# Semi circular prism and light rays

1. Dec 10, 2008

### hallowon

1. The problem statement, all variables and given/known data
So anyway in the diagram of my book there is a ray hitting the centre in the flat part of a semi circular prism. The question in the book states "Ignoring partial reflection, we know that each ray will pass through the flat surface and then pass through the curved surface, Which of the rays refract only once? Explain" I also do not get why it refracts more than once if the light rays do not hit the centre.

2. Relevant equations
Snell's law

3. The attempt at a solution
I know from the centre to the edge it is the radius of the semi circular prism. When the light refracts from the centre onto the edge i know that the normal is perpendicular to that point and the angle of refraction = 0.

2. Dec 10, 2008

### Redbelly98

Staff Emeritus
Welcome to PF

Something is not clear from what you wrote. Is there just a single ray in the figure, hitting the center of the flat side? Or are there more rays (and if yes, please describe their path, i.e. where and at what angle are they incident on the prism?)

It sounds like they are defining "to refract" as "to pass through a surface and exit at an angle different than the incident angle". In other words, if a ray passes through a surface without changing its angle, it is not considered to refract there.

If so, you'll need to think about under what condition a ray does not change its angle when passing through a surface.