1. Properties and Sources of Light
Chapter 16

2. The Nature of Light **As such, light can carry information**
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**

3. 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

4. Electromagnetic Waveforms
The and 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
Since v = c in a vacuum
[11.1]

5. 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.**

6. 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

7. Sources of Light Electric light – Incandescence
Electricity  Heat  Light
Fluorescence
Electricity  UV  Visible Light

8. 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**

9. Intensity of Light (Brightness)

10. Intensity of Light (Brightness)
Intensity at Earth’s surface --
 500W/m2
Intensity at Sun’s surface (given off –
 1360W/m2

11. 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.

12. Visible Light and Energy
Lower Frequency  Longer Wavelength  Lower Energy  Redder Light Higher Frequency  Shorter Wavelength  Higher Energy  Bluer Light
E = hf

13. 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 Wien’s Law

14. Light at Boundaries
Will be both reflected and refracted (But more on this later….)

15. Human Eye

16. 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

17. 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).

18. Color of Objects
Is created by the absorption of OTHER colors and the reflection of the object’s color—this is a Subtractive Process.

19. Color of Objects
Plants appear green because they use more of the red and blue wavelengths in photosynthesis and thus reflect (reject?) green light.

20. 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

21. Colors Light uniform across the spectrum – white
1.0
0.5
400
500
600
700
Reflected or Transmitted Energy
Wavelength (nm)
Red
Green
Blue
Light uniform across the spectrum – white
Not uniform – light appears colored
Primary colors (RGB) beams combine to form white light

22. 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

23. M + Y = (R + B) + (R + G) = W + R or Pink
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

24. 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

25. 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)

26. 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 = x J/Hz or x eV/Hz

27. 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
Bohr made several assumptions, which are critical to his explanation. Later, more exact analysis showed that these assumptions were a reasonable approximation to reality.
1. For most atoms, the chemical, electrical, and optical activity we observe is due primarily to the outermost electron. (For that reason, we call that electron the optical electron.) The remaining electrons sit in closer orbits around the nucleus and don't participate.
2. The energy of the optical electron depends on the size of its orbit. The larger the orbit, the higher the energy.
3. Only certain discrete orbits are permitted for the optical electron. No other orbits are possible. Therefore, the energy of that electron can only take on certain discrete values. This is unlike an artificial satellite, which can be placed in any desired orbit, and therefore can have any desired energy.
4. 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 to it or removed from it. If the electron gains energy, it moves to a higher orbit; if it loses energy, it moves to a lower orbit. But only certain discrete amounts of energy can be added or removed, since only certain orbits are permitted.

28. 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).
Suppose we take a tank of gas and cool it to absolute zero. At that temperature, all atoms are in their ground states, and some may be combined into molecules. All atomic and molecular activity is at the minimum level possible. If light is passed through the gas, the only absorption lines seen will be those produced by electrons jumping up from the ground state, since no other orbits are occupied.
Now let's heat the gas. As the temperature rises, collisions between atoms and molecules become more frequent and more violent. Many atoms have their electrons kicked up from the ground state into higher levels. At the same time, the molecules begin to break up because of the impacts. In the spectrum, absorption lines due to molecules begin to disappear from the spectrum, while lines due to excited atoms appear and grow stronger.
At still higher temperatures, the collisions are violent enough to tear electrons completely off the atoms. Absorption lines due to ions now appear in the spectrum, while lines due to neutral atoms weaken and disappear. This process continues until the gas is either completely ionized, or so highly ionized that nearly all lines are produced at ultraviolet wavelengths, where we can't see them.
The rate and violence of the collisions between atoms and molecules depend primarily on the temperature. Because this is true, when we observe whether the spectrum of a gas contains lines from molecules, from atoms in the ground state, from atoms in excited levels, or from ions, we should be able to judge the temperature of the gas.
In fact, since it turns out that most stars have similar chemical compositions, the temperature becomes the primary property that we can deduce from the spectrum.

29. Atoms and Light

30. Atoms and Light
Most prominent lines in many astronomical objects: Balmer lines of hydrogen

31. 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.**

32. 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.

33. Kirchhoff’s Laws of Radiation (1)
A solid, liquid, or dense gas excited to emit light will radiate at all wavelengths and thus produce a continuous spectrum.

34. 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

35. 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.

36. 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.

37. Measuring the Temperatures of Stars
Measuring the Temperatures of Stars
Comparing line strengths, we can measure a star’s surface temperature!