I am Watching YOU!!
Light
The Electromagnetic Spectrum
Increasing Energy
Energy of Light
10−6 eV
10−4 eV
1 − 2eV
40eV
KeV
MeV
Energy to ionize atom or molecule: 10-1000eV
Where does light actually come from?
Light comes from the
acceleration of
charges.
Accelerating Charges
Oscillating Charges
Radio & Microwave
Frequency of EM wave is the same as the frequency of oscillation.
How is Infrared Light made?
Light comes from the
acceleration of
charges.
Thermal Excitation:
Incandescence
f ~T
Color shifts to shorter wavelengths (higher
frequency) as an object is heated.
increasing temperature
How is Visible Light made?
Light comes from the
acceleration of
charges.
Light is emitted when an electron in an
atom jumps between energy levels
either by excitation or collisions.
Atoms are EM Tuning Forks
They are ‘tuned’ to particular
frequencies of light energy.
Atomic Emission of Light
Each chemical element produces its own
unique set of spectral lines when it burns
Hydrogen Spectra
Light Emission
Incandescent Light Bulb
Full Spectrum of Light
All frequencies excited!
How is UV made?
X-Ray Production
By High Voltage Discharge
X-Ray Production
By High Voltage Discharge
Accelerating Charges
Synchrotron Radiation
X-Ray and Radio
How can we SEE Black Holes?
We see the X Rays produced by matter falling into them.
X Ray Imaging
when X-ray light shines on us, it goes through our skin, but allows shadows of our bones to be projected onto and captured
by film.
When X-ray light shines on us, it goes through our skin, but allows
shadows of our bones to be projected onto and captured by film.
Gamma Ray Production
by Nuclear Decay
when X-ray light shines on us, it goes through our skin, but allows shadows of our bones to be projected onto and captured
by film.
239
P→
U + He + γ
235
46
Gamma Ray Production
Matter-antimatter annihilation
Gamma Ray Burst
Ionizing Radiation:
UV, Xray & Gamma
Energy to ionize atom or molecule: 10-1000eV
Ionizing Radiation:
UV, Xray & Gamma
Light can Kill
Biological Effects of Ionizing
Radiation
Atomic Thermal Blast
IR, VISIBLE, UV
EM Radiation Hazards:
Are Low Frequencies Dangerous?
Sunlight Gives Life!
The Greenhouse Effect
Cosmic EM Radiation
The Atmosphere
Visible Sun: The Photosphere
Atomic Excitations: 400-700 nm at a few thousand Kelvin.
Radio Image of the Sun
Synchrotron Radiation: 10.7 cm – 2800MHz
Infrared Sun
More than half the Sun’s power is radiated in IR.
UV Sun
X-Ray Sun
Bremsstrahlung Radiation (braking, in German)
Gamma Ray Sun
Bremsstrahlung Radiation (braking, in German)
Multiwavelength Solar Flare
Atoms are EM Wave
Tuning Forks
Atoms are Optical Tuning Forks
Light slows down as it travels through
glass because it takes time to be
absorbed and re-emitted.
Transparency
Glass resonates strongly with UV and
infrared, absorbing those frequencies
while transmitting visible frequencies.
Why are most materials Opaque?
(Opaque – Can’t see through)
They absorb light without re-emitting it. Vibrations given
by the light to their atoms and molecules are turned into
random kinetic energy – they become slightly warmer.
Opacity: Mirrors
Free electrons in opaque reflective surfaces
can vibrate, absorb & re-emit at any frequency.
If you pass white light through a prism,
it separates into its component colors.
long wavelengths
short wavelengths
R.O.Y. G. B.I.V
spectrum
ROYGBIV
Why are some materials colored?
Atoms are EM Wave
Tuning Forks
Why are some materials colored?
Colored materials absorb certain colors that
resonate with their electron energy levels and
reject & reflect those that do not.
Additive Primary Colors
Red, Green, Blue
Mixing Colored Light
RGB Color Theory
Radiation of Visible Sunlight
RGB Color Theory
Additive Complementary Colors
Yellow, Cyan, Magenta
The color you have to add to get
white light.
Red + Green = Yellow
Blue + Green = Cyan
Red + Blue = Magenta
Red + Blue + Green = White
White light – red light = ??
White light – yellow light = ??
Look at me with your filters!
What do you see?
Mixing Colored Pigments
Subtractive Colors
Pigments subtract colors from white light.
Mixing Colored Pigments
Subtractive Colors
Pigments subtract colors from white light.
Yellow + Cyan = Green
Cyan + Magenta = Purple
Yellow + Magenta = Red
Yellow + Cyan + Magenta = Black
Subtractive Primary Colors
Yellow, Cyan, Magenta
Printing with Pigments
Yellow + Cyan = Green
Cyan + Magenta = Purple
Yellow + Magenta = Red
Yellow + Cyan + Magenta = Black
Why is the Sky Blue?
Why is the Sky Blue?
Air Molecules absorb and re-radiate blue in all directions
This is called scattering.
So everywhere you look in the sky you see blue.
Why are Clouds White?
A large collection of atoms and molecules vibrate in all frequencies.
All the frequencies (colors) added together make white light.
Why are Sunsets Red?
Why are Sunsets Red?
