# What is a wavefront?

For this picture Anna wrote: "Incandescent light is incoherent because it comes from many sources and the same is true for sunlight. By passing the light through the one slit he created a single coherent wave front."

What represent the drawn lines in the sketch about double slits? Are this wavefronts? What is a wavefront?

A wavefront is the crest of the wave. When you go down to the beach, and see those things called waves, the front is the whole line that is at the same height.

In electromagnetics, it's the same thing. It's the points that are at the same height. In your diagram, the black curves represent a unit of cycle, and the two waves through b and c can either add to each other (to get double height), or can be out of phase (so b=-c and it's nothing).

Basically, S1 produces a single source, and there is only one set of ups and downs. S2 uses two points of S1's radiation to make two points that are using the same cycles. They don't have to be in the same phase, just that their period is the same. So b and c produce waves, and these intersect, and add together based on how they are in phase.

You can see a result by draging a sine wave over another one. The crest of the sine waves are the black arcs, the intensity is found by adding, eg one wave shifted by 0.5 cycle to the other. They cancel out.

• So the lines represent some equal state of an electromagnetic radiation. For example the electric field component of the radiation? – HolgerFiedler Jan 17 '16 at 13:48
• And this wavefronts are moving, of course? – HolgerFiedler Jan 17 '16 at 13:51
• The wave fronts are moving, but the arcs represent the same point in the cycle. So if any of the points are at 0.15 of the cycle, then the next line will be at 1.15 of the cycle and the next at 2.15, as time goes on the .15 becomes .18 then .21 etc. – wendy.krieger Jan 18 '16 at 4:11

A slightly more general construction than Wendy or Bill's is

"A wavefront is a contiguous region of constant phase".

Wendy's choice of "crest" is just the selection of a particular phase ($0 \pm 2\pi n$ for wave represented with $\cos$ for instance), and that is the usual choice when visualizing wavefronts, but there is nothing that prevents you from using the trough or the rising zero-crossing or any other value.

• I like Wendy's answer for the expression in own words. Thanks for replaying the question about moving wave fronts. I'm busy, but will replay later, but today. – HolgerFiedler Jan 18 '16 at 16:04
• "A wavefront is a contiguous region of constant phase." Phase of what? When we talk about light or EM radiation I know only two types of phases, one ist the electric field strength and the other ist the magnetic field strength. Both fields are switching their strength continuously and by this changing periodically their directions (say, if left to right and back for the electric field, then up to down and back for the magnetic field). Is one of this fields represented by the wavefront? – HolgerFiedler Jan 18 '16 at 17:34
• Phase of the wave. In the event that the electric and magnetic fields are out of phase you need to specify, but that is not he case in situations where the "wavefront" formulation is typically applied. And, yes, the phase at any particular point in space is a function of time, which means (in answer to your subsequent question) that wavefronts necessarily move. – dmckee Jan 18 '16 at 17:40
• So let's say, the arc of the wavefront in the sketch represents the electric field strength and a full cycle the electric field made between two arcs. When at the intersection points of two wavefronts the electric field strength doubles (interference of the electric field component of the EM radiation)? Following this point on one of the arcs we get the full cycle of interferenc including the extinction. – HolgerFiedler Jan 18 '16 at 17:52
• This happens without any consequences until the waves "hit" the screen? – HolgerFiedler Jan 18 '16 at 17:54

What represents the lines are coherent photons. The only thing oscillating are photons, not waves. The photons are coherent because their all emitted in phase from the same point source. Because they are in phase gives them the appearance of a wave like this sketch: