Physics

Physics

PHS 5041 OpticsRefraction of Light



Refraction always occurs simultaneously with some partial reflection

Change in direction is with respect to the normal (different angle of incidence and refraction)

Perpendicular rays DO NOT refract (reflection only)


Laws of Refraction:

_Incident ray, refracted ray and the normal lie on the same plane

_ n1sinθ1 = n2 sinθ2 (Snell’s Law)

n1: index of refraction of medium 1

n2: index of refraction of medium 2

θ1: angle of incidence

θ2: angle of refraction


Index of refraction is a measurement of a medium’s optical density


Example:A light ray passes from air into a liquid. If incident angle is 40° and refraction angle is 28.2°. Identify the liquid.

n1sinθ1 = n2 sinθ2

n2 = n1sinθ1 / sinθ2

n2 = (1.003)(sin 40°) / (sin 28.2°) n2 = 1.36 (Ethyl alcohol)


Perpendicular incident rays DO NOT refract (reflection only)


When light goes from a less optically dense medium to a more optically dense medium (n1 < n2), it bends TOWARD the normal


When light goes from a more optically dense medium to a less optically dense medium (n1 > n2), it bends AWAY from the normal


Critical angle: Incident angle for which the refracted angle is 90 degrees

sin θc= n2 / n1

or

sin θc= sin θ1 / sin θ2

(n1 must be higher than n2)


Total Internal Reflection: Occurs when the incident angle is HIGHER than the critical angle

ONLY occurs when light travels from a more to a less optically dense medium


Example:An angle of refraction of 74.6° in air corresponds to an angle of incidence of 40° in crown glass. Which of these two media can produce total internal reflection? Why? Find the value of the critical angle.

_Crown glass is the one that produces TIR for it is more optically dense than air (ng > na). Also incident angle < refracted angle

sin θc= sin θ1 / sin θ2

sin θc= sin 40° / sin 74.6° sin θc= 0.6667

θc= 41.8°


Light velocity is affected as light travels from vacuum into another medium:

nx = c / vx

nx: index of refraction of medium x

vx: velocity of light in medium x

c: velocity of light in vacuum (3 * 108 m/s)


Example:Calculate the index of refraction of Plexiglas knowing that the speed of light in this material is 1.99 * 108 m/s.

np = c / vp

np = (3 * 108 m/s) / (1.99 * 108 m/s)np = 1.51




Applications:


Applications:


Applications:


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