Transcript Light

Chapter 26
Properties of Light
Origin and Nature of Light
Light originates with
accelerated motion of
electrons.
It is an electromagnetic wave
phenomenon.
1. ELECTROMAGNETIC
WAVES
l
James Clerk Maxwell
Electromagnetic Wave Velocity
l
The speed of light (E&M waves) is called c.
l
c = 300,000 km/s = 186,000 mi/s
The Speed of Light
l
Galileo’s Experiment
A
B
c = Round Trip Distance/Total Time
Ole Roemer (1675)
Jupiter
Earth
THE ELECTROMAGNETIC
SPECTRUM
Visible Light
Infrared
Visible Light
Ultraviolet
X-rays
Gamma Rays
Radio Light
Microwaves
Previous Slide
The Visible Spectrum
400nm
500nm
600nm
700nm
Wavelength means COLOR
THE ELECTROMAGNETIC
SPECTRUM
EM Waves
Radio Waves
Microwaves
Infrared
Visible
Ultraviolet
X-rays
Gamma rays
Sources
Vibrating charges
Molecular rotations
Molecular vibrations
Atomic vibrations
Atomic vibrations
Atomic vibrations
Nuclear vibrations
Gamma Rays
Compton Gamma
Ray Observatory
X-rays
Chandra X-Ray
Observatory
Ultraviolet
Ultraviolet Imaging Telescope
Messier 74
Infrared
Radio
Very Large Array
Cygnus A
Speed, Wavelength,
Frequency
c=f
Properties of Light
l
l
Speed (c)
l
Nothing travels faster
l
Absolute constant
Wavelength ()
l
l
l
Frequency (f)
l
l
Gamma rays, X-rays, UV, Visible, IR, Radio
In visible wavelength means color
Number of complete waves per second
Energy of a Photon (E)
l
E = hf
2. TRANSPARENT
MATERIALS
l
l
l
For glass the natural frequency is
in the UV.
Energy of EM wave in UV stays
with atoms longer causing
vibrations with neighboring atoms
with loss of energy.
Thus UV is absorbed.
l
l
l
For visible light the electrons in the glass
are forced into vibration, but at smaller
amplitude.
The atom holds the energy for less time,
with less chance of collision with
neighboring atoms, and less energy
transformed to heat.
All visible gets through clear glass.
Light Speed in a Medium
There is an apparent slow down in the speed
of light.
l
l
Besides the UV, heat waves (IR) don't
get through either.
TRANSLUCENT MATERIALS
l
Frosted glasses are good examples of
translucent materials.
l
They scatter light on transmission.
l
You can’t read or identify objects from light
passing through translucent materials.
3. OPAQUE MATERIALS
l
l
Most things around us are opaque.
Metals are shiny and reflect.
– Because of free electrons - they don’t pass on
energy to atoms.
l
l
Our atmosphere is transparent in visible and
part of IR.
It's good that UV is absorbed in our
atmosphere. UV causes sunburns.
Demo - Radiometer and Filters
4. SHADOWS
When light is blocked, shadows are created.
The kind of shadow depends on the source
of light and the relative positions of the
light source, object, and shadow of the
object.
For instance, a total shadow (umbra) occurs
when an object is illuminated by a point
light source.
Extended light sources cause shadows of
objects to be partial shadows (penumbras).
The shadows may or may not have umbras.
A penumbra will be small if the light source
is far from the object and its shadow.
Sun
Umbra
Full Shadow
Earth
Moon
Partial Shadow
Solar Eclipse
Penumbra
Annular Eclipse
Lunar Eclipse
Sun
Earth
Moon
Earth’s Shadow
The Eye
5. SEEING LIGHT - THE EYE
Cornea - does most of the focusing
Iris - has the eye color and controls light intensity
Pupil - the hole in the eye
Lens - does remainder of focusing
Retina - location of light sensors, has rods and cones
Fovea - center of vision, predominantly cones
Blind spot - optic nerve exit, no light sensors
Color Deficiency
Myopia (Near-Sightedness)
People with near-sightedness, also known as myopia,
cannot see clearly at distance. Myopia occurs when
the eyeball is too long or when the cornea (the clear
part on the front of the eye) is too steep.
Hyperopia (Farsightedness)
or presbyopia
Demo - Blind Spot (in text)
Primates and a species of ground squirrel
are the only mammals that experience
full color vision.
At the periphery of our vision we see
– Motions first
– Colors second
Rods are more sensitive to low light levels
than cones are.
Examples: stars and twilight
Demo - Turn off Room Lights
Rods see blue better. Blue will appear
much brighter than red in dim light, though
the red might be much brighter than the
blue in bright light.
Lateral Inhibition
We don't perceive the actual differences in
brightness in fields of light and dark.
We even out our visual field.
This allows us to discern detail in very bright
areas and in dark areas at the same time.
We accentuate differences rather than
similarities.
Measured Brightness
of Last Slide
Optical Illusions
Show Pinwheel
Chapter 26 Review Questions
You will observe a total eclipse
of the sun when
(a) you stand in the penumbra of the moon’s shadow
(b) you stand in the umbra of the moon’s shadow
(c) sunlight diffracts around the moon
(d) sunlight reflects from the moon to the earth
Which of the following will cast
a shadow that has an umbra but
no penumbra?
(a) the sun
(b) an incandescent lamp
(c) a fluorescent lamp
(d) a point source of light
The speed of light
(a) has never been measured
(b) is about the same as that of sound
(c) is infinitely fast
(d) is very fast, but not infinite
What is the purpose of the pupil
in an eye?
(a) to adjust the focal length of the lens
(b) to adjust the amount of light passing
through the lens
(c) to focus the image
(d) to change the width of the field of view