The angle of refraction of a light ray crossing a boundary between two different media is described by Snell's Law:
n·sinθ = n'·sinθ'
- n = index of refraction of the first medium
- θ = angle of incidence of the light ray, measured from the normal line to the boundary
- n' = index of refraction of the second medium
- θ' = angle of refraction
The only two way to get θ' = θ, meaning the light ray does not refract (bend), is when θ' = θ = 0, meaning the light ray is incident along the normal line at a right angle to the boundary. The refracted ray will pass into the second medium and continue along the normal line without bending.
I can't draw you a picture on this text editor but here's a description:
- Draw a horizontal line representing the boundary between medium 1 and 2
- Label the the area above the line "Air (n)" and the below the line "Glass (n')"
- Draw a line perpendicular to the boundary in the middle of the boundary line. Extend it above and below the boundary
- The incoming (incident ray travels along this line and the refracted ray continues into medium 2 along the same line. Draw the rays right next to the normal line and parallel to it.
