Lecture22 Light Reflection and Refraction

8/3/2019 Lecture22 Light Reflection and Refraction

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Announcements

yTodays material WILL NOT be covered on Exam 3!Material for Exam 3:Chapters 28, 29, 32

y Reminder: Special Homework #8 is due tomorrow.(download and print if you need it)

y Friday, November 11,isthe lastdropday.

y Exam 3 is one week from tomorrow. I will need to know byWednesday of any students who have special needs differentthan for exam 2.


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Announcements

yNote change to syllabushomework for tomorrow:

33: 1, 12, 17, 42, 54, Special Homework #8

(add problem 33.12, do not work problem 33.42)


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Light

How many physicists does it take to change a light bulb?

Normally, light refers to the type of electromagnetic wavethat stimulates the retina of our eyes.

Light acts like a wave except when it acts like particles.


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LightWavesor Particles?*

http://www.nearingzero.net(quantum007.jpg)

*Light is both! Take Physics 107 for further enlightenment!


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Light is a type of electromagnetic wave and travels with thespeed c = 2.9979x108 m/s in a vacuum. (Just use 3x108!)

The Speedof Light

How many physicists does it take to change a light bulb?

Eleven. One to do it and ten to co-author the paper.


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NewsFlash!

Light slows down when it passes froma vacuum into matter!

Details beginning on slide 16!


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Although light is actually an electromagnetic wave, it generally

travels in straight lines (like particles do!).

We can describe many properties of light by assuming that ittravels in straight-line paths in the form of rays.

Aray is a straight line along which light is propagated. Inother contexts, the definition of ray might be extended to

include bent or curved lines.

Geometric Optics

http://www.lab-initio.com (nz341.jpg)


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Alightray is an infinitely thin beam of light. Of course, there

really isnt such a thing, but the concept helps us visualizeproperties of light.

http://www.lab-initio.com (nz342.jpg)

thankfully, there really isnt such a thing


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Light rays from someexternal source strike an

object and reflect off it in alldirections.

We only see those light raysthat reflect in the directionof our eyes.

If you can see something, itmust be a source of light!

Zillions* of rays are simultaneously reflected in all directionsfrom any point of an object. Later, when we study mirrors andlenses, we wont try do draw them all! Just enoughrepresentative ones to understand what the light is doing.

*one zillion = 10a big number


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Todays agenda:

Introduction to Light.You must develop a general understanding of what light is and how it behaves.

ReflectionandRefraction (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.You must understand that the index of refraction of a material is wavelength-dependent.


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Light striking a surface may be reflected, transmitted, orabsorbed. Reflected light leaves the surface at the same angleit was incident on the surface:

U Ui r=

Ui Ur

Reflection

Real ImportantNote: the angles are measured relative to the surface normal.

Possible homework hint: the sum of the angles in a 4-sided polygon is 360r.


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Reflection from asmooth surface is

specular (mirror-like). Reflectionfrom a roughsurface is diffuse(not mirror-like).

http://acept.la.asu.edu/PiN/rdg/reflection/reflection.shtml

http://www.mic-d.com/java/specular/


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Light travels in a straight lineexcept when it is reflected orwhen it moves from onemedium to another.

Refractionthe bending of light rays when light moves fromone medium to a different onetakes place because lighttravels with different speeds in different media.

Refraction

http://id.mind.net/~zona/mstm/physics/light/rayOptics/refraction/refraction1.html


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The speed of light in a vacuum is c = 3x108 m/s. The index ofrefraction of a material is defined by

cn = ,

v

where c is the speed of light in a vacuum and v is the speed oflight in the material.

The speedandwavelengthof lightchange when it passes

from one medium to another, butnotthefrequency, so

.

PPn

cv = and =

n n

If you study light in advanced classes,youll find it is more complex than this.


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Because light never travels faster than c, n u 1.* For water, n= 1.33 and for glass, n }1.5. Indices of refraction for several

materials are listed in your text.

cv =n

8310 m/sv =

2.428v = 1.2410 m/s

Example: calculate the speed of light in diamond (n = 2.42).

