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Light hits Matter: Refraction • Light travels at different speeds in vacuum, air, and other substances • When light hits the material at an angle, part of it slows down while the rest continues at the original speed – results in a change of direction – Different colors bend different amounts – prism, rainbow Application for Refraction • Lenses use refraction to focus light to a single spot Light hits Matter (II): Reflection • Light that hits a mirror is reflected at the same angle it was incident from • Proper design of a mirror (the shape of a parabola) can focus all rays incident on the mirror to a single place Application for Reflection • Curved mirrors use reflection to focus light to a single spot Telescopes From Galileo to Hubble: Telescopes use lenses and mirrors to focus and therefore collect light Rain analogy: Collect light as you collect rain Rain/light collected is proportional to area of umbrella/mirror, not its diameter Telescopes • Light collectors • Two types: – Reflectors (Mirrors) – Refractors (Lenses) Refracting Telescopes Reflecting Telescope Problems with Refractors • Different colors (wavelengths) bent by different amounts – chromatic aberration • Other forms of aberration • Deform under their own weight • Absorption of light • Have two surfaces that must be optically perfect Telescope Size • A larger telescope gathers more light (more collecting area) • Angular resolution is limited by diffraction of light waves; this also improves with larger telescope size Resolving Power of Telescopes Atmospheric Limitations “Light” – From gamma-rays to radio waves • The vast majority of information we have about astronomical objects comes from light they either emit or reflect • Here, “light” stands for all sorts of electromagnetic radiation • A type of wave, electromagnetic in origin • Understanding the properties of light allows us to use it to determine the – temperature – chemical composition – (radial) velocity of distant objects Waves • Light is a type of wave • Other common examples: ocean waves, sound • A disturbance in a medium (water, air, etc.) that propagates • Typically the medium itself does not move much crest Wave Characteristics wavelength 2 x amplitude trough direction of wave motion • Wave frequency: how often a crest washes over you • Wave speed = wavelength () frequency (f) Electromagnetic Waves • Medium = electric and magnetic field • Speed = 3 105 km/sec Electromagnetic Spectrum Energy: low medium high Electromagnetic Radiation: Quick Facts • There are different types of EM radiation, visible light is just one of them • EM waves can travel in vacuum, no medium needed • The speed of EM radiation “c” is the same for all types and very high ( light travels to the moon in 1 sec.) • The higher the frequency, the smaller the wavelength ( f = c) • The higher the frequency, the higher the energy of EM radiation (E= h f, where h is a constant) Visible Light • Color of light determined by its wavelength • White light is a mixture of all colors • Can separate individual colors with a prism Three Things Light Tells Us • Temperature – from black body spectrum • Chemical composition – from spectral lines • Radial velocity – from Doppler shift Temperature Scales Fahrenheit Centigrade Kelvin 459 ºF 273 ºC 0K 32 ºF 0 ºC 273 K Human body temperature 98.6 ºF 37 ºC 310 K Water boils 212 ºF 100 ºC 373 K Absolute zero Ice melts Black Body Spectrum • Objects emit radiation of all frequencies, but with different intensities Ipeak Higher Temp. Ipeak Ipeak Lower Temp. fpeak<fpeak <fpeak Cool, invisible galactic gas (60 K, fpeak in low radio frequencies) Dim, young star (600K, fpeak in infrared) The Sun’s surface (6000K, fpeak in visible) Hot stars in Omega Centauri (60,000K, fpeak in ultraviolet) The higher the temperature of an object, the higher its Ipeak and fpeak 14 Wien’s Law • The peak of the intensity curve will move with temperature, this is Wien’s law: Temperature * wavelength = constant = 0.0029 K*m So: the higher the temperature T, the smaller the wavelength, i.e. the higher the energy of the electromagnetic wave Example • Peak wavelength of the Sun is 500nm, so T = (0.0029 K*m)/(5 x 10-7 m) = 5800 K • Instructor temperature: roughly 100 °F = 37°C = 310 K, so wavelength = (0.0029K*m)/310 K = 9.35 * 10-6 m = 9350 nm infrared radiation ≈ 10 μm = 0.01 mm Measuring Temperatures • Find maximal intensity Temperature (Wien’s law) Identify spectral lines of ionized elements Temperature Color of a radiating blackbody as a function of temperature • Think of heating an iron bar in the fire: red glowing to white to bluish glowing