Part II

Electromagnetic Waves

1

So far

2

We have discussedThe nature of EM wavesSome of the properties of EM waves.

Now we will discussEM waves and optics

MirrorsLensesApplications

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Reflection & Refraction

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Reflection

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Light Reflection

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Angles of incidence measured from the NORMAL to the mirror.

For reflection – The angle of incidence equals the angle of reflection.

surface normal

sameangleincident ray exit ray

Principle of Least Time

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A

Btoo longshortest path;equal angles

8 PHY102

Your eye focuses the diverging rays reflected by the mirror.

The light rays appear to come from behind the mirror.

An image is virtual if the light rays from a point on the object are directed as if they diverged from a point on the image, even though the rays do not actually pass through the image point.

Mirror, Mirror, on the wall, how big do you have to be to see it all? The strange world of images.

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“image” you

“real” you

The speed of light

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Light travel’s at a speed c in a vacuum.In an actual material, it travels a bit slower,

at a velocity of v.The speed of light depends on the material

through which it is traveling.DEFINITION – INDEX OF REFRACTION (or

refractive index):1

mediumnt transparaain light ofvelocity

vacuumain light ofvelocity n

Index of Refraction

11

Notice

12

vacuum

f

c

f

cn

fvv

cn

1

For two materials

1

2

2

1

22

11

n

n

divide

n

n

vacuum

vacuum

Huygen’s Principle

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Each point on a wavefront actsas a secondary source of sphericalwaves that progress from thesource at the speed of light (whateverit may be).

A spherical wave with very large radius,behaves as a plane wave.

14

15 PHY102

.

Not there yet!

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The Wave Nature of LightThe law of refraction is explained by

Huygen’s Principle

The little wavelets move slower in medium 2 than in medium 1. Doing the tangent shows how the wave fronts bend.

Some Geometry

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0

0

90

90

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The Wave Nature of Light

ADC2

BAD1

sin1 v1tAD

and sin 2 v2tAD

2

1

1

2

1

2

1

2

1

2

2211

1

2

2

1

2

1

2

1

/

/

sinsin

/

/

sin

sin

n

n

nc

nc

v

v

tv

tv

nn

n

n

nc

nc

v

v

If medium 1 is air, then

n1 1 and v1 c and 1 n /n

Snell’s Law of Refraction

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2211 sinsin nn Law of Reflection

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Another ViewOf Reflection(Huygen’s)

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Both Together

21

22

23

a

bcritical

bbcriticala

ab

n

nSin

nnSinn

nn

)90sin( 0

At the critical angle andbeyond, only reflection is

possible.

Total Reflection – Optical Fiber

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A cross section of a submarine communications cable.1 - Polyethylene2 - Mylar tape3 - Stranded steel wires4 - Aluminium water barrier5 - Polycarbonate6 - Copper or aluminum tube7 - Petroleum jelly8 - Optical fibers

Lengths of 100 KM are possible before amplification is necessary.

Thousands of conversations can be carried on a single fiber.

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Waves on a string.

dichroism

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80%

1% of opposite polarization

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29

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Mirage

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