Download E = mc2

Astronomy 110 Announcements:
• Homework #2 due Tuesday, June 7th –note change!
• Reading for Monday: pp. 117 – 125, 131 – 133
• There will be a reading quiz Monday since homework due date has
changed. Note change in reading assignment.
• Observing night (Extra Credit) – Thursday, June 9th
at Kapiolani Park at 8:00pm (rain date—Friday June
10th)
Mass-Energy
Review of Types of Energy
• Kinetic – energy of motion
– Thermal energy: related to temperature and density
• Radiative Energy – energy of light
• Potential Energy – stored energy
– Gravitational Potential Energy: depends on mass
(m), gravity (g), and height (h)
– Mass Energy: E=mc2
• Orbital Energy: total kinetic + potential
energy for an object in orbit.
Recap: Conservation of Energy
• Mass itself is a form of potential energy
E =
• A small amount of mass can
release a great deal of energy
• Concentrated energy can
spontaneously turn into particles
(for example, in particle
accelerators)
mc2
• Energy can be neither created nor destroyed.
• It can change form or be exchanged between
objects.
• The total energy content of the Universe was
determined in the Big Bang and remains the
same today.
How do gravity and energy together explain orbits?
• Orbits cannot change spontaneously (because of
conservation of energy).
• An object’s orbit can only change if it somehow
gains or loses orbital energy =
kinetic energy + gravitational potential energy
• If an object gains enough orbital energy, it may
escape (change from a bound to unbound orbit)
•escape velocity from Earth ! 11 km/s from sea
level (about 40,000 km/hr)
! So what can make an object gain or lose orbital
energy?
• Propulsion
• Friction or atmospheric drag
• A gravitational encounter.
Chapter 5
Light: The Cosmic Messenger
Escape and
orbital
velocities don’t
depend on the
mass of the
cannonball
5.1 Basic Properties of Light and Matter
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•
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Our goals for learning
What is light?
What is matter?
How do light and matter interact?
What is light?
A wave transmits energy
without carrying material
along it.
Light is an
electromagnetic wave
– affects charged particles
and magnets
Properties of Waves
Speed = wavelength x frequency
• Speed of light = 300,000 km/s (3x105 km/s)
• Since speed of light is a constant, lower
wavelength of light has a higher frequency
and vice versa
• Wavelength/ frequency of visible light is
directly related to its color:
– Longer wavelength = redder light
– Shorter wavelength = bluer light
Light comes in many forms
Light is also a particle
Photons: “pieces” of light, each with precise wavelength,
frequency, and energy.
Energy of a photon is directly related to its frequency
(indirectly related to wavelength):
• higher frequency (smaller wavelength, bluer light)
= higher energy
• Visible light: ~400 –
700nm (1nm = 10-9m)
• Infrared (IR): 1,000 –
100,000 nm (0.1mm)
• Radio waves: >1mm (>
1,000,000 nm)
• Ultraviolet (UV): ~100 –
400nm
• X-rays: ~0.01 – 0.1 nm
• Gamma rays: < 0.01 nm
How do light and matter interact?
5.2 Learning from Light
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•
•
•
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•
•
•
Emission
Absorption
Transmission
Reflection or Scattering
Our goals for learning
What types of light spectra can we observe?
What does light tell us about composition?
How does light tell the temperatures of
planets and stars?
• How does light tell us the speed of a distant
object?
What types of light spectra can we observe?
Example: Solar Spectrum
Review of Atoms
• Atoms consist of protons, neutrons, and
electrons
• Atomic number = # of protons
Atomic mass
• Atomic mass = # protons + # neutrons
4
• Isotope = elements of the same atomic
2
number, but different masses (Ex: 12C,
Atomic
number
13C)
• Ion = an element that has lost at least one
electron (given enough energy to
completely rip it away from the atom)
He
!Absorption line spectrum
!
1. How does light tell us the composition of
different objects?
• Electrons in
atoms have distinct
energy levels.
• Each chemical
element, ion,
molecule, has a
unique set of energy
levels.
Chemical Fingerprints
• Every atom, ion, and molecule has a unique
spectral “fingerprint”
• We can identify the chemicals in gas by their
fingerprints in the spectrum.
• With additional physics, we can figure out
abundances of the chemicals, and much more.
Distinct energy levels lead to distinct emission or
absorption lines.
Hydrogen
Energy
Levels
Thought Question
Which letter(s) labels absorption lines?
A
B
C
D E
Which letter(s) labels absorption lines?
A
B
C
D
E
Thought Question
Which letter(s) labels the peak (greatest
intensity) of infrared light?
A
B
C
D E
Which letter(s) labels the peak (greatest
intensity) of infrared light?
A
B
C
D E
Thought Question
Which letter(s) labels emission lines?
Which letter(s) labels emission lines?
A
B
C
D E
2. How does light tell us the temperatures of
planets and stars?
Thermal Radiation
• Nearly all large or dense objects emit thermal
radiation, including stars, planets, you…
• An object’s thermal radiation spectrum depends on
only one property: its temperature
!Exception: diffuse gas clouds—photons readily pass
through them. Emission or absorption lines seen
instead of continuous thermal spectrum.
A
B
C
D E
Two Properties of Thermal Radiation:
1. Hotter objects emit more light at all frequencies per unit area.
2. Hotter objects emit photons with a higher average energy.
Thought Question
Which is hotter?
a) A blue star.
b) A red star.
c) A planet that emits only infrared light.
Thought Question
Why don’t we glow in the dark?
a) People do not emit any kind of light.
b) People only emit light that is invisible to our eyes.
c) People are too small to emit enough light for us to
see.
d) People do not contain enough radioactive material.
Which is hotter?
a) A blue star.
b) A red star.
c) A planet that emits only infrared light.
Why don’t we glow in the dark?
a) People do not emit any kind of light.
b) People only emit light that is invisible to our
eyes.
c) People are too small to emit enough light for us to
see.
d) People do not contain enough radioactive material.
3. How does light tell us the speed of a distant
object?
Infrared radiation = heat
Visible radiation
(reflected light)
The Doppler Effect.
Infrared radiation
(emitted light)
The Doppler Effect
• Object moving towards you produces blueshift =
shift to shorter wavelengths
• Object moving away from you produces redshift =
shift to longer wavelengths
The amount of blue or red shift tells us an object’s
speed toward or away from us: