
When we put monochromatic and coherent light through a diffraction grating we'll have a number of maximum points produced. These, we refer to as maximum orders (you get a first order maximum produced on either side of the zero order) - the angle between an order beam and the zero order (the normal line - the most intense maxima) is dependent on the wavelength of light, the distance from the screen to the grating and the slit separation within the grating, s.

n / d = sin( )

We can see this when we rearrange the diffraction grating equation.
We can therefore say as we increase diffraction increases (the waves spread out more) and so is greater - all just from looking at the grating equation
5. What does coherent light mean?
6. (Original post by Jennifer50)
What does coherent light mean?
It means that the 2 waves produced have: a fixed phase difference, same frequency and same wavelength. Having coherent waves makes the interference pattern a lot clearer.

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