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Chapter 28 Chapter 28 Reflection and Refraction 1. REFLECTION Most objects we see reflect light. 2. PRINCIPLE OF LEAST TIME Fermat's principle: Light travels in straight lines and will take the path of least time. Fermat says take this path. 5 4 1 2 3 3. LAW OF REFLECTION

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reflection and refraction

Chapter 28

Chapter 28

Reflection and Refraction

1 reflection
1. REFLECTION

Most objects we see reflect light.

2 principle of least time
2. PRINCIPLE OF LEAST TIME

Fermat's principle:

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

3 law of reflection

5

4

1

2

3

3. LAW OF REFLECTION
  • Using Fermat's principle one can show the law of reflection.

A

B

Mirror

slide7
The law of reflection states that

the angle of incidence (qi) equals

the angle of reflection (qr).

  • This is true for specular reflection.

(Specular reflection is “mirror-like” reflection.)

slide8
Included in the law of reflection is the fact that the incident ray, the normal, and the reflected ray all lie in the same plane.

Normal

Reflected Ray

Incident Ray

qr

qi

Specular Reflecting Surface

slide9

Demo - Candle Burning Under Water

Video - Candle Burning Under Water

Video - Water Wave Reflections

Demo - Mirror Track Tracing – Take home exercise for PQ Points

slide10

Plane Mirror

Ray Diagramming

Plane Mirror

slide11

Plane Mirror

Ray Diagramming

Plane Mirror

plane mirrors
Plane Mirrors

Using ray diagramming one finds that the image is

1. Upright

2. Same size as the object

3. Virtual

diffuse reflection
Diffuse Reflection
  • Video - Diffuse Reflection
  • If successive elevations of a surface are no more than l/8 apart, then

the surface is said to be polished at that wavelength.

  • Most objects are seen by diffuse reflection.
slide14

Magnification of a surface

Diffuse Reflection

Colored lines are for the purpose of

distinguishing reflected rays from

incident rays.

slide15
Something that is barely polished for red light would not be polished for blue light.
  • Consider microwave satellite dishes.

They are polished for microwaves but not for visible.

  • The metal screen in a microwave oven serves what purpose?
reflection from curved surfaces concave shown here

f

Reflection from Curved Surfaces(Concave shown here)

Ray Diagramming

The law of specular reflection is still obeyed.

Principal axis

Reflecting Surface

slide17

f

Reflection from Curved Surfaces(Concave shown here)

Ray Diagramming

Principal axis

Reflecting Surface

The law of specular reflection is still obeyed.

slide18
Demo - Coin Mirage
  • Demo – Deep Concave Mirror
  • Demo - Make-up Mirror Images
  • Concave mirrors can produce real inverted images that are magnified, diminished, or the same size as the object.
  • Concave mirrors can produce virtual upright images that are magnified.
more on mirrors
More on Mirrors
  • Convex mirrors always produce a diminished, upright, virtual image.
  • Demo - Convex Mirror
  • Demo - Two Perpendicular Mirrors
  • Demo - Three Perpendicular Mirrors
  • Demo – Road Reflectors
  • Question - Why is it hard to see at night after or during a rain?
what type of mirror would you use to produce a magnified image of your face
What type of mirror would you use to produce a magnified image of your face?

(a) flat

(b) concave

(c) convex

(d) you could use a concave or a convex mirror

what is are the purpose s of the wire screen in the door of a microwave oven
What is (are) the purpose(s) of the wire screen in the door of a microwave oven?

(a) to absorb microwaves

(b) to allow you to see what's cooking

(c) to reflect microwaves

(d) all of the above

(e) only (b) and (c)

4 refraction
4. REFRACTION
  • Fermat's principle can also explain refraction (remember the beach).
  • Refraction of light is the bending of light as it passes obliquely from one medium to another.
  • It is due to the different speeds of light in the two different media.
index of refraction
Index of Refraction

Index of refraction of a material equals

the speed of light in a vacuum divided by

the speed of light in the material.

slide25
Demo – Green Laser Refraction in an Aquarium Demo - Twinkling with Laser

Slide - Twinkling Cartoon

slide26
Video - Water Wave Refractions
  • Video - "Broken" Pencil Refraction
slide27

Because of atmospheric refraction,

we have lingering, elliptical sunsets.

Sun

Earth

Sun

slide28

Mirage

Cool air

Warm air

Surface of water?

