Chapter 26 Chapter 26 Properties of Light
Dec 18, 2015
Origin and Nature of LightOrigin and Nature of Light
Light originates with accelerated motion of
electrons.
It is an electromagnetic wave phenomenon.
Electromagnetic Wave VelocityElectromagnetic Wave Velocity
The speed of light (E&M waves) is called c.
c = 300,000 km/s = 186,000 mi/s
THE ELECTROMAGNETIC SPECTRUM
EM Waves SourcesRadio Waves Microwaves
InfraredVisible
UltravioletX-rays
Gamma rays
Vibrating charges Molecular rotations Molecular vibrations
Atomic vibrations Atomic vibrations Nuclear vibrations
Atomic vibrations
Properties of LightProperties of Light Speed (c)
Nothing travels faster Absolute constant
Wavelength () Gamma rays, X-rays, UV, Visible, IR, Radio In visible wavelength means color
Frequency (f) Number of complete waves per second
Energy of a Photon (E) E = hf
2. TRANSPARENT MATERIALS 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.
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 MediumLight Speed in a Medium
There is an apparent slow down in the speed of light.
Besides the UV, heat waves (IR) don't get through either.
TRANSLUCENT MATERIALS
Frosted glasses are good examples of translucent materials.
They scatter light on transmission. You can’t read or identify objects from light
passing through translucent materials.
3. OPAQUE MATERIALS
Most things around us are opaque. Metals are shiny and reflect.
– Because of free electrons - they don’t pass on energy to atoms.
Our atmosphere is transparent in visible and part of IR.
It's good that UV is absorbed in our atmosphere. UV causes sunburns.
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.
5. SEEING LIGHT - THE EYE
Cornea - does most of the focusing
Iris -
Pupil -
has the eye color and controls light intensity
Lens -
the hole in the eye
does remainder of focusing
Retina - location of light sensors, has rods and cones
Blind spot -
Fovea - center of vision, predominantly cones
optic nerve exit, no light sensors
Myopia (Near-Sightedness)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.
Demo - Blind Spot (in text)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 LightsDemo - 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 InhibitionLateral 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.
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
(b) you stand in the umbra of the moon’s shadow
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
(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
(d) is very fast, but not infinite