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
1
B
2
5
4
3
Mirror
• 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.)
Normal
Reflected Ray
Incident Ray
qi
qr
Specular Reflecting Surface
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.
Demo - Candle Burning Under Water
Video - Candle Burning Under Water
Video - Water Wave Reflections
Demo - Mirror Track Tracing – Take home
exercise for PQ Points
Plane Mirror
Ray Diagramming
Plane Mirror
Plane Mirror
Ray Diagramming
Plane 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 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.
Magnification of a surface
Diffuse Reflection
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)
Ray Diagramming
f
Principal axis
Reflecting Surface
The law of specular reflection is still obeyed.
Reflection from Curved Surfaces
(Concave shown here)
Ray Diagramming
f
Principal axis
Reflecting Surface
The law of specular reflection is still obeyed.
• 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
• 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?
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.
c
n
v
Demo – Green Laser Refraction in an Aquarium
Demo - Twinkling with Laser
Slide - Twinkling Cartoon
• Video - Water Wave Refractions
• Video - "Broken" Pencil Refraction
Because of atmospheric refraction,
we have lingering, elliptical sunsets.
Sun
Sun
Earth
Mirage
Cool air
Warm air
Surface of water?
Highway Mirage
Looming
Warm air
Cool 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
Wave fronts of light
qi
Air
Water
qR
• This bending produces illusions.
• Example: Objects in water appear closer
and nearer to the surface.
Eye
Air
Water
Demo – Bow Fishing
Fish can see everything above the surface of
water within a 960 cone.
960
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
Dispersion in a Prism
Slit
White Light
Source
Prism
Rainbow
•
•
•
•
•
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.
Secondary Rainbow
Double rainbows are dimmer, higher, and
have colors reversed. Link to picture.
Green Flash
Sun
Earth
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
• 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.
Two Prisms in a Monocular
Flex Cam – Monocular
• Fiber optic devices make use of TIR.
• Demo - Laser and Light Pipe
• Video - Woman at Edge of Pool
• Flex 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
A Convex Lens Converges Light Rays
f
Imaging with a Convex Lens
is bent upon entering the lens. and passes through a point
called the focal point.
Arrow as
Upon exiting the lens it is bent again
Object A ray parallel to the principal axis
sees an
image here.
f
Principal Axis
An eye placed here
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
is bent upon entering the lens.
Upon exiting the lens it is bent again
Arrow as
and passes through a point
A ray parallel to
the principal axis
Object
called the focal point.
f
sees an
image here.
Principal Axis
Convex Lens
An eye placed here
A ray passing through the center of the lens is basically undeflected.
Farsighted
people
usemagnified
lenses similar
to these.
This arrangement produces
an upright,
virtual,
image.
It is a simple magnifying glass.
A Concave Lens Diverges Light Rays
f
Imaging with a Concave Lens
is bent upon entering the lens.
Arrow as
Object A ray parallel to the principal axis
Upon exiting the lens it is bent again
such that is appears to have come
from a point called the focal point.
f
Principal Axis
sees an
image here.
An eye placed here
Concave Lens
A ray passing through the center of the lens is basically undeflected.
Nearsighted
peoplean
use
lenses virtual,
similar to
these. image.
This arrangement
produces
upright,
diminished
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
Farsighted people use lenses similar to these.
8. LENS DEFECTS
Spherical Aberration
Convex Lens
Cure – Diaphragm or lens combination
Chromatic Aberration
Convex Lens
Cure – Diaphragm or lens combinations
(achromatic lens)
Demo – Overhead Chromatic Aberration
Astigmatism – due to barrel-shaped cornea
Eyeglasses
• Picture - Farsightedness
• Picture - 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