Double-Slit Interference

Double-Slit Interference

History

Thomas Young

In 1799 Thomas Young proposed that light was was wave, contradicting Newton's belief that light was made up of tiny particles. Young's experiment showed that light does indeed have wave properties.

Set-Up

Young's experiment has a source of light illuminating two narrow, closely spaced slits. This allowed the light to project an image of a screen in front of the two slits.

Results

Because light is a wave, Young found that the image on the screen was an alternating series of light and dark bars. This was caused by interference between the two slits of light. If Newton had been right, there would have only been two bars of light on the screen directly in front of where the slits were placed.

m the "spectral order"

What does m do?

By changing the value of m we can see at what angles constructive interference occurs. We start with m=0, which means we get an angle of zero degrees (regardless of wavelength) and constructive interference at this point. When m equals zero, this called the direct central maximum.

Changing m

Leading from one side of the direct central maximum, m increases by one each time: +1, +2, +3..., and on the other side, m decreases by one each time: -1, -2, -3....By increases or decreasing m in this manner, we can calculate the angles at which the next constructive interference occurs.

Wavelength and Distance

Manipulation of lambda

Different outcomes also arise when the wavelength is either increased or decreased. If lambda is increased (by moving the slits further from the screen), there will be an increase in the interference pattern spacing. Likewise, if lambda is decreased (by moving the slits closer to the screen), there will be a decrease in the interference pattern spacing.

Manipulation of d

By changing d in the constructive interference equation different outcomes arise. By decreasing the distance between the slits, the interference pattern spacing (distance between alternating light and dark patches on the screen) increases. By contrast, if you increase the distance between the slits, the interference pattern spacing decreases.

Constructive Interference Equation

The equation for constructive interference for double-slit interference is: dsin(theta)=m(lambda), where d is the distance between the two slits, theta is the angle at which a bright region appears on the screen, m is an integer, and lambda is the wavelength of the light.