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Lecture #11 Reflection, Refraction, and Dispersion. February, 23 rd. Agenda 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 .

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Lecture #11Reflection, Refraction, and Dispersion

February, 23rd


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  • Agenda

  • 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


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Geometric Optics (tracing rays) We are interested about the direction of travel of the electromagnetic wave (propagation direction).Light does cool things1. Reflection2. Refraction 3. Dispersion


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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.


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Reflection

light waves can be reflected at

a surface between two media

refl

inc

Law of Reflection

inc = refl


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5

4

1

2

3

LAW OF REFLECTION

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

A

B

Fermat's principle:

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


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Plane Mirrors

Using ray diagramming one finds that the image is upright, the same size as the object, and is virtual.


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Ray Diagramming

Plane Mirror


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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:


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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


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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.


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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.


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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.


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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?


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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.


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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.

n1sin 1=n2sin 2


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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.


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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.


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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.


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Supplementary Materials:

Chapter 23rd,

pp. 633-634, 642-644, 645-646

Look at examples

Homework assignment:

Chapter 23,

Practice problems!!!

#1, #5,