Transcript Class 7 Ch 5a(Feb. 1)

Chapter 5
Light: The Cosmic Messenger
5.1

Outline Ch 5 Light: The Cosmic Messenger
Basic Properties of Light and Matter
Light: electromagnetic waves
1. Velocity (c = speed of light), wavelength and frequency (colors),
energy.
2. Electromagnetic spectrum, visible spectrum, atmospheric windows

5.2
Matter: Atoms. How do light and matter interact?
Learning from Light: Origin of Starlight (some not in book)
1. How photons are produced
2. Relation temperature  motion of atoms
3. Blackbody Radiation (hot iron example). Wien’s Law:
hotter  brighter, cooler  dimmer
hotter  bluer, cooler  redder (max ~1/T)
4. Colors of Stars: redder are cooler, bluer are hotter
5. Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and
Emission
6. Radial Velocity: Doppler effect
5.3
Telescopes: reflecting and refracting, ground, airborne, space.
5.1 Basic Properties of Light and Matter
•
•
•
•
Our goals for learning
What is light?
What is matter?
How do light and matter interact?
What is light?
•Light is an
electromagnetic wave
•Light is also a particle
Photons: “pieces” of light, each
with precise wavelength,
frequency, and energy.
Speed of light “c” is a
constant in a vacuum =
300,000 km/sec
The Electromagnetic Spectrum
The Electromagnetic Spectrum
Atmospheric “Windows”
1. Visible Window (plus some UV and some infrared)
2. Radio Window
Question
The higher the photon energy…
a) the longer its wavelength.
b) the shorter its wavelength.
c) energy is independent of wavelength.
The higher the photon energy…
a) the longer its wavelength.
b) the shorter its wavelength.
c) energy is independent of wavelength.
What is matter?
Atomic structure:
Atomic Terminology
• Atomic Number = # of protons in nucleus
• Atomic Mass Number = # of protons + neutrons
Atomic Terminology
• Isotope: same # of protons but different # of
neutrons (4He, 3He)
• Molecules: consist of two or more atoms (H2O, CO2)
How do light and matter interact?
•
•
•
•
Emission
Absorption
Transmission
Reflection or Scattering
Terminology:
• Transparent: transmits light
• Opaque: blocks (absorbs) light
Interactions of light and matter
Question
Why is the rose red?
a)
b)
c)
d)
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.
Why is the rose red?
a)
b)
c)
d)
The rose absorbs red light.
The rose transmits red light.
The rose emits red light.
The rose reflects red light.
What have learned?
• What is light?
• Light is an electromagnetic wave that also comes
in individual “pieces” called photons. Each
photon has a precise wavelength, frequency and
energy.
• Forms of light are: radio waves, microwaves,
infrared, visible light, ultraviolet, X-rays and
gamma rays
What have we learned?
• What is matter? Ordinary matter is made of
atoms, which are made of protons, neutrons
and electrons.
• How do light and matter interact? Matter
can emit light, absorb light, transmit light or
reflect light
5.2. Learning from Light
• Our goals for learning
• What types of light spectra can we observe?
• How does light tell us what things are made
of?
• How does light tell the temperatures of
planets and stars?
• How does light tell us the speed of a distant
object?
5.2
Learning from Light: Origin of Starlight
(much of 5.2 not in book)
1. How photons are produced
2. Relation temperature  motion of atoms
3. Blackbody Radiation (hot iron example). Wien’s Law:
hotter  brighter, cooler  dimmer
hotter  bluer, cooler  redder (max ~1/T)
4. Colors of Stars: redder are cooler, bluer are hotter
5. Types of spectra (Kirchhoff’s 3 laws ): Continuous,
Absorption and Emission
a.
b.
c.
Model of atoms: energy levels
Continuous spectrum
Emission lines and absorption lines
