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Chapter 16

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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**

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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

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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]

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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.**

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Velocity of Light c = 3 x 10 8 m/s (In a vacuum) Slower values in other mediums, even air slows down light, but frequency will stay the same

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Sources of Light Electric light Incandescence Electricity Heat Light Fluorescence Electricity UV Visible Light

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Intensity of Light (Brightness) Defined as the power of light hitting a surface area in W/m 2. 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**

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Intensity of Light (Brightness)

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Intensity at Earths surface -- 500W/m 2 Intensity at Suns surface (given off 1360W/m 2

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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.

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

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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 = T 4 Stefan-Boltzmann Law Wiens Law

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

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Human Eye

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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 (n c 1.376) Below the cornea is aqueous humor (n ah 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

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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).

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

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

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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

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Colors Light uniform across the spectrum white Not uniform light appears colored Primary colors (RGB) beams combine to form white light 1.0 0.5 0 400500600700 Reflected or Transmitted Energy Wavelength (nm) Red Green Blue

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Colors Any two colored light beams that together produce white are said to be complementary: M + G = W C + R = W Y + B = W Overlapping three primary colors in different combinations: R + B + G = W R + B = Magenta (M) B + G = Cyan (C) R + G = Yellow (Y)

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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

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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

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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)

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Energy Quanta Each quantum of electromagnetic radiation (a photon) has energy proportional to its frequency. E = hf The constant of proportionality is Plancks constant h = 6.626 x 10 -34 J/Hz or 4.136 x 10 -15 eV/Hz

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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

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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).

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Atoms and Light

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Most prominent lines in many astronomical objects: Balmer lines of hydrogen

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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.**

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Scattering Vs. Absorption If 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.

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Kirchhoffs 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.

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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 states Transition back to lower states emits light at specific frequencies

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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 states Frequencies corresponding to the transition energies are absorbed from the continuous spectrum.

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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.

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Measuring the Temperatures of Stars Comparing line strengths, we can measure a stars surface temperature!