*A

ctually, not true but dont worry about it unless you take advanced courses in optics.


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Because n u 1, wesee this:

If n < 1, we wouldsee this:

Thanks to Dr. Xiaodong Yang for the images.


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When light moves from one medium into another, some isreflected at the boundary, and some is transmitted.

The transmitted light is refracted (bent).

Ua is the angle of incidence, and Ub is the angle of refraction.

Snells Law

air (na)

water (nb

)

Ua

Ub

nb>na

air (nb)

water (na

)

Ub

Ua

na>nb

a a b bn sin = n s i n


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air (na)

water (nb)

Ua

Ub

nb>na

Light passing from air (n } 1) into water (n } 1.33).

Light bends towards the normal to the surface as it slowsdown in water.

a b(1) sin = ( 1 . 3 3 ) s i n

a b>

a a b bn sin = n s i n


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air (nb)

water (na)

Ub

Ua

na>nb

Light passing from water (n } 1.33) into air (n } 1).

Light bends away from the normal to the surface as itspeeds up in air.

a b(1.33) sin = ( 1 ) s i n

a b UC

U1 U1

U1


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n2

n1>n2

Ray incident normal to surface is not bent.

Ray incident normal to surface is not bent. Some is reflected,Ray incident normal to surface is not bent. Some is reflected,some is transmitted.


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n2

n1>n2

Increasing angle of incidence


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n2

n1>n

2

Increasing angle of incidencemore


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n2

n1>n

2

Increasing angle of incidencemorecritical angle reachedIncreasing angle of incidencemorecritical angle reachedsome of incident energy is reflected, some is transmittedalong the boundary layer.


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n2

n1>n

2

Light incident at any angle beyond UC is totally internallyreflected.


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application: fiber optics

http://laser.physics.sunysb.edu/~wise/wise187/janfeb2001/reports/and

rea/report.html


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Ui

Uf

ni=1 (air)

nf>1

Light is incident atan angle Ui on atransparent fiber.

The light refracts at an angle Uf.

i i f f n sin = n s i n

i f fsin = n s i n

Example: determine the incident angle Ui for which light strikesthe inner surface of a fiber optic cable at the critical angle.


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Ui

Uf90r-Uf

ni=1 (air)

nf>1

Light strikes the fiberwall an an angle of

90-Ufnormal to thesurface.

At the critical angle, instead of exiting the fiber, the refractedlight travels along the fiber-air boundary. In this case, 90r-Uf isthe critical angle.

!f f f c an sin 90 - = n s i n = n s i n 9 0 1

ff

1sin 90- = n

90r

Solve the above for Ufand use to solve forUi.

i f fsin = n s i n


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Ui

Uf90r-Uf

ni=1 (air)

nf>1

Numerical example:what is critical angle if

nf=1.4?

f1

sin 90- = 1.4

90r

rf90 - = 4 5 . 5 8 rf = 4 4 . 4 1

ri f1 sin = 1 . 4 s i n

= 1 . 4 s i n 4 4 . 4 1 = 0 . 9 8 0

ri = 7 8 . 5

This is a very large angle of incidence! If you want the incident light to benearly parallel to the fiber axis, you must surround the fiber with a coating

with noutside


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application: swimming underwater

If you are looking up from underwater, if your angle of sight(relative to the normal to the surface) is too large, you see anunderwater reflection instead of whats above the water.


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application: perfect mirrors

(used in binoculars)

application: diamonds


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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.You must understand that the index of refraction of a material is wavelength-dependent.


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Dispersion

Weve treated the index of refraction of a material as if it had asingle value for all wavelengths of light.

In fact, the index of refraction is generally wavelength- (or

color-) dependent.


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Picture from the Exploratorium (http://www.exploratorium.edu/).

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Lecture22 Light Reflection and Refraction