A right-angle glass prism turns a ray of light through 90 degrees. The ray enters one face along the normal, so it does not bend. It strikes the slanted face at 45 degrees, which is greater than the critical angle for glass (about 42 degrees), so it is totally internally reflected. It then leaves the third face along the normal. Two such prisms placed together make a periscope.
MissionFollow the ray through the prism, then judge what happens where it meets the slanted face.Streak 0Best 0
The ray meets the slanted face at 45 degrees, which is more than the critical angle. So it is:
angle at the slanted face45°
critical angle for glassabout 42°
turn of the ray90°
45 degrees is greater than 42 degrees, so the ray is totally internally reflected and turns through 90 degrees.