Diffraction and polarisation

In a nutshell

Diffraction occurs when waves spread out as they pass through a gap. Diffraction patterns are produced on a screen when light diffracts through a gap. Polarisation is when particles only oscillate along one direction perpendicular to direction the wave travels. Waves that reflect off a surface become partially polarised. Polarising filters can polarise electromagnetic waves.

Diffraction

Diffraction occurs when waves spread out as they pass through a gap or go around an obstacle. Any type of wave can be diffracted and their speed, wavelength and frequency does not change due to diffraction.

The amount the wave is diffracted depends on the wavelength of the wave and the size of the gap it's passing through or obstacle its moving round.

The most diffraction occurs when the size of the gap (or obstacle) is the same as the wavelength of the wave. Diffraction is unnoticable when the gap is much larger than the wavelength. When the gap is much smaller than the wavelength, most of the waves are reflected.

The distance between wavefronts is always equal to one wavelength, as the wavelength of diffracted waves does not change.

Note: In an exam, the gap may be referred to as an aperture!

Example

The wavelength of sound waves are a similar size to that of a doorway. This means that diffraction of sound is noticeable and is why you can hear conversations even if you cannot see the person talking.

However, the wavelength of light is much smaller than the size of a doorway. This means that diffraction of light is unnoticeable which is why you cannot see the person talking.

To observe diffraction of light a much smaller gap is needed.

Diffraction patterns

A monochromatic, coherent light source (e.g. a laser) can be used to observe a diffraction pattern. The laser light diffracts through a narrow slit and produced a pattern of light and dark fringes. This is caused by constructive and destructive interference. 

1. Monochromatic coherent light source (laser) 2. Single slit ​​ 3. Screen 4. Pattern on the screen

The pattern produced by interference consists of a central bright fringe, known as the central maximum, surrounded by alternating light and dark fringes. 

White light is made up of different wavelengths of light. Using white light instead of monochromatic light produces a spectra of colours on the screen.  

The diffraction grating equation ($$d\sin{\theta} = n\lambda$$) shows that the wavelength of light, $$\lambda$$, is proportional to the angle at which is diffracts, $$\theta$$. As white light is made up of many wavelengths, $$\lambda$$, the wavelengths will all have differing angles of diffraction, $$\theta$$. Hence, the light splits up and forms a spectra of colours on the screen.

Red wavelengths diffract more than violet wavelengths.

1. White light source 2. Single slit 3. Screen 4. Pattern on the screen

The width of the central maximum for any diffraction pattern depends on:

• The wavelength of the light used. A longer wavelength increases the amount of diffraction. The central maximum becomes wider, which decreases the intensity.
  
• The slit width of the slit. A larger slit width decreases the amount of diffraction. The central maximum becomes narrower, which increases the intensity.

Polarisation

The oscillations of transverse waves can occur in any orientation that is perpendicular to the direction of wave propagation. Ordinary light from an unpolarised source oscillates in many different directions. This light is said to be unpolarised.​

Polarisation is when a transverse wave is restricted to oscillate in one plane/axis only. This is also known as plane polarisation. 

​Note: A plane is a two-dimensional, flat surface!

Partial polarisation occurs when transverse waves are reflected off surfaces. This causes more waves to oscillate in one specific plane, but there are still waves oscillating in other planes.

Example

Light reflected off water is partially polarised in the horizontal direction. Sunglasses contain polarising filters which don't allow horizontally polarised light to pass through. This reduces the glare off surfaces of water.

Polarisation of light

Electromagnetic (EM) waves are mostly unpolarised when they naturally occur. The electric field oscillates in many directions.

Unpolarised light can be polarised by using a polarising filter. They only allow light pass through if they oscillate in the specific plane that they are aligned to. The axis that the filter is aligned to is called the transmission axis.

1. Unpolarised light 2. Plane polarised light​​ 3. Polarising filter 4. Transmission axis (light oscillating this way passes through)

When two polarising filters at right angles are placed on after the other, no light will be able to pass through both of them.

Polarisation of microwaves

Microwaves can be polarised using a metal grille. This is because microwaves have a longer wavelength than visible light. 

1. Microwave transmitter 2. Metal grille 3. Microwave reciever

The free electrons in the metal bars absorb the electric field that is aligned in the same direction as the grille. This means that horizontally aligned waves can pass through a vertical metal grille and vertically aligned waves can pass through a horizontal grille.

Tip: As an exam tip, remember that this is the opposite to what happens in visible light polarisation!

Uses of polarising EM waves

The polarisation of EM waves is used in communications transmitters.

Some transmitters transmit vertically plane polarised radio waves and others transmit horizontally plane polarised radio waves. Receivers will only receive the signals in the direction that they are aligned in. 

Doing this reduces interference from both signals.