Why does light change direction?

Answer 1

A useful analogy in explaining the refraction of light would be to imagine a marching band as they march from pavement (a fast medium) into mud (a slower medium) The marchers on the side that runs into the mud first will slow down first. This causes the whole band to pivot slightly toward the normal (make a smaller angle from the normal).

I borrowed this from Wikipedia. This is how I think of it but I couldn't think of how to say this as well as the above statement.

Answer 2

The light changes its path/direction due to the phenomenon on of diffraction. When a light beam passes from a lightly dense medium to more dense medium it bends towards the normal(perpendicular[drawn at the point of intersection of the light beam with the medium]) but it bends away from normal when passing from a more denser medium to light dense medium.

Answer 3

It has everything to do with the fact that light is a wave. And a wave travels in the direction that is perpendicular to its wave front.

In short, what happens when a wave enters a slower medium at an angle it that the direction of its wavefront changes.

Before continuing to read look at the image at the link "Image of wave fronts" which is in the related links section below.

Now...

To understand this it might be useful to imagine a row of people walking shoulder to shoulder. Now imagine they approach a swamp in which they will end up traveling at half speed. Imagine they approach it at an angle so that the guy on the right reaches the swamp first, then the guy next to him and so on down the line. Now when they were approaching the swamp they were traveling in a direction "perpendicular to their wave front". However imagine the moment when the guy on the right just reaches the swamp. While he is in the swamp he will be traveling at half speed but the guy farthest on the left will be traveling at full speed until he hits the swamp later. This means the guy on the left will travel a greater distance in that time. Now imagine they individually keep traveling in the same direction they traveling on dry land until all of them are in the swamp. The guy on the right will have only traveled half as far as the guy on the left. They will no longer be traveling "perpendicular to their wave front", which is to say they will no longer be shoulder to shoulder. However imagine that they decide to "make like a wave" and do exactly that. Imagine they all rotate their shoulders so that they are again shoulder to shoulder and then they begin walking shoulder to shoulder in the new direction perpendicular to the new line that they form. They "direction of the wave front" has changed.

Now imagine the same thing again but this time they don't wait until they are all in the swamp before walking in the new direction and instead as they enter the swamp they one at a time turn their shoulders and begin walking in the new direction. In doing this they would very much be behaving like a wave.

The above is not the greatest analogy but it does give you a little bit of an idea of what is happening. To fully understand what is happening you want to read up on Huygen's Principle.

In the related links below, begin by trying to understand the page at the link.

"Snell's law and Huygen's principle"

Answer 4