# Refraction and Snell's Law

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Snell's Law In the previous sections we studied light reflecting off various surfaces. What happens when light passes from one medium into another? The speed of light, like that of all waves, is dependent on the medium through which it is travelling. When light moves from one medium into another (for example, from air to glass), the speed of light changes. If the light ray hits the boundary of the new medium (for example the edge of a glass block) at any angle which is not perpendicular to or parallel with the boundary, the light ray will change its direction through the next medium, or appear to `bend'. This is called refraction of light. It is important to note that while the speed of the light changes when it passes into the new medium, the frequency of the light remains the same. Definition 1: Refraction Refraction occurs at the boundary of two media when light travels from one medium into the other and its speed changes but its frequency remains the same. If the light ray hits the boundary at an angle which is not perpendicular to or parallel to the surface, then it will change direction and appear to `bend'. Refraction is nicely demonstrated when you look from above at an angle at a straw in a glass of water. The straw appears bent in the liquid. This is because the light rays leaving the straw change direction when they hit the surface between the liquid and the air. Your eyes trace the light rays backwards as straight lines to the point they would have come from if they had not changed direction and as a result you see the tip of the straw as being shallower in the liquid than it really is. Figure 1: Due to refraction, a straw in a glass of water appears bent when an observer looks down at an angle from above the water surface. Refractive index The speed of light and therefore the degree of bending of the light depends on the refractive index