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This text is meant to accompany class discussions. It is not everything there is to know about energy. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations linked below under unit resources.
Click here for questions that go along with this energy section.
Index of Refraction ...Continued
Sometimes when "the speed of light" is reported it is not the speed of the photons being reported, but the AVERAGE SPEED of light from location "A" to location "B." Below is an animation showing this. As a photon travels from the left to the right in a piece of glass, it strikes many molecules and gets redirected. This takes time at every collision. In between collision with the molecules, the photon does travel at 3.00 x 108 m/s.

This video can be found on YouTube at


The average speed of light through a material is related by the "index of refraction."

  • n = the index of refraction, [ No units ] (Always greater than 1 in this course.)
  • νc= the speed of light in vacuum, 3.00 x 108m/s. (Memorize this number.)
  • νm= the speed of light in medium it is passing through measured in m/s.

This also means

Where the λ's are the wavelengths.


What happens to light as it travels between mediums?



Snell's Law
As light travels between mediums with different indexes of refraction, it path will be altered at the boundary to meet the condition below.
The angles are all measured from the normal line. The "normal line" is perpendicular the boundary between two substances.


This video can be found on YouTube at


As light passes between two substances,

  • it does not bend if the the two indexes of refraction are the same or if it impacts the boundary perpendicular to the surface.
  • It bends AWAY from the normal line if it travels from a low index to a higher index of refraction.
  • It bends CLOSER to the normal line if it travels from a low index to a higher index of refraction.
  • The bigger the ratio of the indices's of refraction, (higher/lower,) the great the bending. [Compare the two images on the left above.]



What is the refracted angle of the light after passing between the two different materials?



This video can be found on YouTube at

Critical Angle


This video can be found on YouTube at




Will the incident light ray reflect or refract and at what angle?



Find the critical angle: If the 28° is great than this angle the the ray is reflected at 28°. If 28° is smaller than the critical angle, then the light is refracted.


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by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)