Conceptual Physics

In Concept 2, you see the wave front created by the wavelets moving toward a barrier

with a gap in it. Before the wavelets reach the barrier, each wavelet has another

wavelet adjacent to it.

Huygens’ principle states that the position of the wave front at any time can be found by

drawing a line tangent to the leading surface of each wavelet. As each wavelet moves

forward, the wave front moves forward as well.

Let’s consider what happens when the wave front meets the barrier. The barrier

prevents most of the wave front from moving forward (for simplicity’s sake, we ignore

the reflection of the wave). One section of the wave front (and the wavelets that make it

up) passes through the opening. They continue to move forward after passing through.

As Huygens mentions in the quote above, you can think of the sources of the wavelets

as a set of vibrating particles. After the wave front passes through the slit, the oscillating

particles that now make up the wave front are free to interact with other particles above

and below them. The interaction of the particles with their formerly stationary neighbors

and the spherical form of the wavelets account for the spherical expansion of the wave

front. This is illustrated in Concepts 3 and 4.

If you like, you can think of the wavelets as a group of subway passengers exiting a

subway car. Their “wave” is constrained by the width of the subway doors, but when

they exit the subway and enter the subway station, their wave can expand amongst the

people awaiting the subway.

Wave front made up of spherical

wavelets

Huygens' principle

Wave front tangent to wavelets, same

speed

Diffraction

Portion of wave passes through slit

After the slit, particles at edges interact

with nearby particles

Diffraction

Result is a spherical wave front

34.7 - Single-slit diffraction

Waves, including light waves, diffract as they pass through a single narrow slit. When a screen is placed on the other side of the slit, a

Single-slit diffraction pattern

Light and dark fringes

(^632) Copyright 2000-2007 Kinetic Books Co. Chapter 34