Diffraction

Physics: AQA A Level, topic 3.2.4

Single Slit Diffraction

When light is shone through a single slit, it diffracts and produces a distinct pattern.

Laser light is monochromatic and coherent.
Monochromatic means that the light is all the same wavelength.
Coherent means the light is in phase and has the same frequency.
If the wavelength of the laser light is roughly the same as the width of the single slit, we see a diffraction pattern.

$Laser diffraction pattern$

The image shows the pattern we expect to see in a successful single slit diffraction experiment.
We see a bright central fringe with alternating dark and bright fringes either side.
The bright fringes are caused by constructive interference.
The dark fringes are caused by destructive interference.

$White light diffraction pattern$

If we use white light instead of a laser, we see a different but similar diffraction pattern.
White light is made up of all the visible colours of light. It is not monochromatic.
Each colour is diffracted by different amounts.
Red has the longest wavelength, so is diffracted the most and appears on the outside of the fringes.
Blue has the shortest wavelength, so is diffracted the least and appears on the inside of the fringes.

The width of the central diffraction maximum varies with slit width and wavelength.

If we increase the slit width, the width of the central maximum will decrease.
This is because the diffraction effects will decrease.
Imagine if the width becomes very large. The light will just pass straight through without being diffracted at all.
The intensity of the central maximum will increase because the photons are less spread out.

If we increase the wavelength of the incident light, the width of the central maximum will increase.
This is because diffraction effects will increase.
Think of longer wavelength light (red) being on the outside of white light fringes compared with shorter wavelength light (blue) being on the inside.
The intensity of the central maximum will decrease because the photons are more spread out.

When light is shone through a grating with slit widths comparable to its wavelength, the light is diffracted into a pattern of bright and dark lines.

The bright and dark lines correspond to where constructive or destructive interference has taken place.
The positions of maxima are called 'orders'.
This diffraction grating has many slits.

On a screen there will be a central point. This point is called the "zero order".
The zero order line has the largest brightness out of all the other lines.
Either side of the central line lies the first order lines. The zero order line lies in the middle of the two.
The further away from the central point, the dimmer the lines are and the more orders that are visible.

The more slits in the grating, the sharper the pattern of lines on the screen.
The pattern produced is the same as that of Young's double slit experiment, except that the lines are sharper and more easily measurable.

What are the bright fringes of a diffraction pattern caused by?
In the single slit diffraction experiment, what effect does increasing the wavelength of incoming light have on the width of the central maximum?
In the single slit diffraction experiment, what effect does decreasing the wavelength of incoming light have on the width of the central maximum?
Explain what effect increasing the wavelength of incoming light in the single slit diffraction experiment will have on the diffraction pattern produced.
What is the zeroth order maxima?