# Fermat principle and light

1. Jan 9, 2009

### feynmann

In Feynman lectures, Feynman says if light goes from air to denser medium,light goes more in air and less in denser medium so that it takes shortest time to reach its destination.

My question are

How light knows that there is a denser medium ahead in its path?

Does light get information about the denser medium?

what is information?

2. Mar 10, 2009

### destro47

The light doesn't "know" which path/time is the shortest....The fact is that light has wave characteristics as well as ray properties. The light waves propagate along ALL paths....the waves that are traveling along paths that are not minimized will reach the point exactly out of phase and undergo destructive interference, while the waves on the minimal path will interfere constructively. So basically any light waves traveling along a path that does not represent minimal time will cancel each other out.

3. Mar 11, 2009

### mn4j

So what happens to the energy of the photons when they destructively interfere? Doesn't this go against the law of conservation of energy?

4. Mar 11, 2009

### DeShark

There's only one photon and it interferes with itself. As such, the "wave" which interferes with itself is just a probability to find the photon at a given point. The photon is either found at a point or not found at a point. The photon never splits up. But if you make a measurement along one of the paths to find out which way it went, you'll destroy the interference effect. The question of "what happens to the photon in between its emission from a source and its detection" is the subject of the so-called interpretations of quantum mechanics. i.e. quantum theory doesn't tell us anything apart from measurable phenomena.

Edit: The wave itself is not directly the probability. If you've done complex valued functions, the square of the amplitude of the wave gives the probability density.