Physics 1230: Light and Color
Exam 1 is tomorrow, Wed. June 9, in class.
Covers material from Chapter 1, pgs 1-25,
Lectures and Homework 1-3.
HW4 will be up soon. Due Thursday, 5PM
Lecture 5:
Shadows, eclipses. Exam 1 Review.
Reading: Chapter 1 for review. Chapter 2,
pgs. 29-68
1
Which level of physics is needed to
explain what properties of light?
• Image formation – ray theory
• Wavelength color,
polarization and
diffraction – wave theory (electricity and
magnetism)
• Interaction of light with
atoms -
• Constant speed of light
no matter how fast the
source or observer is
moving – special theory of relativity
We have to stay focused
to make progress!
– quantum theory of photons
RAY THEORY for now.
Chapter 2 – Geometrical Optics
Geometrical optics is the theory of RAYS (straight lines)
and how they reflect and refract (bend). Lots of similarity
to GEOMETRY of lines and triangles.
Main Topics
We
are
here
1.
2.
3.
4.
Shadows
Reflection
Refraction
Dispersion
3
1. Shadows
a.
b.
c.
d.
Point source or diffuse source?
Umbra and penumbra
Solar and lunar eclipses
Pinhole cameras
4
A point light source emits rays in all
directions radially outwards
The rays from two
point light sources
look like this
The rays only tell us which direction
the light goes in. We know that the
light gets dimmer as you move further
away from the light source. (Think of
the sun. It would be blinding if we
were closer to the sun)
Shadows appear when rays are blocked
Wall
Wall
Rays that are NOT
blocked by the book
Rays that ARE blocked
by the book
Point light
source
Book
unblocked
A
B
2 point light
sources
blocked
umbra
Book
unblocked
If we move the wall back from the
book, the shadow gets…
A) Smaller B) Bigger
C) no change
The two parts of the penumbra each get light
from only one of the two bulbs. The umbra
gets no light from either of the two bulbs. The
bright region gets light from both of the bulbs.
Concept question
Shadows tell us:
A) What direction the
light is shining from
B) That something is
blocking the light
C) That light travels in
straight lines
D) A, B, & C
E) A & C
We can extend the definition of the umbra and penumbra to
exist in space even without a wall or screen!
Wall
The light from B doesn't reach this penumbra
A
B
Book
umbra
The light from A doesn't reach this penumbra
We can think about large light sources as being composed of many small light sources
An "extended object" consists of many points. Each
point on the object emits or reflects rays in all
directions (unless the object is a mirror)
MANY reflected rays come
from each point on Alex.
This is diffuse reflection
Incident rays from a
frosted light bulb
The more rays that reach a
point the brighter the point
• This is why regions outside
the penumbra and umbra
are brighter
– These regions get light rays
from both point light sources
• The more lights you turn on
the brighter the reflected
light from objects in the
room
Light
source 2
Light
source 1
Reflected rays
from light 1
Reflected rays
from light 2
– See rays at right
Your eye sees
a brighter nose
than with either
light source alone
An extended light source such as the sun (or a large
light bulb) also produces an umbra and penumbra in
empty space behind the Earth (or another object)
• All rays coming from point A on the sun
between the two dashed rays are blocked by
Earth
• All rays coming from point B on the sun
between the two dotted rays are blocked by
Earth
Rays from this part of the sun
DO reach the upper penumbra
A
• The umbra gets no light from any
portion of the sun
• The umbra gets smaller not larger further behind
Earth since the Sun is larger than Earth
• The penumbra gets light from part of
Sun
Penumbra
Umbra
Penumbra
B
the sun
– If you look back from the penumbra you can see part
of the sun
• When the moon passes completely into the
umbra there is a total eclipse of the moon.
– When the moon passes into the penumbra there is a
partial eclipse of the moon
Rays from this part of the sun
DON'T reach the upper penumbra
because they are blocked by Earth
Solar eclipse Geometry:
moon
NOTE: The umbra is usually about 200km wide
http://www.mreclipse.com/Special/SEprimer.html
Total solar eclipse
During a solar eclipse, the shadow of the Moon
passes over the surface of the Earth. From the
Earth, we can see the moon blocking the light of
the Sun. Looking at the demonstration, you may
think that solar eclipses happen very often. The
Sun, Earth, and Moon must be lined up just
right, in order for a solar eclipse to take place.
