Wave property of light Waves can carry energy Wavelength ( ) : distance between successive crests (or troughs) Frequency (f): # of waves passing a point in 1 second Wave speed (v): wavelength x frequency v = x f v sound = 340 meters/second v light = 3x10 8 meters/second

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

Electromagnetic Spectrum Visible: nm Usable: 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

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 cm 2 covered = L/4  R 2 B ~ L/R 2 … Inverse Square Law of Light

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: E photon ~ f High frequency = high energy photon Low frequency = low energy photon Bright light = lots of photons

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

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 max = 3x10 6 nm / T (T in Kelvin)

Properties of thermal radiation Hotter objects emit more photons, so hotter objects are brighter objects Energy emitted per unit surface area ~ T 4 Double an object’s temperature, and it emits 16 times as much energy! (16 = 2 4 ) Triple the temperature, and it emits 81 times as much energy!! (81 = 3 4 ) Luminosity of an object depends on?

Properties of thermal radiation Stefan-Boltzmann Law Luminosity depends on temperature and surface area L ~ R 2 T 4