Sunday, May 21, 2006


Snell's Law of Refraction

If light hits the surface of a media at less than a 90° angle, the angle formed between the line representing the path of light and a line that is perpendicular to the surface (the so called normal line), is called the angle of incidence. The line representing the light that emerges on the other side of the interface, measured from the normal line, is called the angle of refraction.
Click on image to enlarge.

n sin i = n’ sin r where: i = angle of incidence as measured from the normal

r = angle refracted as measured from the normal

When light enters a denser media at an angle, it slows down so that the path becomes a bit more perpendicular. Therefore light assumes a more nearly perpendicular path when passing from a less dense, into a denser medium, but assumes a less perpendicular path when passing from a denser medium into a less dense one.

  • When moving from rarer to denser medium, light is bent towards the normal. (Figure 16 A)

  • When moving from a denser to a rarer medium, light is bent away from normal. (Figure 16 B)

Question: Light leaves a medium of n = 1.55 at an angle of 30 o to the normal, how much does the angle change in air?

Answer: Using n sin i = n’ sin r where

n = index of refraction of the medium where light is coming from = 1.55

i = angle of incidence = 30°

n’ = index of refraction of air = 1

r = angle of refraction = (n sin i)/n’ = (1.55 sin 30°)/1 = 50.81° . Therefore the angle would change 50.81° – 30° = 20.81°.

Question: Define refraction of light.

Answer: Refraction is the bending of light between media and is a function of the incident angle. This is based on Snell’s Law and is not dependent on the speed of light. Snell’s Law is the relationship between the incident and refracted angles of the light ray. It has no bearing on the Law of Reflection.

Question: What happens to light as it travels from a less dense to a denser medium?

Answer: It is refracted towards the normal. If it travels from a more dense to a less dense medium, it is refracted away from the normal.

Question: What happens to a beam of light, perpendicular to the interface between two media, as it emerges from the more a dense medium?

Answer: It is transmitted at a higher speed.


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