# PHY C15: Double Slit Interference

Today we’re going to cover:

• Interference patterns
• Conditions required for 2-source interference
• Equation λ = ax/D

Let’s go!

What happens when a wave is sent through 2 slits?
It forms an INTERFERENCE PATTERN like so:

What are the conditions required in order for this 2-source interference pattern to be observed?

• Waves must be COHERENT
• 2 sources emit circular waves (radially)

What is coherence?
2 wave sources are said to be coherent sources when they:

• Emit only 1 frequency (for light, monochromatic)
• Have the same frequency
• Have a constant phase relationship (they are in phase)

How do you ensure 2 sources are coherent?

1 classic example of 2-source interference is Young’s Double Slit Experiment.

Here, a monochromatic light source is put behind a single slit, diffracting the light waves further into 2 slits (which act as the 2 sources).

The interference pattern can be seen on a screen – this experiment was one of the first to show that light had wave properties.

The pattern is described as a FRINGE pattern, consisting of bright & dark fringes:

What affects the distance between fringes on this pattern?
This distance is labelled as x, & is affected by:

The equation relating them all:

OR, rearranged to express x:

A brief recap on the reason behind the pattern:

What if we use non-monochromatic light (white light) for the double slit experiment?
A clear fringe pattern will not be seen.

White light is made up of many different frequencies of light.
When all of these pass through the double slit, they form their own individual patterns.

The patterns overlap on each other, resulting in mostly white light surrounding coloured fringes near the centre.

⇐ Previous in Physics: Measuring the Speed of Sound using Stationary Waves
⇒ Next in Physics: Diffraction

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