# Q & A: refraction of light, bending and speed change

Q:
Hi,When light ray in air hits a glass surface perpendicularly, it will move straight but the speed of light will decrease.Does this mean there is 'refraction'?? After all, refraction means bending of light.Some University textbooks stated that there is refraction in the above case because there is a change in speed. Direction does not matter.I'm very confused now. Please help me understand this! Thank you in advance!!
- Kevin (age 30)
Singapore
A:

Actually, it sounds as if you understand this very well. The light slows down as it enters the glass. When it enters at some non-perpendicular angle, that slowing causes a change of direction. The reason is that the wave-fronts of the incoming and transmitted waves need to match up. Since the wave in glass is going overall slower, to keep its sideways velocity as big as that of the incoming wave it must be heading at a bigger sideways angle.

As for whether somebody wants to call all of this refraction or only the non-perpendicular case, there's nothing to understand. It's just names. I prefer calling the whole thing refraction rather than separating out the perpendicular case for a special name.

Mike W.

(published on 01/05/2015)

## Follow-Up #1: single photon refraction

Q:
does this allow a single photon to refract
- Michael (age 14)