I had read that when light enters a glass( or any other medium) its speed changes which lead to the bending of its path. But why doesn't light bend when it is incident normal to the surface. It is still entering a new medium and likewise, its speed will change but why doesn't it refract then?
I just can't understand it!
[Physics] Why a light ray does not refract when it is incident normally to the surface
opticsrefractionsymmetry
Related Solutions
The wave only refracts if it enters the medium at an angle. Follow a single wavecrest; if the wave is entering the medium at an angle, then part of the wavecrest enters the medium first, and starts to slow down, while the other part of the wavecrest is still going fast, and therefore the wavecrest must bend. If the wave enters at a right angle, then the entire wavecrest is slowed down simultaneously and no refraction occurs.
A good question, and one that confused me for a long time.
In the absense of matter the light wave is massless. However in the presence of polarisable matter the light interacts with the matter and forms a composite system. Now you have a single wavefunction that describes the combined system and you can no longer separate it into a light bit and matter bit. This composite system propagates with a velocity of less than $c$.
The composite system of light and matter is called a polariton although I think strictly speaking the term polariton is reserved for strongly interacting systems such as Bose-Einstein condensates, where the speed of light can be reduced to only a few metres per second.
Best Answer
Gilbert's answer is great, but if you want a different approach just look at Snell's law: $$n_1sin(\theta _1)=n_2sin(\theta _2)=$$
Where $n_i$ is the index of refraction for medium $i$, and $\theta _i$ is the angle the light ray makes with respect to the normal in each medium.
If light enters from medium $1$ to $2$ perpendicular to the boundary between the two media, then $\theta _1=0$. This means than $sin(\theta _2)=0$. The only values $\theta _2$ can have are then $0$ or $\pi$, but an angle of $\pi$ wouldhave the light ray not in medium $2$. Hence it must be that $\theta _2=0$.