EWN - Diffraction and Double Slit Interference
Diffraction and Double Slit Interference
Introduction
For hundreds of years it was debated whether light traveled as a stream of particles or as a wave. We now know the remarkable answer that it actually does both! To determine the nature of light you have to examine how light interacts with its surroundings. In this unit we will focus on some of the properties of light that represent its wave nature.
Diffraction occurs when a wave passes by the edge of an obstruction that causes the wave to bend around the obstruction. Particles passing the edge would either go straight by (because they didn't strike it) or strike it and bounce away. The only way for something to end up in the "shadow region" behind the obstacle is to diffract around the edge and pass behind the object. This is why diffraction is a defining characteristic of waves. If the wave passes through a narrow opening (each side of the opening acting as an obstruction) you can get the wave to diffract and move in multiple directions. In this image you see a series of plane waves diffracting as they pass through two small opening of varying widths. To achieve the type of circular wave diffraction you see in the right image, the width of the opening has to be comparable to the wavelength of the wave.
Interference
To fully understand the experiments that proved light's wave nature, you must first have a working knowledge of how waves behave when they try to occupy the same space at the same time. As you learned in a previous course, two mechanical waves occupying the same space at the same time interfere, meaning their amplitudes add together causing the medium oscillate at an amplitude equal to the sum of the original amplitudes. When waves are in phase
their amplitudes add to create a larger amplitude wave. This is constructive interference. When wave our out of phase their amplitudes subtract creating a smaller amplitude wave. This is destructive interference. Electromagnetic waves can also interfere. When two monochromaticwaves strike the same spot on a screen the spot can range from brighter (constructive interference) to completely dark (complete destructive interference).
Young's Double Slit Experiment
In 1801, Thomas Young performed an experiment that would test the wave nature of light.
The pattern that the waves make on the screen after passing through the two slits is an interference pattern. As waves pass through the slits, each slit becomes its own wave source. When the waves meet on the screen the pattern is the result of interference between the two sources at the location of the screen. The bright fringes are called maxima and are the result of constructive interference. The dark fringes are called minima and are the result of destructive interference.
When Young allowed sunlight to shine on two narrow, closely spaced slits he observed an interference pattern similar to the one below.
The presence of the wave interference pattern is part of the proof of the wave nature of light.
Double Slit Interference Practice
IMAGES AND VIDEOS SOURCED FROM PUBLIC DOMAIN