# Light passing through the normal of a surface

1. Oct 16, 2009

### zero_kilo

When light passes from one medium to other there is refraction but at the normal to the surface no refraction why? We know refraction occured due to change in speed of light in the medium.

Also why frequency of light do not change in going from one medium to other medium (eg from air to glass or vice-versa)

thanks

2. Oct 16, 2009

### Bob S

There usually always is both refraction and reflection for normal incidence. For an index of refraction n=1.5 from air (n=1), the fraction reflected is

F = (n-1)2/(n+1)2 = 0.04

The frequency of the light radiation is conserved, but in the medium, atoms absorb and re-radiate the light, causing a delay or reduction of the velocity.
Bob S

3. Oct 16, 2009

### zero_kilo

thanks Bob

i understand what you said but when drawn a normal to the incident light its straight.At the normal lets have an incident light (normal incident) we always draw straight in the diagram or shall we say its just a diagram and let know that there is still refraction.Refarction changes normal path.Why draw straight

can you please elaborate the conservation of frequency.

thanks again

4. Oct 16, 2009

### Bob S

"Normal" means perpendicular to the surface. There are certain special angles where there is no relection of polarized light. See
http://en.wikipedia.org/wiki/Brewster's_angle
Bob S

re frequency: the light slows down and the wavelength gets shorter when it enters a refrective medium, but frequency is unchanged. In a sense it is like a driven electrical or mechanical circuit, where the driving term is A sin(wt). No matter what the circuit is, the frequency is unchanged.

Last edited: Oct 16, 2009
5. Oct 16, 2009

### zero_kilo

thanks Bob you are a great help

but this is still my question:

Why? Is not there are a refraction when it passes from one medium to another? Refraction changes direction of path.

thanks

6. Oct 16, 2009

### pseudophonist

Consider a series of wavefronts incident on a medium at an angle. The rate (eg. 1 per second) at which the waves arrive at and leave the surface of the medium must be the same. Hence frequency is conserved and the wavelength changes.

If we represent the wavefront by a line then part of the wavefront in medium 1 has one wavelength a and in in medium 2 it has wavelegth b. The line has to be continuous (there is no break) so the angle with respect to the interface changes.

However, if the waves are incident normal to the interface then the change in wavelength can be accomodated without changing the angle. Indeed, why would they bend one way and not another?

7. Oct 17, 2009