Total Internal Reflection - PHET

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Total Internal Reflection

| Rutgers University Physics |




Total Internal Reflection

 St. Mary's University



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Did you know ...


Skipping a rock across water has one thing in common with 'skipping' light off a media boundary.



Total Internal Reflection






A stone will skip across a pond only at wide angles
from the normal
(about 70 degrees)






2. Critical Angle θc– angle of incidence for which the angle of refraction equals 900 when light travels inside one medium that borders on air or a vacuum

Deriving Critical Angle Equation


n1sinθ1 = n2 sinθ2


    Substitute θc, θ2 = 90, n2 = 1


n1sinθc = 1sin90


n1sinθc = 1


 sinθc = 1/n1







Ex) Find θc for Lucite



sin θc = 1/n1


From reference table, n1 = 1.50 


sinθc = 1/1.50



θc = 41.8






3. Total Internal Reflection


Total Internal Reflection

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What is this called? 

Rutgers University Physics 


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(These kinds of fibers are coated with materials that have a low index of refraction (fast).
These materials are called cladding.)



St. Mary's University's Astronomy
 and Physics Dept.




When θ1 > θc
Total Internal Reflection

(Outside Medium Must Be Faster)

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| Total Internal Reflection |

Rutgers U. Physics




Ex) Lucite

θc = 41.8










When θ1 = θc


θ2 = 900






When θ < θc Refraction





Refraction of Light

Rick Reed




a) Total Internal Reflection?







Choice 2

(Outside Medium Must be Fast)




b) Refraction?







Choice 3








c) Impossible?





Choice 1

(Light always crosses over normal)









What's wrong with this picture?





Part of the ray
would be refracted.
Total Internal Reflection only occurs when the outside
 medium is very fast.








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