Highway Mirage

slide29

Looming

Warm air

Cool air

an oar partially immersed in water appears broken because of
An oar partially immersed in water appears "broken" because of

(a) refraction

(b) diffraction

(c) polarization

(d) interference

(e) absorption

5 cause of refraction
5. CAUSE OF REFRACTION
  • When light passes from one medium to another, its speed changes which in turn causes a bending of the light.
  • Examples:

Car running onto shoulder of road

Light passing from air into water

slide34

This bending produces illusions.

  • Example: Objects in water appear closer and nearer to the surface.

Eye

Air

Water

dispersion
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.
  • Demo - Aquarium Prism
dis pe rsio n i n a pr ism
Dispersion in aPrism

Slit

White Light

Source

Prism

slide42
Picture - Rainbow
  • Individual drops act as dispersers.
  • The 42o cone
  • Demo– Rainbow Model
  • A single eye can only see a small range of colors from a single raindrop.
slide43

SecondaryRainbow

Double rainbows are dimmer, higher, and have colors reversed. Link to picture.

green flash
Green Flash

Earth

Sun

Dispersion occurs causing

multiple images of the sun.

The last to set would be blue,

but most of the blue has been

scattered which leaves green.

6 total internal reflection
6. TOTAL INTERNAL REFLECTION
  • Video - Laser Under Water
  • Critical angle is the angle where total internal reflection (TIR) begins.
  • TIR is possible only when light is entering a medium of lesser index of refraction.
  • Binoculars make use of TIR.
slide46

Two Prisms in a Monocular

Flex Cam – Monocular

slide47

Demo - Laser and Light Pipe

  • Video - Woman at Edge of Pool
  • Flex Cam – Aquarium and Finger Dip in Water
  • Fiber optic devices make use of TIR.
a lingering sunset is
A lingering sunset is

(a) a looming effect

(b) caused by an elliptical (oval) sun

(c) due to atmospheric refraction

(d) caused by reflections from the upper atmosphere

when a beam of light emerges at a nonzero angle from water to air the beam
When a beam of light emerges at a nonzero angle from water to air, the beam

(a) bends away from the normal

(b) continues in the same direction

(c) bends toward the normal

(d) changes frequency

(e) slows down

7 lenses
7. LENSES
  • Lenses use refraction to form images.
  • Demo - Fresnel Lens
imaging with a convex lens

f

Principal Axis

An eye placed here

Imaging with a Convex Lens

and passes through a point

called the focal point.

is bent upon entering the lens.

Arrow as

Object

Upon exiting the lens it is bent again

sees an

image here.

A ray parallel to the principal axis

Convex Lens

A ray passing through the center of the lens is basically undeflected.

This arrangement produces an inverted, real, diminished image.

more imaging with a convex lens

f

Principal Axis

An eye placed here

More Imaging With a Convex Lens

is bent upon entering the lens.

Upon exiting the lens it is bent again

Arrow as

Object

and passes through a point

called the focal point.

A ray parallel to the principal axis

sees an

image here.

Convex Lens

A ray passing through the center of the lens is basically undeflected.

Farsighted people use lenses similar to these.

This arrangement produces an upright, virtual, magnified image. It is a simple magnifying glass.

imaging with a concave lens

f

Principal Axis

An eye placed here

Imaging with a Concave Lens

is bent upon entering the lens.

Arrow as

Object

Upon exiting the lens it is bent again

A ray parallel to the principal axis

such that is appears to have come

from a point called the focal point.

sees an

image here.

Concave Lens

A ray passing through the center of the lens is basically undeflected.

Nearsighted people use lenses similar to these.

This arrangement produces an upright, virtual, diminished image.

types of lenses
Types of Lenses

Plano

Convex

Plano

Concave

Double

Convex

Double

Concave

Convex Meniscus

Concave Meniscus

Convex lenses are positive converging lenses.

Concave lenses are negative diverging lenses.

Nearsighted people use lenses similar to these.

Farsighted people use lenses similar to these.

slide58

8. LENS DEFECTS

Spherical Aberration

Convex Lens

Cure

– Diaphragm

or lens combination

slide59

Chromatic Aberration

Convex Lens

Cure – Diaphragm or lens combinations

(achromatic lens)

Demo – Overhead Chromatic Aberration

Astigmatism – due to barrel-shaped cornea

eyeglasses
Eyeglasses
  • Picture - Farsightedness
  • Picture - Nearsightedness
slide63
One way to reduce the problem of spherical aberration in a positive lens is to join a negative lens to it to form a compound lens.

(a) True

(b) False

slide64

If you wish to take a picture of your image while standing 5 ft in front of a plane mirror, for what distance should you set your camera to provide the sharpest focus?

(a) 10 ft

(b) 5 ft

(c) 2.5 ft

(d) it can't be done