# Lecture #11 Reflection, Refraction, and Dispersion by QUxwB7

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```									         Lecture #11

Reflection, Refraction, and
Dispersion

February, 23rd
Agenda

1. Review Reflection

2. Refraction (…the bending of light…).
Refraction occurs when light is traveling in
one material and then enters a new
material.

3. Dispersion

4. Total Internal Reflection
Geometric Optics (tracing rays)

We are interested about the direction of travel of the
electromagnetic wave (propagation direction).

Light does cool things

1. Reflection

2. Refraction

3. Dispersion
Reflection
One obvious property of light is that it reflects off of surfaces.
Among other things, this gives rise to the images we see in mirrors.
Reflection
light waves can be reflected at
a surface between two media

inc refl

Law of Reflection

inc = refl
LAW OF REFLECTION

• Using Fermat's principle one can show the
law of reflection.

A                                                    B
5
1              4
2    3

Fermat's principle:

Light travels in straight lines and will take the path of least time.
Plane Mirrors

Using ray diagramming one finds
that the image is upright, the same
size as the object, and is virtual.
Ray Diagramming
Plane Mirror
Refraction
Light refracts, which means that it bends when passing from one
medium to another. When light enters a more dense medium from one
that is less dense, it bends towards a line normal to the boundary
between the two media.

Example:
5. CAUSE OF REFRACTION
• When light passes from one medium to
another, its speed changes which in turn causes
a bending of the light.
• Example: car running onto shoulder
• This bending produces illusions.
• Example: objects in water appear closer and
nearer the surface
Refraction:

1. At surface, the direction of the ray
changes.

2. When speed of ray decreases, the ray
is bent towards the normal.

3. Usually, a more dense
medium…decreases the light speed.
Refraction

Snell found (in 1621) a law called the Law of Refraction (Bending)

Its qualitative form:

If a ray of light enters a more dense medium, then the ray is bent
towards the normal.

If it enters a less dense medium, the ray is bent away from
the normal.

This explains the direction of bending.
Refraction

Snell found (in 1621) a law called the Law of Refraction (Bending)

Its qualitative form:

If a ray of light enters a more dense medium, then the ray is bent
towards the normal.

If it enters a less dense medium, the ray is bent away from
the normal.

This explains the direction of bending.
Question:

Why does the wave-speed of light get slower, in a material
medium?

Model:
The atoms of the material absorb (temporarily)
the light. Then they re-emit it.

On its way from one atom to another, light travels with speed
c. But, there is a time-delay between the absorption of light by
an atom, and its re-emission.

Problem: Suppose white light is sent through a prism. It then is
dispersed into a rainbow of colors. Which takes longer to get
through the prism – red light or green light?
The Speed of Light:

In empty space, there is just one value for the speed
of light. It is 300,000 km per second. This value is
denoted as c.

This value of c does not change, no matter what
the frequency or wavelength.

In a material, the speed of light is less than c.

Examples: In water, v=0.75c; In glass, v=0.67c;
In diamond, v=0.41c, perhaps the slowest speed of
light in any simple material.
Index of Refraction (Refractive index)
Index of refraction of a material equals
the speed of light in a vacuum divided by
the speed of light in the material.
Note that v < c always. So n is always less
than 1. From experiment, n is also a measure
of the strength of light-bending of the material,
in going from air to the material.

c
n              n1sin 1=n2sin 2
v
Taking Advantage of
Refraction
The greater the density difference between the two materials, the more
the light bends. One place where this is used is in lenses for a variety of
optical devices, such as microscopes, magnifying glasses, and glasses for
correcting vision. An example of an image formed from a lens is shown
below.
Dispersion
Another aspect of light that is quite familiar is dispersion. If a beam of
white light enters a glass prism, what emerges from the other side is a
spread out beam of many colored light. The various colors are refracted
through different angles by the glass, and are ``dispersed'', or spread
out.
Dispersion
• Different frequencies are bent different
amounts which causes a separation of white
light into its constituent colors.
• This is the basic principle behind the operation
of a prism. We say that a prism disperses the
light.
• The higher frequencies interact most (slow
down the most) and thus are bent the most.
Supplementary Materials:
Chapter 23rd,
pp. 633-634, 642-644, 645-646
Look at examples

Homework assignment:
Chapter 23,
Practice problems!!!
#1, #5,

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