•Reading:
–Ch. 5
•Homework:
–None
1
Electromagnetic Spectrum
Radio (0.4 GHz)
Atomic Hydrogen
Radio (2.7 GHz)
Molecular Hydrogen
Infrared
Mid-Infrared
Near-Infrared
Visible
X-Ray
Gamma ray
http://adc.gsfc.nasa.gov/mw/mmw_images.html
2
Nature of Light
• The exact nature of light was an enigma to
scientists for many centuries
• Light has properties similar to waves
–Young’s double-slit experiment
• Light sometimes behaves like a particle
–Photoelectric effect
3
Photoelectric Effect
• Light shining on a piece of
metal can cause electrons to
be ejected from the surface
– only happens if the light has a
wavelength shorter than some
value (depends on the metal)
• As intensity of light increases,
number of electrons increases,
but not their energy
• As the wavelength of light
decreases, number of
electrons remains the same
but their energy increases
4
Quantization of Light
• Electromagnetic radiation is quantized
hc
E=
λ
E
= energy of photon
h
= Plank� s constant = 6.625 × 10−34 J s
c = speed of light
λ
= wavelength of light
• Einstein used the idea of the quantization of light to explain the
photoelectric effect
–light is quantized into packets of energy (called photons)
–energy of each photon is determined by its wavelength
–only photons with enough energy are able to eject electrons
–higher intensity light means more photons, not more energy per photon
–won Nobel prize in physics in 1921
5
Electromagnetic Spectrum
http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html
6
How Light is Produced
7
Kirchhoff’s Laws
1.A hot, opaque body, such as a perfect
blackbody or a hot, dense gas, produces a
continuous spectrum of radiation
• a complete rainbow of colors without any
spectral lines
2.A hot, transparent gas produces an
emission line spectrum
• a series of bright lines against a dark
background
3.A cool, transparent gas in front of a source
of continuous spectrum produces an
absorption line spectrum
• a series of dark lines among the colors of a
continuous spectrum
8
Kirchhoff’s Laws
• http://bcs.whfreeman.com/universe9e
–Animation 5-1
9
Blackbody
• An idealized object
–Does not exist in nature
• A perfect blackbody absorbs all of the
radiation that falls on it.
–Does not reflect any light
• A blackbody emits radiation only according
to its temperature
10
Temperature
• Measure of relative heat contained in 2
objects
• Three common temperature scales
Fahrenheit (° F) Celsius (° C) Kelvin (K)
Ice point
32
0
273
Steam point
212
100
373
Note: A change of
1° C is the same as
a change of 1 K
11
Temperature
12
Temperature
• Temperature is a measure of the average
random motion of atoms or molecules in a
substance
• The atoms & molecules in all substances are
moving
–gas molecules in the air
–water molecules in a cup of water
–even the molecules in a solid are vibrating with
some amount of energy
13
Blackbody Radiation
• Any hot opaque body produces a continuous
spectrum of radiation
–All such objects emit radiation at all wavelengths
(gamma rays to radio waves)
–Such objects do not emit at the same intensity at
all wavelengths
–Peak wavelength depends on the temperature
• Sun emits mostly in the visible
• Room-temperature objects emit mostly in the infrared
14
Blackbody Curve
•Hotter blackbody
–higher intensity at all
wavelengths
–peak intensity at shorter
wavelength
• hotter blackbody is bluer
•Wien’s Law
15
Sun as a Blackbody
• Stars, such as the
Sun, are approximate
blackbodies
• Using Wien’s law, can
estimate the surface
temperature of a star
just by observing its
blackbody spectrum
• Anyone notice
anything curious
about the Sun’s
intensity curve?
16
Sun as a Blackbody
• Wien’s law does
not depend on the
chemical
composition of the
blackbody
• All perfect
blackbodies at a
given temperature
produce the same
spectrum
17