Light enters a plane-parallel block of glass from the vacuum of space and leaves again into space through the opposite side (e.g., in a space-borne instrument). What is the speed of light after it leaves the glass?
A) same speed as it had before it entered the glass
B) The exit speed will depend on the thickness of the glass, becoming slower the thicker the glass.
C) faster than when it entered because it has passed through the dense glass
D) slower because it has passed through the glass and been slowed by it

Question

Quizplus · Accepted Answer

The speed of light in a vacuum is a constant (approximately 299,792 kilometers per second), regardless of its previous medium. When light exits the glass and re-enters space (a vacuum), its speed returns to this constant value, unaffected by the glass's thickness or density.

Light Enters a Plane-Parallel Block of Glass from the Vacuum