© 2001-2005 Shannon W. Helzer. All Rights Reserved. Unit 18 Reflection & Refraction of Light.

$: © 2001-2005 Shannon W. Helzer. All Rights Reserved. Unit 18 Reflection & Refraction of Light.$
© 2001-2005 Shannon W. Helzer. All Rights Reserved.

Unit 18Reflection & Refraction

of Light


Reflection Observe the reflection of a laser beam due to the mirror. The Normal is an imaginary line drawn at a 90 angle with the surface of the

mirror at the point where the beam strikes the mirror. The angle between the incident ray and the normal is known as the angle of

incidence (i ).

The angle between the reflected ray and the normal is known as the angle of reflection (r ).

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Mirror

Incident Ray Reflected Ray

Norm

al

i r


Mirror Types There are three types of mirrors:

plane mirror, concave mirror, and convex mirror.

Each of these three mirror types form images with different properties.

A plane mirror forms an image that is the same size as the object.

A convex mirror forms an image that is smaller than the object.

A concave mirror forms an image that is larger than the object.

What type of mirror is shown in the figure to the right?

Convex Mirror

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???M

irro

r T

ype?

????

Object Reflected Image


Mirror Types

What type of mirror is shown in the figure below? Plane Mirror.

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????

?Mir

ror

Typ

e???

??Object Reflected Image


Mirror Types

What type of mirror is shown in the figure below? Concave Mirror.

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????

?Mir

ror

Typ

e???

??

Object Reflected Image


Diffuse Scattering.

Imagine that several parallel laser beams are fired at a mirror and a ceiling tile.

The reflected beams from the mirror surface would still be parallel, and i and r

would be equal. However, the reflected beams from the tile surface would not be parallel. The reflected beams from the tile surface exhibit diffuse scattering.

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Mirror Ceiling Tile


Sound Wave Reflection The picture to the right shows the TV room

of a disappointed viewer. He is very disappointed due to the poor

sound quality of his TV. The rays represent the path the sound travels

in the room. Sound waves, like light waves, reflect off of

surfaces. He is sitting in an acoustic “dead zone.” Such a zone is typically the result of

destructive interference of sound waves. What could he do in order to improve the

sound quality? If he moves his chair, then he will be able to

better hear and enjoy his TV.

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Simply put, “Refraction” means bends. When discussing light beams, light

bends when it goes from one medium (glass, water, air, etc.) to another.

If it goes from a more dense medium to a less dense medium, then light speeds up and bends away from the Normal.

If it goes from a less dense medium to a more dense medium, then light slows down and bends towards the Normal.

Consider the wheel and axel to the right. It rolled from pavement (less dense) to

gravel (more dense) The left wheel slowed down causing the

axel to turn towards the normal.

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Pavement

Gravel

Refraction


Refraction

When light undergoes refraction while traveling from a less dense to a more dense medium, the light bends towards the normal.

This is because light travels faster in a less dense medium than in a more dense medium.

Is the fish safe from the laser if we point the laser at the fish?

When laser light travels from air, a less dense medium, and is shined onto the surface of water, a more dense medium, in an fish tank, the light bends towards the normal as shown.

As a result, the light will bend and travel below the fish.

This bending of light at the interface of two mediums is known as refraction.

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Normal


Refraction

Refraction applies to Lenses as well. Which laser path to the right would the laser

beam follow? When the beam first strikes the lens, it goes

from a less dense to a more dense medium. As a result, the beam will slow down and

turn towards the normal. When the beam strikes the boundary from

the lens back to air, it goes from a more dense to a less dense medium.

As a result, the beam speeds up and will turn away from the normal.

It will stay on the same side of the normal. Therefore, the center beam path would be the

correct path followed by the laser beam.

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Refraction

Which of the three paths (A, B, or C) is the one the beam would actually travel?

Remember, lights bends towards the normal when it goes from a less dense to a more dense medium because it slows down in the more dense medium.

Conversely, light bends away from the normal when it goes from a more dense to a less dense medium because it speeds up in the less dense medium.

Which is the correct path?

A B C

Air

Glass

Water

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Light Spectrum

Previously, we learned that light travels in waves known as “Transverse” waves. Light is composed of massless particles known as photons. The spectrum of electromagnetic radiation spans several wavelengths

(frequencies) of the photons. Of particular interest to us is the visible spectrum. To remember the visible spectrum, use the name “ROY G BIV.”

VisibleInfrared Ultra Violet GammaRadio Waves Microwaves X-rays

Red

Oran

ge

Yellow

Green

Blu

e

Ind

igo

Violet

Frequency Increases

Speed Increases 18-12


Dispersion of Light Dispersion is the separation of light

according to its frequencies into its different colors.

A prism is a device often used to demonstrate dispersion of white light.

Light with higher frequency travels slower in a medium; therefore, it will bend more with respect to the normal.

Light with lower frequency travels faster; therefore, it will bend less with respect to the normal.

One of the exit beams is orange and the other is green.

Based on the above information, which exit beam is orange? Top or bottom?

Red

Oran

ge

Yellow

Green

Blu

e

Indigo

Violet

Frequency

Speed

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Refraction & Fishing

Dr. Physics is watching a fish swimming in a pond.

He sees the fish indicated in the position shown.

Which position, top or bottom, is the actual position of the fish?

Why? When light beams travel from a

more dense to a less dense medium, they speed up and bend away from the normal.

As a result, the actual position of the fish is deeper than the apparent position of the fish.

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Lenses and Magnification – Diverging Lens

When the beam first strikes the lens, it goes from a less dense to a more dense medium.

As a result, the beam will slow down and turn towards the normal.

When the beam strikes the boundary from the lens back to air, it goes from a more dense to a less dense medium.


Will the resulting image be larger or smaller than the original image?

