When light propagates through two adjacent materials that have different optical properties, some interesting phenomena occur at the interface separating the two materials. Forexample, consider a ray of light that travels from air into the water of a lake. As the ray strikes the air$-$water interface $($the surface of the lake$),$ it is partly reflected back into the air andpartly refracted or transmitted into the water. This explains why on the surface of a lake sometimes you see the reflection of the surrounding landscape and other times the underwatervegetation.$$

$=1\mathrm{.}33$ nw $=1\mathrm{.}00$ na m m $[(10\mathrm{.}6)/1]100$ n f v n$1$ f n n$1$ n$1$ n n$1$ n$1$ v n n$1$

These effects on light propagation occur because light travels at different speeds depending on the medium. The index of refraction of a material, denoted by $,$ gives an indication ofthe speed of light in the material. It is defined as the ratio of the speed of light in vacuum to the speed in the material, or