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

Wave Characteristics Wave frequency: how often a crest washes over you Wave speed = wavelength ( )  frequency (f)

Electromagnetic Waves Medium = electric and magnetic field Speed = 3  10 5 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 FahrenheitCentigradeKelvin Absolute zero  459 ºF  273 ºC 0 K Ice melts32 ºF0 ºC273 K Human body temperature 98.6 ºF37 ºC310 K Water boils212 ºF100 ºC373 K

Black Body Spectrum Objects emit radiation of all frequencies, but with different intensities Higher Temp. Lower Temp. I peak f peak <f peak <f peak

Cool, invisible galactic gas (60 K, f peak in low radio frequencies) Dim, young star (600K, f peak in infrared) The Sun’s surface (6000K, f peak in visible) Hot stars in Omega Centauri (60,000K, f peak in ultraviolet) The higher the temperature of an object, the higher its I peak and f peak 14

Wien’s Law The peak of the intensity curve will move with temperature, this is Wien’s law: Temperature * wavelength = constant = 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 = ( K*m)/(5 x m) = 5800 K Instructor temperature: roughly 100 °F = 37°C = 310 K, so wavelength = (0.0029K*m)/310 K = 9.35 * 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