By the time the light reaches your eye, all the blue
and most of the green have been scattered out.
White light minus green and blue is red light.
Why is the Ocean Cyan?
White light minus cyan is red. Ocean water absorbs red.
Light Rays
We can consider light waves to be
RAYS because if the object it interacts
with are several times larger than the
wavelength of light, it acts like it travels
in straight lines and acts like a ray.
Principle of Least Time
Why does a mirror reflect light?
Mirrors are metallic (conductors) and have free electrons.
These free electrons can absorb and re-emit a continuous
spectrum of EM frequencies and so more of the incident light
is reflected back and not absorbed by the material.
Why does polishing
a mirror make it
more reflective?
Law of Reflection
Mirror Object & Virtual Image
Virtual Image: An image that cannot be projected onto a surface.
A virtual image only appears like light rays came from the
location of the image, they are not really there.
Mirrors make Virtual Images.
Mirror Object & Virtual Image
If the candle is ½ a meter in front of the mirror,
how far behind the mirror is the image located?
What is the minimum length of a
plane mirror in which you can
see a full view of yourself?
Mirror Reflection
Convex & Concave
Which is convex? A or B?
Refraction:
Bending Light into Focus
Refraction: Bending of Light
Transmitted through Materials
Light Bends because it Slows Down.
Atoms are Optical Tuning Forks
If the frequency of incident light resonates with the electron energy
levels in the atom, then the atoms absorb and re-emit the light, and
that takes time.
Light refracts toward the normal
when it slows down (air to water)
and refract away from the normal
when it speeds up (water to air)
Apparent Depth
25°
Total Internal Reflection
How are Rainbows Formed?
Different Colors bend different amounts!
If you pass white light through a prism,
it separates into its component colors.
long wavelengths
short wavelengths
R.O.Y. G. B.I.V
How much it bends depends on
WAVELENGTH.
long wavelengths
short wavelengths
R.O.Y. G. B.I.V
Atoms are Optical Tuning Forks
The glass is more tuned to higher frequencies and so the Blue takes
more time to be absorbed and re-emitted. It takes longer to interact
so it travels slower through the glass. Therefore it bends more.
Dispersion
Shorter wavelengths bend more through the glass.
The speed and wavelength change but
the FREQUENCY does NOT.
Fr
Frequency depends on the oscillating source!
long wavelengths
short wavelengths
R.O.Y. G. B.I.V
Rainbows
Dispersion & Internal Reflection
Lenses
Bringing Light into Focus
Converging Lens
Diverging Lens
Focal Point
Every lens has a focal point – the distance away from the
lens that rays come into focus. The focal point depends
on the shape and thickness of the lens.
Real Images
Virtual Images
Lenses
Bringing Light into Focus
Converging Lens
Diverging Lens
Pin Hole Camera
Camera Lens
Why aren’t images produced on the wall without a lens or hole?
Human Vision
Human Retina
Sharp Spot: Fovea
Blind Spot: Optic Nerve
Optical Antennae: Rods & Cones
Rods: Intensity
Cones: Color
Accomodation
Vision Defects
Nearsighted
Far-Sighted
Another Way to Bend Waves
Diffraction depends on SLIT WIDTH: the smaller the width,
relative to wavelength, the more bending and diffraction.
Double Slit is VERY IMPORTANT because it is evidence
of waves. Only waves interfere like this.
Double Slit
Double Slit for Electrons
shows Wave Interference
Dispersion: Diffraction Gratings
Waves can be bent by diffraction.
Light can be dispersed by diffraction.
The greater the wavelength, the greater the angle.
How does this compare to dispersion with a prism?
Dispersion: Diffraction Gratings
How does this compare to dispersion with a prism?
Longer wavelength light is bent more with a grating.
Shorter wavelength light is bent more with a prism.
Thin Film Interference
Thin Films
When reflecting off of different surfaces and traveling through
different thickness, different frequencies take different times to
travel and can undergoes a phase shift in wavelength. When the
waves rejoin, they interfere destructively canceling out different
frequencies of light.
Polarization
The EM fields are aligned in specific directions.
Polarization of Light
Plane Polarized
Circular Polarized
Polarization upon
Reflection
Circularly Polarized EM Wave
Elliptically Polarized EM Wave
Two crossed polarizers cut all light.
But add a third polarizer in between and
Light shines through! How is this so?
Light is in a Superposition of
Polarization States!
This is a Quantum Effect.
Three Polarizers Paradox
You
You have
have aa field
field with
with
cows.
cows. To
To make
make sure
sure
that
that now
now cows
cows get
get out,
out,
you
you put
put up
up two
two fences.
fences.
They
They stay
stay in
in their
their field.
field.
But
But you're
you're really
really
paranoid,
paranoid, so
so you
you put
put aa
third
third fence
fence in
in between
between
the
the two.
two. Now,
Now, all
all of
of aa
sudden,
sudden, one
one fourth
fourth of
of
your
your cows
cows are
are
wandering
wandering in
in your
your
neighbor's
neighbor's field.
field.
Where are the black dots?
Are the bricks crooked?
Keep staring at the black dot in the center. After a
while the gray haze around it will appear to shrink.
Which arrow is longer?
Which inner box is larger?