# PHY C15: Diffraction Grating

Last for the chapter!

• Diffraction Grating
• Equation: nλ = d sin θ
• White light through a diffraction grating

Let’s go!

What is a diffraction grating?
A plate with a large number of closely spaced parallel slits.

What happens when monochromatic light is sent through a diffraction grating?
A pattern of narrow bright fringes is formed.

This pattern is caused, once again, by interference of light waves.

What equation is used to describe the fringe pattern?

If you have sharp eyes, you’ll notice that this is similar to the equation used to describe single-slit interference.

HOWEVER, the purpose of this equation is DIFFERENT:
This describes the pattern of BRIGHT MAXIMA, not the minima in the equation for single-slit interference.

How do you use this equation?

d = spacing between adjacent slits
θ = angle from centre line to maxima
n = order of maximum (0,1,2,3,…)
λ = wavelength of light

The 0th order maximum is the bright maximum at the centre, the 1st order maxima are the 2 maxima closest to the centre, etc.

Let’s try a classic example.
Given:

• number of lines per metre = 5.00 x 105
• 3rd-order maximum observed at an angle of diffraction of 78°

Find λ.

What happens when you send white light through a diffraction grating?
The different frequencies (colours) making up white light will be diffracted by a different amount, since they have different wavelengths.

Since violet & blue have the shortest wavelengths, they will have smaller angles.

Since red has the longest wavelengths, it has the largest angle.

It should look like this:

A real-life image would look like this:

What’s an application of diffraction grating?
It can be used as a SPECTROMETER.

This is a tool used to identify the wavelength(s) of light that is sent through it.

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