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Wave property of light

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 v sound = 340 meters/second v light = 3x10 8 meters/second.

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Wave property of light

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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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