Transcript Wednesday, October 7

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