Download Properties and Sources of Light

Chapter 16
The Nature of Light
 Travels straight and fast
 Reflects and Refracts at boundaries (and is also
absorbed
 Has color and intensity
 Behaves as BOTH a wave AND a particle (photon)
**As such, light can carry information**
Wave and Particles
 The wave nature of light is needed to explain various
phenomena
 Interference
 Diffraction
 Polarization
 The particle nature of light was the basis for ray
(geometric) optics
Electromagnetic
Waveforms


 The
E and B fields are
perpendicular to each other
 Both fields are
perpendicular to the
direction of motion
 Therefore,
electromagnetic waves
are transverse waves
 With all periodic waves
v  f
 Since v = c in a vacuum
c  f
[11.1]
Electromagnetic Waves, Summary
 A static electric charge produces an electric field.
 A uniformly changing (moving) electric field produces
an magnetic field
 A uniformly changing (moving) magnetic field
produces a electric field
**But NONE of these produces an EM WAVE.
For this you need an accelerating charge.**
Velocity of Light
c = 3 x 108m/s (In a vacuum)
Slower values in other mediums,
even air slows down light, but
frequency will stay the same
Sources of Light
Electric light –
• Incandescence
Electricity  Heat  Light
• Fluorescence
Electricity  UV  Visible Light
Intensity of Light (Brightness)
Defined as the power of light hitting a
surface area in W/m2.
• Since light propagates in a spherical
fashion, this is related by the inverse
square of the distance between the source
and the observer.
•
**JUST LIKE GRAVITY**
Intensity of Light (Brightness)
Intensity of Light (Brightness)
•
Intensity at Earth’s surface --
 500W/m2
• Intensity at Sun’s surface (given off –
 1360W/m2
Visible Light
Visible light consists of a range of wavelengths (400 –
700nm), spanning violet to red in color. When all
wavelengths are present, white light is observed.
Visible Light and Energy
Lower Frequency  Longer Wavelength  Lower
Energy  Redder Light
Higher Frequency  Shorter Wavelength  Higher
Energy  Bluer Light
E = hf
Visible Light and Energy
When materials gain heat energy, their atoms become
more active/excited and give off light. This light
contains all wavelengths but has a “peak” wavelength
which depends upon the temperature.
 Cooler = Redder
 Hotter = Bluer
E = sT4
Stefan-Boltzmann Law
max
3,000,000 nm

TK
Wien’s Law
Light at Boundaries
Will be both reflected and refracted
(But more on this later….)
Human Eye
Human Eye
 Eye is almost spherical (24 mm x 22 mm)
 Flexible shell – the sclera
 Most of the bending of the rays entering the eye take
place at the air-cornea interface (nc ≈ 1.376)
 Below the cornea is aqueous humor (nah ≈ 1.336) and the
iris – a variable diaphram
 Behind the iris – crystalline lens (~ 9 mm dia, 4 mm
thick) surrounded by an elastic membrane
 Provides fine-focusing via changes in shape
Human Eye
Photoreceptors –
 Cones – three types “tuned” to react to Red, Blue and
Green light and send the appropriate signals to the
brain.
 Rods – react to Black/White and are more sensitive.
Brain – conducts an additive process in which the
various intensities of each primary color are put
together to produce a range of colors (millions).
Color of Objects
Is created by the absorption of OTHER colors and the
reflection of the object’s color—this is a Subtractive
Process.
Color of Objects
Plants appear green because they use more of the red
and blue wavelengths in photosynthesis and thus
reflect (reject?) green light.
White, Black, and Gray
 A reflecting surface is white when it diffusely
scatters a broad range of frequencies under
white illumination
 Diffusely reflecting surface that absorbs
somewhat uniformly across the spectrum
reflects a bit less than a white surface and
appears gray
 A surface that absorbs almost all the light
appears black
Reflected or Transmitted Energy
Colors
1.0
Green
Red
Blue
0.5
0
400
500
600
700
Wavelength (nm)
 Light uniform across the spectrum – white
 Not uniform – light appears colored
 Primary colors (RGB) beams combine to form white
light
Colors