A diverging lens produces a larger image.

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Lenses and Magnification – Converging Lens

When the beam first strikes the lens, it goes from a less dense to a more dense medium.

As a result, the beam will slow down and turn towards the normal.

When the beam strikes the boundary from the lens back to air, it goes from a more dense to a less dense medium.


Will the resulting image be larger or smaller than the original image?

A converging lens produces a smaller image.

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Internal Reflection

Suppose we had a laser submerged in water. If it was pointing straight up, then the beam would pass through the boundary

interface without reflecting or refracting. If we rotate the laser, then some of the beam will reflect and some will refract. As we continue to rotate the laser, we would eventually observe the refracted

beam traveling along the boundary interface. This phenomena occurs at the critical angle. As the laser is rotated past the critical angle, all of the beam will be reflected and

none will be refracted. This phenomena is known as total internal reflection.

c

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Lens Vocabulary

A – Center of Curvature B – Focal Point C – Focal Length D – Principal Axis

A AB B

C C

D

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Magnification Magnification is the enlargement or reduction of the size of the image of

an object produced by a lens. If the object is beyond twice the focal length (2f), then the image will be

smaller and real (upside down) If the object is at 2f, the it will be the same size and real. Between f and 2f, the object will be larger and real. At f the image will focus at infinity (not be visible). In between f and the lens, the image will be much larger, virtual (right side

up), and will appear on the same side of the lens as the object. This situation occurs with a magnifying glass.


The Anatomy of an Eye

Label the following eye anatomy (parts) in the figure below. A – Cornea B – Lens C – Iris D – Blind Spot

A

B

C

D

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Nearsighted

A nearsighted person sees objects nearby clearly; however, far away objects appear blurry.

The light rays from far away objects form the image within the eye before the rays reach the retina.

This problem is corrected using a diverging lens. This diverging lens separates the rays before they reach the eye lens. The eye lens then focuses these rays on the retina thereby producing a high

quality image.

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Farsighted

A farsighted person sees objects far away clearly; however, near objects appear blurry.

The light rays from near objects would form the image beyond the eye in the brain cavity well beyond the retina.

This result, of course, is impossible. This problem is corrected using a converging lens. This converging lens brings the rays together before they reach the eye lens. The eye lens then focuses these rays on the retina thereby producing a high

quality image.

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Index of Refraction, n The index of refraction is a ratio of the speed of light in a vacuum (c) to the speed

of light in a medium (v) like glass, oil, water, quartz, diamond,…. The top figure shows a laser aimed into a evacuated cylinder (nothing is inside it). The speed of light in this cylinder is c = 3 x 108 m/s. The bottom figure shows a laser aimed into a cylinder of water. The light will travel slower in this cylinder, and its speed (v) can be measured in a

lab. The ratio of these speeds gives the index of refraction (n) of the material through

which the light is shined (in this case water).

1

cn

v


Snell’s Law

Snell’s law allows us to determine the angle of refraction given the angle of incidence of a light ray on a boundary interface.

n1 is the index of refraction of medium 1, and 1 is the angle of incidence on the boundary between the two interfaces.

n2 is the index of refraction of medium 2, and 2 is the angle of refraction.

Based upon the picture to the right, which medium us more dense: 1 or 2? Why?

Since the beam bent towards the normal when going from medium 1 to medium 2, medium 2 is more dense than medium 1.

n1 sin 1 = n2 sin 2

2

1n1

n2

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Snell’s Law Given the following information,

determine the type of surrounding material and the lens material.

i = 65R, Use N1, nS = 1.36, nL = 1.54.

Begin by measuring the incident angle and drawing the incident beam.

Use Snell’s Law to calculate the refracted angle.

Draw the refracted beam to the next material interface.

Draw the normal as shown and measure the new incident angle.

This step not shown. Use Snell’s Law to calculate the

refracted angle, and draw the refracted beam.

The material is determined based upon where your second refracted beam stops.

What are your materials for this problem?

Air with diamond lens

Air with window glass lensWater with

diamond lens

Ethanol with Quartz lens

N1N2N3

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Assignment:

1. i = 65R, Use N1, nS = 1.36, nL = 1.54.2. i = 45R, Use N2, nS = 1.33, nL = 2.42. 3. i = 30L, Use N3, nS = 1.00, nL = 1.61.4. i = 30L, Use N3, nS = 1.00, nL = 2.42.


Snell’s Law

Air: n1 = 1.003Diamond: n2 = 2.42

1 = 60Snell’s Law

n1 sin 1 = n2 sin 2

60 21 9 22


Snell’s Law

Water: n1 = 1.33Glass: n2 = 1.52

1 = 40Snell’s Law

n1 sin 1 = n2 sin 2

40 34 26 6218-26


Refraction

Refraction is responsible for the “broken straw” effect you observe when viewing a straw in a drinking glass.

In the next slide, we will use this effect in order to determine the index of refraction of glass.

18-27


Lab – Determination of ni for Glass

Follow the procedure on your lab in order to find the index of refraction for the piece of glass provided by your instructor.

You will do this experiment twice using different angles.

P3

P2

P1

N1

N2

1

2

P4

2

1

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Where we are committed to Excellence In Mathematics And ScienceEducational Services.


A A

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N3

Air with window glass lens

N1N2

Glass around diamond lens

Water around Quartz lens


A

A

18-1


A

Keep Me

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Air with diamond lens

Air with window glass lens

Water with diamond lens

N1N2N3

Ethanol with Quartz lens

© 2001-2005 Shannon W. Helzer. All Rights Reserved. Unit 18 Reflection & Refraction of Light.

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Transcript of © 2001-2005 Shannon W. Helzer. All Rights Reserved. Unit 18 Reflection & Refraction of Light.