How do you calculate interference fringes spacing?

Interference fringe spacing can be calculated using the formula λD/d, where λ is the wavelength, D is the distance to the screen, and d is the slit separation.

In more detail, the phenomenon of interference fringes is a result of the wave nature of light. When light passes through two closely spaced slits, it diffracts and the diffracted waves interfere with each other. This interference can be constructive (resulting in bright fringes) or destructive (resulting in dark fringes). The pattern of these fringes can be observed on a screen placed some distance away from the slits.

The formula to calculate the spacing (y) between these fringes is given by y = λD/d. Here, λ is the wavelength of the light used, D is the distance from the slits to the screen, and d is the separation between the slits. This formula is derived from the path difference between the two waves and the condition for constructive interference.

To use this formula, you need to know the values of λ, D, and d. The wavelength of light is usually given in the problem or can be found in a reference table. The distance to the screen and the slit separation can be measured directly in the experiment.

It's important to note that this formula assumes that the angle θ (between the central maximum and the fringe being considered) is small. For larger angles, a more complex formula involving the sine of the angle is used.

IB Physics Tutor Summary: To find the spacing between interference fringes, use the formula y = λD/d, where y is the fringe spacing, λ is the light's wavelength, D is the screen distance, and d is the slit gap. This shows how light waves, when passing through slits, create a pattern of bright and dark bands on a screen due to their wave-like behaviour.