Overlapping three
primary colors in
different combinations:
R+B+G=W
R + B = Magenta (M)
B + G = Cyan (C)
R + G = Yellow (Y)
 Any two colored light beams that together produce
white are said to be complementary:
M+G=W
C+R=W
Y+B=W
Colors

Overlap beam of magenta and yellow
M + Y = (R + B) + (R + G) = W + R or Pink


A color is saturated (deep and intense)
when it does not contain any white light
Pink is unsaturated red
Colors




Yellow stained glass –
absorbs blue
White light (RGB) will pass
red and green (yellow) and
absorb blue
This is subtractive
coloration
Additive coloration results
from overlapping light
beams
Photons and Atoms
Photons – small “bundles” of energy that have definite
frequencies.
 Higher Frequency  Higher Energy
 Lower Frequency  Lower Energy
Intensity of Light – depends upon…
 The energy of the individual photons (frequency)
 The density of the photons (number hitting a receptor
per unit time)
Energy Quanta
 Each quantum of electromagnetic radiation (a
photon) has energy proportional to its
frequency.
E = hf
 The constant of proportionality is Planck’s
constant
 h = 6.626 x 10-34 J/Hz or 4.136 x 10-15 eV/Hz
Atoms and Light
 For most atoms, the chemical, electrical,
and optical activity we observe is due
primarily to the Optical (outermost)
Electron.
 The energy of the optical electron depends
on the size of its orbit.
 Atoms at low temperature – in ground state
 As the temperature rises atoms are excited
above ground state
Atoms and Light
 Only certain discrete orbits are permitted for
the optical electron.
 The optical electron can jump from one
orbit to another, provided that an amount
of energy exactly equal to the energy
difference between the two orbits is supplied
or removed.
 When the downward atomic transition is
accompanied by the emission of light, the
energy of the photon (hf) exactly matches
the quantized energy decrease of the atom
(∆E).
Atoms and Light
Atoms and Light
Most prominent
lines in many
astronomical objects:
Balmer lines of
hydrogen
Scattering
Scattering is an interaction of photons and atoms.
 A single atom can interact with a single photon at one
time
 Depending upon the atoms in a given material, certain
frequency photons are absorbed, then re-emitted. In
most materials, the energy re-emitted is transferred as
heat.
 All other frequency photons are reflected.
**Special materials re-emit photons in a delayed
fashion, known as Photo-Luminescence.**
Scattering Vs. Absorption
 If the photon’s frequency matches (is “right” for) the
atom and can excite its Optical Electron, its energy is
Absorbed, redirected to neighboring atoms and
converted to heat.
 If the photon’s frequency DOES NOT match (isn’t
“right” for) the atom, it will reflect, or “bounce off”
the atom’s electron cloud. This will be the
frequency/wavelength/color that we see.
Kirchhoff’s Laws of Radiation (1)
1.
A solid, liquid, or dense gas excited to emit
light will radiate at all wavelengths and thus
produce a continuous spectrum.
Kirchhoff’s Laws of Radiation (2)
2. A low-density gas excited to emit light will do
so at specific wavelengths and thus produce
an emission spectrum.
Light excites electrons in atoms
to higher energy states
Transition back to lower states emits
light at specific frequencies
Kirchhoff’s Laws of Radiation (3)
3.
If light comprising a continuous spectrum
passes through a cool, low-density gas, the
result will be an absorption spectrum.
Light excites electrons in
atoms to higher energy states
Frequencies corresponding to the
transition energies are absorbed
from the continuous spectrum.
The Spectra of Stars
Inner, dense layers of a star
produce a continuous
(blackbody) spectrum.
Cooler surface layers absorb light at specific frequencies.
=> Spectra of stars are absorption spectra.
Measuring the Temperatures of
Stars
Comparing line strengths, we can
measure a star’s surface temperature!