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Friday
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Snell’s Law of Refraction
•
n
1
sin
θ
1
=
n
2
sin
θ
2
–
θ
1
is the angle of incidence
–
θ
2
is the angle of refraction
2
• The experimental discovery of this
relationship is usually credited to
Willebrord Snell and is therefore known as
Snell’s law of refraction
Example
Example
Huygens’s Principle
• All points on a wave front are point sources
for spherical secondary waves, called
wavelets, which propagate with the
characteristic speed of waves in that
medium
• The new position of the wave front is the
surface tangent to the wavelets
Huygens’s Construction –
Examples
Plane wave
Spherical wave
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Huygens’s Principle and the Law
of Reflection
•
AD
=
BC
=
c
Δ
t
Huygens’s Principle and the Law
of Reflection, cont.
• Triangle
ABC
is
congruent to triangle
ADC
cos
γ
=
BC
/
AC
• cos
=
/
• cos
’ =
AD
/
AC
• Therefore, cos
= cos
’ and
=
’
• This gives
θ
1
=
θ
1
’
• This is the law of
reflection
Huygens’s Principle and the Law
of Refraction
• Ray 1 strikes the
surface and at a time
interval
Δ
t later, ray 2
strikes the
surface
• During this time
interval, the wave at
A
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 Spring '09
 Physics, Snell's Law, refractive index, Total internal reflection, Law of Refraction

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