Light not bending

[flano]

why does light not bend when it enters perpendicular to a transparent surface

[Hippy]

The bending you see when a beam of light passes from air into a liquid is called refraction. It is a result of the different speed of the light wave as it crosses from one medium to another of different density. This is how lenses work. The water or glass is denser than air so slow down the wave. If the beam of light hits the boundary from air to water at exactly 90 degrees then the beam slows down uniformly across its width. However, imagine the beam hitting the boundary a glancing blow. The front of the beam is a square edge and it arrives at the air to water boundary and one corner hits the boundary before the opposite edge and so experiences the slowing first and by doing so it "drags" the beam around at this point. The beam then travels through the denser medium in a straight line, provided it is of uniform density.

To calculate the angles of refraction is a complex an non-linear business and a clever mathematician called Snell devised a formula to explain this phenomenon. Snell's law applies to light refraction in any situation, regardless of the two media densities. For the determined, Snell's law is given as:
Ni * sine(THi) = Nr * sine(THr)
where Ni = refractive index of incident medium; THi = Theta i the angle of incidence; Nr = refractive index of the refractive medium; and THr = Theta r the angle of refraction.

The clever thing here is that if any three of the quantities are known then the fourth can be calculated.

[Hippy]

As a postscript, the refractive index of air is 1.0, and that of water 1.33. The higher the number the more the medium will slow (and therefore refract) the light beam. So, glass is around 1.55.