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.

3. LAW OF REFLECTION

• Using Fermat's principle one can show the

law of reflection.

A B

1 2 345

Mirror

• The law of reflection states that

the angle of incidence (i) equals

the angle of reflection (r).

• This is true for specular reflection.

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

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

Specular Reflecting Surface

Incident Ray Reflected Ray

i r

Demo - Candle Burning Under WaterDemo - Candle Burning Under Water

Video - Candle Burning Under WaterVideo - Candle Burning Under Water

Video - Water Wave ReflectionsVideo - Water Wave Reflections

Demo - Mirror Track Tracing – Take home Demo - Mirror Track Tracing – Take home

exercise for PQ Pointsexercise for PQ Points

Plane Mirror

Ray DiagrammingPlane Mirror

Plane Mirror

Ray DiagrammingPlane Mirror

Plane Mirrors

Using ray diagramming one finds

that the image is

1. Upright

2. Same size as the object

3. Virtual

Diffuse Reflection

• Video - Diffuse ReflectionVideo - Diffuse Reflection

• If successive elevations of a surface are

no more than /8 apart, then

the surface is said to be polished at that

wavelength.

• Most objects are seen by diffuse

reflection.

Diffuse Reflection

Magnification of a surface

Colored lines are for the purpose of distinguishing reflected rays from

incident rays.

• 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)

The law of specular reflection is still obeyed.

Principal axis

Reflecting Surface

Ray Diagramming

f

Reflection from Curved Surfaces(Concave shown here)

The law of specular reflection is still obeyed.

Principal axis

Reflecting Surface

Ray Diagramming

f

• Demo - Coin MirageDemo - Coin Mirage

• Demo – Deep Concave Mirror Demo – Deep Concave Mirror

• Demo - Make-up Mirror ImagesDemo - 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

• Convex mirrors always produce a diminished, upright, virtual

image.

• Demo - Convex Mirror Demo - Convex Mirror

• Demo - Two Perpendicular MirrorsDemo - Two Perpendicular Mirrors

• Demo - Three Perpendicular MirrorsDemo - Three Perpendicular Mirrors

• Demo – Road ReflectorsDemo – Road Reflectors

• Question - Why is it hard to see at night Question - Why is it hard to see at night after after

or during a rain?or during a rain?

Chapter 28 Review Questions

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?

(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

• 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 of a material equals

the speed of light in a vacuum divided by

the speed of light in the material.

v

cn

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

Slide - Twinkling Cartoon Slide - Twinkling Cartoon

• Video - Water Wave RefractionsVideo - Water Wave Refractions

• Video - "Broken" Pencil Refraction Video - "Broken" Pencil Refraction

Earth

Sun

Sun

Because of atmospheric refraction, we have lingering, elliptical sunsets.

Mirage

Warm air

Cool air

Surface of water?

Highway Mirage

Cool air

Looming

Warm air

Chapter 28 Review Question

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

(a) refraction

(b) diffraction

(c) polarization

(d) interference

(e) absorption

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

Air

Water

i

R

Wave fronts of light

• This bending produces illusions.

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

Air

Water

Eye

Demo – Bow FishingDemo – Bow Fishing

Fish can see everything above the surface of

water within a 960 cone.

960

Demo - Disappearing Pyrex Demo - Disappearing Pyrex

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 Demo - Aquarium Prism

Prism

Slit

White Light Source

Dispersion in a Prism

Rainbow

• Picture - RainbowPicture - Rainbow

• Individual drops act as dispersers.

• The 42o cone

• DemoDemo – Rainbow Model– Rainbow Model

• A single eye can only see a small range of colors from a single raindrop.

Secondary Rainbow

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

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 beenscattered which leaves green.

6. TOTAL INTERNAL REFLECTION

• Video - Laser Under WaterVideo - 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.

Flex Cam – Monocular Flex Cam – Monocular

Two Prisms in a Monocular

• Fiber optic devices make use of TIR.

• Demo - Laser and Light PipeDemo - Laser and Light Pipe

• Video - Woman at Edge of PoolVideo - Woman at Edge of Pool

• Flex Cam – Aquarium and Finger Dip in WaterFlex Cam – Aquarium and Finger Dip in Water

Chapter 28 Review Questions

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

(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

• Lenses use refraction to form

images.

• Demo - Fresnel Lens Demo - Fresnel Lens

A Convex Lens Converges Light Rays

f

Imaging with a Convex Lens

sees animage here.

Convex Lens

f

Principal Axis

Arrow asObject A ray parallel to the principal axis

is bent upon entering the lens.

Upon exiting the lens it is bent again

and passes through a point called the focal point.

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

An eye placed here

This arrangement produces an inverted, real, diminished image.

More Imaging With a Convex Lens

sees animage here.

Convex Lens

Principal Axis

Arrow asObjectA ray parallel to the principal axis

is bent upon entering the lens.Upon exiting the lens it is bent again

and passes through a point called the focal point.

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

An eye placed here

This arrangement produces an upright, virtual, magnified image. It is a simple magnifying glass.Farsighted people use lenses similar to these.

f

A Concave Lens Diverges Light Rays

f

f

Imaging with a Concave Lens

sees animage here.

Concave Lens

Principal Axis

Arrow asObject A ray parallel to the principal axis such that is appears to have come

from a point called the focal point.

is bent upon entering the lens.

Upon exiting the lens it is bent again

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

An eye placed here

This arrangement produces an upright, virtual, diminished image.Nearsighted people use lenses similar to these.

Convex lenses are positive converging lenses.Concave lenses are negative diverging lenses.

Farsighted people use lenses similar to these.Nearsighted people use lenses similar to these.

Types of Lenses

DoubleConvex

DoubleConcave

Concave Meniscus

PlanoConvex

PlanoConcave

Convex Meniscus

Convex Lens

Spherical Aberration

8. LENS DEFECTS

or lens combination Cure – Diaphragm

Convex Lens

Chromatic Aberration

Cure – Diaphragm or lens combinations(achromatic lens)

Demo – Overhead Chromatic AberrationDemo – Overhead Chromatic AberrationAstigmatism – due to barrel-shaped cornea

Eyeglasses

• Picture - FarsightednessPicture - Farsightedness

• Picture - NearsightednessPicture - Nearsightedness

Chapter 28 Review Questions

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

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