6. Radial Velocity: Doppler effect
5.2.1 How photons are produced?
When the motion of an electron is disturbed
5.2.2 Relation temperature  motion of
atoms (from Ch.4)
•The higher the temperature the faster the atoms in a
substance will be moving
•As atoms collide the electrons collide and their motion is
disturbed
• When the motion of electrons gets disturbed they
produce photons
•The higher the temperature, the more collisions, the more
photons
Temperature Scales (from Ch.4)
5.2
Learning from Light: Origin of Starlight
1. How photons are produced
2. Relation temperature  motion of atoms
3. Blackbody Radiation (hot iron example). Wien’s Law:
hotter  brighter, cooler  dimmer
hotter  bluer, cooler  redder (max ~1/T)
How does light tell us the
temperatures of planets and stars?
Cooler
Hotter
Properties of Blackbody Radiation:
1. Hotter objects emit more light (per unit area) at all wavelengths.
i.e. hotter  brighter, cooler  dimmer
2. Hotter objects emit photons with a higher average energy.
i.e. hotter  bluer, cooler  redder
Properties of Blackbody Radiation:
3. Wein’s Law (max ~1/T) i.e., if we can measure the maximum
radiation emitted by an object we can determine its temperature
Maximum emission
max1 max2 max3
5.2
Learning from Light: Origin of Starlight
1.
How photons are produced
2. Relation temperature  motion of atoms
3. Blackbody Radiation (hot iron example). Wien’s Law:
hotter  brighter, cooler  dimmer
hotter  bluer, cooler  redder (max ~1/T)
4. Colors of Stars: redder are cooler, bluer are
hotter
Stars behave like “blackbodies” so we can use their
colors to determine their temperatures
5.2
Learning from Light: Origin of Starlight
1.
How photons are produced
2. Relation temperature  motion of atoms
3. Blackbody Radiation (hot iron example). Wien’s Law:
hotter  brighter, cooler  dimmer
hotter  bluer,
cooler  redder (max ~1/T)
4. Colors of Stars: redder are cooler, bluer are hotter
5. Types of spectra(Kirchhoff’s 3 laws ): Continuous,
Absorption and Emission (page 117-119 of book)
a.
b.
c.
Model of atoms: energy levels
Continuous spectrum
Emission lines and absorption lines
What types of light spectra can we observe?
This process produces an emission spectrum
This process produces an absorption spectrum
Kirchhoff’s Laws (p117-119 in
book)
1.
2.
3.
1
Continuous Spectrum (thermal radiation
spectrum)
Emission Spectrum
Absorption spectrum
3
2
Continuous Spectrum
Emission Spectrum
Emission Spectrum
Absorption Spectrum
Absorption Spectrum
Solar Spectrum
How does light tell us what things are made of?
• Electrons in atoms have distinct energy levels.
• Each chemical element, ion, molecule, has a unique set of
energy levels.
•We can identify the chemicals in gas by their fingerprints in
the spectrum.
Distinct energy
levels lead to
distinct emission
or absorption
lines.
Question 1
.
.
.
.
.
If the temperature of a star goes from 6000 K to 5000 K, what happens
to its light?
1.
It becomes brighter
2.
It becomes bluer
3.
It becomes fainter
4.
It becomes redder
5. It remains constant
.
.
.
.
.
The correct answer is:
A. 3 only
B. 4 only
C. 5 only
D. 1 and 2
E.
3 and 4
Question 2
Can one use the visible color of the Moon to
determine its temperature?
A.
B.
C.
D.
Yes, because the Moon is similar to stars
Yes, because the Moon does not reflect light
Yes, because the Moon orbits Earth
None of the above are correct
Which is hotter?
a) A blue star.
b) A red star.
c) A planet that emits only infrared light.
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.
5.2.6 Doppler Effect
Radial Velocity
• Approaching stars: more energy,
• Receding stars: less energy,
• Remember Exam 1 (Ch. 1-5) on Thursday
Feb. 10
• HW due next Monday: Ch. 4 and 5.