This happens only two to five times a year.
Since the Moon's shadow is so small, compared
to the size of the Earth, a solar eclipse can be
seen from only small portions of the Earth.
http://micro.magnet.fsu.edu/primer/java/scienceopticsu/solar/index.html
Map of the total solar eclipse, Aug 1, 2008
http://en.wikipedia.org/wiki/Image:Solar_eclipse_animate_(2008-Aug-01).gif
Schedule:
http://antwrp.gsfc.nasa.gov/apod/image/0311/112003lunareclipse_koehn.gif
Lunar eclipse (partial and total)
During a lunar eclipse, the moon passes
through the shadow of the Earth. As we
look at the moon from the Earth, it looks
to us as if the shadow of the Earth is
slowly covering the moon. You may think
that lunar eclipses happen very often.
However, the Sun, Earth, and Moon must
be lined up just right, in order for a lunar
eclipse to take place. This happens very
rarely. In most years there are only two
lunar eclipses that can be seen only from
certain places on Earth. In a partial lunar
eclipse, the moon passes through the
penumbra or part of the umbra. In a total
lunar eclipse, the moon is completely
within the umbra.
Based on what we know about eclipses, how do we prove this?
A pinhole camera works by blocking rays
(demo)
• What is an image?
•
A real image is formed on a screen when one or
more rays from each point on the object reach the
corresponding points on the screen and no other
rays from other points on the object reach those
points
blocked rays
Pinhole Camera
Image of
light bulb
Light bulb
• Notice that this image is upside down and
left-right reversed.
Using shadows…
The object photographed with a pinhole
camera does not have to be self-luminous!
One of many rays of light shining on Alex
Pinhole Camera
blocked rays
Alex
• Once again this image is upside down and leftright reversed. Early photographs
(daguerreotypes) were always left-right reversed;
• Note the correspondence between the distances
object-camera-screen and image vs. object sizes
Reflected rays
off the real Alex
go through the hole
and make the image
Image of
Alex
Finding an image by using rays is called ray tracing.
Trace rays from the object through the pinhole in the
camera to find the image rather than trusting your intuition!
Is the image of Alex smaller or
larger than the real Alex?
Is the image of Alex smaller or
larger than the real Alex?
a)Smaller
b)Larger
c)Same size
a)Smaller
b)Larger
c)Same size
Extra Credit Project (20 points):
Construct and use a camera on your own
(see textbook for details, pages 35 & 36)
Review (Chapter 1)
• Light is an electromagnetic wave
• EM waves have both traveling electric and
magnetic fields
• EM waves are created by accelerating charge.
• EM waves can make other charges move.
• EM waves travel outward like waves in a pond,
with electric and magnetic vectors perpendicular
to the direction of motion.
• EM waves carry energy (and momentum).
25
Review (Chapter 1 too)
• c = 300,000 km/s =3 x 108 m/s
• c = fl, units are Hz for f and meters for l
• wavelength of light spans 400 – 700 nm
(blue to red)
• Atmosphere transmits light and radio,
is opaque to UV, x-rays, some IR
• Amplitude of a wave is the half-height
• Wavelength is distance from crest to crest
• Period is the time for the wave to go up and
down once at some location.
26
Review
27
Review
The rotating mirror rotates 530 revolutions per
second. Therefore, the frequency of revolution is:
A) 530 seconds B) 84 Hz C) 530 Hz
D) None of these.
28
Review
The rotating mirror rotates 530 revolutions per second.
Therefore, the period for a single revolution is:
A) 1.9 seconds B) 530 seconds C) 1.9 milliseconds
D) 1.9 nanoseconds E) Something else
29
Review
The rotating mirror rotates with a period of 1.9 msec.
Therefore, the time needed to rotate from one flat
mirror to the next flat mirror is:
A) 1.9 msec B) 238 msec C) 1.9 ksec
D) 238 nsec E) Something else
30
Review
The rotating mirror requires 238 msec to rotate from
one flat mirror to the next. During that time, the
reflected light ray travels out 22 miles are returns. The
speed of light in miles per second is:
A) 92,500 miles/sec B) 185,000 miles/sec
C) 300,000 miles/sec E) None of these
31
That’s IT!
Good Luck on
Exam 1
32