Properties and Sources of Light

Properties and Sources of LightChapter 161The Nature of LightTravels straight and fastReflects and Refracts at boundaries (and is also absorbedHas color and intensityBehaves as BOTH a wave AND a particle (photon)**As such, light can carry information**2Wave and ParticlesThe wave nature of light is needed to explain various phenomenaInterferenceDiffractionPolarizationThe particle nature of light was the basis for ray (geometric) optics3Electromagnetic WaveformsThe and fields are perpendicular to each otherBoth fields are perpendicular to the direction of motionTherefore, electromagnetic waves are transverse wavesWith all periodic waves

Since v = c in a vacuum

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4Electromagnetic Waves, SummaryA static electric charge produces an electric field. A uniformly changing (moving) electric field produces an magnetic fieldA uniformly changing (moving) magnetic field produces a electric field**But NONE of these produces an EM WAVE. For this you need an accelerating charge.**5Velocity of Lightc = 3 x 108m/s (In a vacuum)

Slower values in other mediums, even air slows down light, but frequency will stay the same6Sources of LightElectric light Incandescence

Electricity Heat Light

Fluorescence

Electricity UV Visible Light

7Intensity 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**8Intensity of Light (Brightness)

9Intensity of Light (Brightness) Intensity at Earths surface -- 500W/m2

Intensity at Suns surface (given off

1360W/m210Visible LightVisible light consists of a range of wavelengths (400 700nm), spanning violet to red in color. When all wavelengths are present, white light is observed.

11Visible Light and EnergyLower Frequency Longer Wavelength Lower Energy Redder Light

Higher Frequency Shorter Wavelength Higher Energy Bluer LightE = hf12Visible Light and EnergyWhen 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 = BluerE = sT4

Stefan-Boltzmann Law Wiens Law13Light at BoundariesWill be both reflected and refracted

(But more on this later.)

14Human Eye

15Human EyeEye is almost spherical (24 mm x 22 mm)Flexible shell the scleraMost 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 diaphramBehind the iris crystalline lens (~ 9 mm dia, 4 mm thick) surrounded by an elastic membraneProvides fine-focusing via changes in shape16Human EyePhotoreceptors 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).

17Color of ObjectsIs created by the absorption of OTHER colors and the reflection of the objects colorthis is a Subtractive Process.

18Color of ObjectsPlants appear green because they use more of the red and blue wavelengths in photosynthesis and thus reflect (reject?) green light.

19White, Black, and GrayA reflecting surface is white when it diffusely scatters a broad range of frequencies under white illuminationDiffusely reflecting surface that absorbs somewhat uniformly across the spectrum reflects a bit less than a white surface and appears grayA surface that absorbs almost all the light appears black20ColorsLight uniform across the spectrum whiteNot uniform light appears coloredPrimary colors (RGB) beams combine to form white light1.00.50400500600700Reflected or Transmitted EnergyWavelength (nm)RedGreenBlue

21ColorsAny two colored light beams that together produce white are said to be complementary:M + G = WC + R = WY + B = WOverlapping three primary colors in different combinations:R + B + G = WR + B = Magenta (M)B + G = Cyan (C)R + G = Yellow (Y)

22ColorsOverlap 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 lightPink is unsaturated red23ColorsYellow stained glass absorbs blueWhite light (RGB) will pass red and green (yellow) and absorb blueThis is subtractive colorationAdditive coloration results from overlapping light beams

24Photons and AtomsPhotons small bundles of energy that have definite frequencies.Higher Frequency Higher EnergyLower Frequency Lower Energy

Intensity of Light depends uponThe energy of the individual photons (frequency)The density of the photons (number hitting a receptor per unit time)25Energy QuantaEach quantum of electromagnetic radiation (a photon) has energy proportional to its frequency.

E = hf

The constant of proportionality is Plancks constanth = 6.626 x 10-34 J/Hz or 4.136 x 10-15 eV/Hz26Atoms and LightFor 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 stateAs the temperature rises atoms are excited above ground state27Bohr 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.

Atoms and LightOnly 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).28Suppose 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. Atoms and Light

29Atoms and Light

Most prominent lines in many astronomical objects: Balmer lines of hydrogen30ScatteringScattering is an interaction of photons and atoms.A single atom can interact with a single photon at one timeDepending 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.**31Scattering Vs. AbsorptionIf the photons 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 photons frequency DOES NOT match (isnt right for) the atom, it will reflect, or bounce off the atoms electron cloud. This will be the frequency/wavelength/color that we see.32

Kirchhoffs 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.033

Kirchhoffs 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 statesTransition back to lower states emits light at specific frequencies034

Kirchhoffs 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 statesFrequencies corresponding to the transition energies are absorbed from the continuous spectrum.035

The Spectra of StarsInner, dense layers of a star produce a continuous (blackbody) spectrum.Cooler surface layers absorb light at specific frequencies.=> Spectra of stars are absorption spectra.036Measuring the Temperatures of Stars

Comparing line strengths, we can measure a stars surface temperature!037