Today

1
Todays agenda Introduction to Light. You must
develop a general understanding of what light is
and how it behaves. Reflection and Refraction
(Snells Law). You must be able to determine the
path of light rays using the laws of reflection
and refraction. Total Internal Reflection and
Fiber Optics. You must be able to determine the
conditions under which total internal reflection
occurs, and apply total internal reflection to
fiber optic and similar materials. Dispersion. Yo
u must understand that the index of refraction of
a material is wavelength-dependent.
2
Reflection
Light striking a surface may be reflected,
transmitted, or absorbed. Reflected light leaves
the surface at the same angle it was incident on
the surface
?i
?r
Real Important Note the angles are measured
relative to the surface normal.
Possible homework hint the sum of the angles in
a 4-sided polygon is 360?.
3
Reflection from a smooth surface is specular
(mirror- like). Reflection from a rough surface
is diffuse (not mirror-like).
http//acept.la.asu.edu/PiN/rdg/reflection/reflect
ion.shtml
http//www.mic-d.com/java/specular/
4
Refraction
Light travels in a straight line except when it
is reflected or when it moves from one medium to
another.
http//id.mind.net/zona/mstm/physics/light/rayOpt
ics/refraction/refraction1.html
Refractionthe bending, or change of direction
of light rays when light moves from one medium to
a different onetakes place because light travels
with different speeds in different media.
5
The speed of light in a vacuum is c 3x108 m/s.
The index of refraction of a material is defined
by
If you study light in advanced classes, youll
find it is more complex than this.
where c is the speed of light in a vacuum and v
is the speed of light in the material.
The speed and wavelength of light change when it
passes from one medium to another, but not the
frequency, so
6
Because light never travels faster than c, n ?
1. For water, n 1.33 and for glass, n ?1.5.
Indices of refraction for several materials are
listed in your text.
Example calculate the speed of light in diamond
(n 2.42).
Actually, not true but dont worry about it
unless you take advanced courses in optics.
7
Because n ? 1, we see this
If n lt 1, we would see this
Thanks to Dr. Xiaodong Yang for the images.
8
Snells Law
When light moves from one medium into another,
some is reflected at the boundary, and some is
transmitted. Toy
The transmitted light is refracted (bent).
?a is the angle of incidence, and ?b is the angle
of refraction.
9
Light passing from air (n ? 1) into water (n ?
1.33).
Light bends towards the normal to the surface
as it slows down in water.
10
Light passing from water (n ? 1.33) into air (n ?
1).
Light bends away from the normal to the surface
as it speeds up in air.
11
Snells law, also called the law of refraction,
gives the relationship between angles and indices
of refraction
air (na)
air (na)
?a
?a
?b
?b
water (nb)
water (nb)
You are free to choose which is a and which is
b.
? is the angle the ray makes with the normal!