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Understanding Light: Properties, Waves, and Color Spectrum

Learn about the properties of light waves, including wavelength, frequency, and speed. Discover how light can be analyzed into different colors based on their wavelengths. Explore the electromagnetic spectrum, measuring light, and the particle nature of light known as photons. Understand thermal radiation, Wien's Law, and the Stefan-Boltzmann Law.

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Understanding Light: Properties, Waves, and Color Spectrum

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  1. Wave property of light Waves can carry energy Wavelength (l) : distance between successive crests (or troughs) Frequency (f): # of waves passing a point in 1 second Wave speed (v): wavelength x frequency v = l x f vsound = 340 meters/second vlight = 3x108 meters/second

  2. More Newton… Showed that light is composed of many colors Different l means different color (and vice versa) Visible light has short wavelengths: Blue: l = 4 x 107 meters (400 nm) Green: l = 5.5 x 107 meters (550 nm) Red: l = 7 x 107 meters (700 nm) 1 nanometer = 10-9 meters Visible light range = 400 to 700 nm

  3. Electromagnetic Spectrum • Visible: 400-700 nm • Usable: 0.001 nm – 10 km! • Atmospheric Windows: • Earth’s atmosphere absorbs and reflects radiation at several wavelengths • From the ground, we only detect visible and radio • For other wavelengths, we must observe in a place above most or all of the atmosphere: • Mountains: Near IR • Planes: Far IR • Balloons: UV, X-ray • Space: everything including gamma ray

  4. Measuring Light • Luminosity • Total rate of energy emission • Intrinsic to the star • L = energy/time “ergs per second” • Brightness • Depends on distance from object • Rate of energy passage through a fixed area • B = (ergs/second) / # of cm2 covered = L/4pR2 • B ~ L/R2 … Inverse Square Law of Light

  5. The particle nature of light Late 1800s – Early 1900s: Quantum Mechanics Light can be thought of as being made up of particles called PHOTONS The energy of a photon is proportional to frequency: Ephoton ~ f High frequency = high energy photon Low frequency = low energy photon Bright light = lots of photons

  6. Continuous Spectrum • Emitted by hot bodies (also called thermal radiation or black body radiation) • A blackbody is a perfect radiator: • Emits photons at all energies (a continuous spectrum) • Shape of the spectrum is solely determined by the object’s temperature

  7. Properties of thermal radiation Increase temperature: particles move faster interact at higher energies more of the higher energy photons produced Wien’s Law: Hotter objects emit higher energy (bluer) photons lmax = 3x106 nm / T (T in Kelvin)

  8. Properties of thermal radiation Hotter objects emit more photons, so hotter objects are brighter objects Energy emitted per unit surface area ~ T4 Double an object’s temperature, and it emits 16 times as much energy! (16 = 24) Triple the temperature, and it emits 81 times as much energy!! (81 = 34) Luminosity of an object depends on?

  9. Properties of thermal radiation Stefan-Boltzmann Law Luminosity depends on temperature and surfacearea L ~ R2T4

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