# S-Cool Revision Summary

## S-Cool Revision Summary

#### Diffraction

A wave will diffract (spread out) as it goes through a gap or past an obstacle.

*Note:* The wavelength remains the same before and after the gap.

*Remember this:* The nearer the slit size is to the wavelength, the more the wave will diffract.

- The smaller the gap the greater the diffraction.
- The longer the wavelength the greater the diffraction.

**You should be able to describe experiments such as the ripple tank or microwave kit that will show diffraction.**

#### Single Slit Diffraction Pattern

If a wave goes through a slit a diffraction pattern can be detected on the other side, with regions where the wave is intense and regions where the intensity falls to zero. A graph of intensity against distance from the centre of the pattern can be drawn:

*Coherence:*

**Coherent waves are waves with a constant phase difference.** (*Note:* They don't have to be in phase for this to be true.) They will have the same frequency and wavelength (they are normally produced from one source).

*Young's Double Slits:*

The pattern formed is of close, bright **"fringes"** of light.

A bright fringe occurs at P if S_{2}P - S_{1}P = nλ

A dark fringe occurs at P if S_{2}P - S_{1}P = nλ + ½ λ

#### Interference and Superposition

When two waves meet they will **interfere** and **superpose**. After they have passed they return to their original forms. This is true if they are coherent or not.

#### Path Difference

You will need to be able to work out whether there will be constructive or destructive interference at a point. We do this by comparing how far the two waves have travelled to reach the point. The difference in the distances will tell us if the waves are in phase or not.

#### Finding the Wavelength of Light

**Using Young's double slits to find the wavelength of light:**

λ = wavelength

a = distance between slits

x = fringe spacing

D = distance from slits to screen

#### Diffraction from a Diffraction Grating

**Using a diffraction grating to find the wavelength of light:**

nλ = dsinθ

d = slit spacing

θ = angle from centre

n = order of maximum

The pattern that you get with a large number of slits (a **diffraction grating**) is similar to the double slit pattern in that there are bright fringes on a dark background, but there are far fewer fringes and the gaps between them are much larger.

**Double slit pattern:**

(closely spaced bright fringes on a dark background)

**Grating pattern:**

(widely spaced bright fringes on a dark background)