# Interference

### National 5 Definitions

##### Transverse

A type of wave which oscillates perpendicular to the wave's direction of energy transfer.

##### Longitudinal

A type of wave which oscillates parallel to the wave's direction of energy transfer, and travels through compression and decompression of the particles in a medium.

##### Wavelength

The distance between two peaks of a wave.

##### Frequency

The number of waves which are emitted or pass a point in a fixed unit of time.

##### Amplitude

The distance from the centre line of a wave to the peak of the wave.

##### Period

The time it takes for one complete wave cycle to be emitted or pass a point.

##### Reflection

The rebounding of an object, particle or wave against a surface.

##### Refraction

The bending of a wave as it passes from one medium to another, caused by the difference in speed between the two mediums.

##### Diffraction

The spreading out of a wave as it passes through a gap, caused by the wave's interference with itself.

### Interference

Superposition is the adding together of multiple waves, causing them to interfere with one another.

##### Constructive Interference

Constructive interference is where the waves are in phase, and therefore combine to form a wave with a greater amplitude.

##### Destructive Interference

Destructive interference is where the waves are out of phase, and cancel each other out.

##### Maxima

When two wave peaks meet at a point, it is known as a maximum (plural maxima) - a point with the greatest possible amplitude.

In an interference pattern, the central maximum is known as the zero order maximum, and for each maximum away from it, the order number increases by one.

##### Minima

When two opposing wave peaks meet at a point, it is known as a minimum (plural minima) - a point with the smallest possible amplitude.

### Double-Slit Experiment

The double-slit experiment was first performed by Thomas Young in 1802, and demonstrates the wave-like properties of light. It accomplishes this by making light diffract through two narrow slits, creating an interference pattern.

### Path Difference

The path difference between two waves is the difference in length from two coherent wave sources to a common meeting point.

If the path difference between two waves is an integer number of wavelengths, the meeting point will be a maximum. Similarly, if the path difference is a half-integer number of wavelengths, the meeting point will be a minimum.

#### Formula

$\text{Path difference}=m\lambda$
Variable Key

• $\text{Path difference}$ is in any consistent unit of length.
• $m$ is the order number of the maximum.
• $\lambda$ is the wavelength of the wave, in any consistent unit of length.

### Diffraction Gratings

A diffraction grating is a panel which consists of many slits spaced close together. They can be used to produce interference patterns, similar to a double slit.

#### Formula

$d\mathrm{sin}\theta =m\lambda$
Variable Key

• $d$ is the slit separation (distance between slits), in any consistent unit of length.
• $\theta$ is the angle from the central maximum.
• $m$ is the order number of the maximum.
• $\lambda$ is the wavelength of the wave, in any consistent unit of length.