image of Eye on retina

Graphic from dorling kindersley books website





D. Index of Refraction (n) – a comparison of speed of light in a substance to its speed in a vacuum



If a substance has an index of refraction of 2.00 it means light is twice as fast in a vacuum than it is in that medium


n = c/v


c = speed of light in a vacuum
3.00 x 108 m/s)

    v = speed of light in a medium



nvacuum = 1.00  


nair ≈ 1.00  , v ≈ 3 x 108 m/s






Which substance slows down light the most when light enters from air?





(highest (n) index of refraction)













(smallest (n) index of refraction) 



Any device with a lens refracts light





Ex 1) Speed of light in alcohol?    


n = 1.36 (Reference Table)


    n = 1.36 = c/v


1.36 = (3.00 x 108 m/s)/V     



3.00 x 108 m/s = 1.36V


    V = 2.21 x 108 m/s



Magnifying glasses enlarge objects with refraction





 n & v relationship?

n = c/v








The more light is
 slowed down

or sped up,

the more
it changes direction


Refraction in Glass



The human eye refracts light




1. Prediction θref from θin






a. Snell's Law

n1sinθ1 = n2sinθ2


1 – incident medium


2 – refractive medium





n1sinθ1 = n2sinθ2



1.00 sin53 = (1.50) sinθ2  



.798 = 1.50sinθ2




.798/1.50 = sinθ2



.532 = sinθ2


(inverse sine .532)


θ2 = 320  






Ex) Light refracts as it moves from water to flint glass. Which angle is bigger, qin or qref?


Faster medium has bigger angle


Water 1.33?? or Flint glass 1.66??


Angle of Incidence bigger, Water faster





Ranking Task

Rank the media below from
fastest to slowest







Refraction Relationships




NOVA (PBS) explains how refraction makes diamonds sparkle




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