3.2.4

# Diffraction

Test yourself

## Single Slit Diffraction

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

### Laser light

• 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.

## Width of Central Diffraction Maximum

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

### Slit width

• 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.

### Wavelength

• 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.

## Diffraction Gratings

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.

### Maxima and minima

• 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.

### Maxima and minima 2

• 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.

### Number of slits

• 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.