We know that light is a wave as it diffracts. Diffraction is the bending or spreading out of waves around obstacles/gaps

Maximum diffraction occurs when the size of the obstacle/gap is the same order of magnitude as the wavelength of the wave.

For example, sound waves can diffract around a door, but light can't, as light has a much smaller wavelength. This is why we can hear someone from the other room, but we cannot see them.

Young's double slit experiment shows us the wave properties of light as the waves diffract from the two slits and superpose when they meet. If directed at a screen, an interference pattern of alternate dark and bright fringes can be seen.

The interference pattern can only be observed if the light waves are coherent. This means that the waves emerging from the slits have the same frequency/wavelength and have a constant phase difference. To achieve this, a laser beam is used instead of a bulb or candle as it provides monochromatic light (light of a single colour/wavelength).

If the two waves are exactly in phase, a central maximum is visible on the screen, where constructive interference occurs because the path difference of the waves is zero.

Formula:

Constructive interference, forming a bright fringe occurs if the path difference between the two waves is or , where is an integer and the waves meet exactly in phase, . This would occur for example if the two waves both arrive at a peak.

If the path difference is then we get destructive interference. A dark fringe is formed as the two waves are in antiphase, . This would occur for example if one wave arrives as a peak and the other as a trough.

To summarise,

Practical Observations

• If slit separation, , is increased then fringe spacing, , will decrease as seen from the equation, as they are inversely proportional. The fringes will be closer together and thus harder to measure, which is why a smaller slit separation is ideal.

• If violet light is used instead of red light, the fringes will become much closer together, as violet light has a shorter wavelength than red light. This is because wavelength is directly proportional to fringe separation.

• If we increased the distance between the slits and the screen, the fringe pattern would widen. In practice, this is the usual adjustment to make, as when measuring fringe spacing, we want the fringes to be as far apart as possible and to measure across multiple fringes to reduce the percentage uncertainty in our results. In practice, you would measure the distance across multiple fringes using a vernier calliper, as they have better resolution than a ruler.