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Telescopes & Light: Part 2
All About Light
What is light?
• In the 17th Century, Isaac Newton argued
that light was composed of little particles
while Christian Huygens suggested that light
travels in the form of waves.
• In the 19th and 20th centuries Maxwell,
Young, Einstein and others were able to
show that Light behaves both like a particle
and a wave depending on how you observe
it.
What does light do?
• Light transfers energy from place to
place.
• Light transfers information from place to
place.
• Everything we know about astronomical
objects, we have learned through the
analysis of light.
Scottish physicist James Clerk Maxwell showed
mathematically in the 1860s that light must be a
combination of electric and magnetic fields.
In 1905, Einstein calculated the energy of
a particle of light (photon) and
proposed the photoelectric effect.
Ephoton = hc/l
photon
e-
But, where does light actually
come from?
Light comes from
the acceleration of
charged particles
(such as electrons
and protons)
Waves
• Wave - (general definition) a pattern that repeats itself cyclically
in both time and space
• Electromagnetic radiation travels through space in the form of
waves - a wave is a way in which energy is transferred from
place to place without physical movement of material from one
location to another.
• A wave is not a physical object.
• Wave period - number of seconds needed for the wave to repeat
itself at some point in space (time from crest to crest).
• Wavelength - the number of meters needed for the wave to
repeat itself at a given moment in time (length from crest to
crest).
• Amplitude - maximum departure of the wave from the
undisturbed state (maximum height).
• Frequency - number of crests passing any given point per unit
time (= 1 / period).
Wave Motion
• Period is given in seconds, frequency is 1/seconds or
Hz (Hertz).
• Wave velocity - a wave moves a distance equal to
one wavelength in one period.
• Velocity = wavelength x frequency
• Speed of light in a vacuum is constant and is called
“c” - it equals 3.0 x 108 m/s.
Lecture Tutorial: EM Spectrum (p. 45)
• Work with a partner!
• Read the instructions and questions
carefully.
• Discuss the concepts and your answers
with one another. Take time to
understand it now!!!!
• Come to a consensus answer you both
agree on.
• If you get stuck or are not sure of your
answer, ask me or another group.
Diffraction and Interference
• Diffraction - bending of a wave around a barrier (light
shining around a corner, etc.).
• Interference - when two or more waves interact.
Interference can be constructive (the waves add) or
destructive (the waves subtract, or even cancel each
other out completely).
So how do we actually get
light?
An atom
consists of a
small, dense
nucleus
(containing
protons and
neutrons)
surrounded
by electrons
- Model
Proposed by
Niels Bohr
1913
The electron
should be
thought of as a
distribution or
cloud of
probability
around the
nucleus that, on
average,
behaves like a
point particle on
a fixed circular
path.
Interactions Between Charged
Particles
•
•
•
•
Electrons - negatively charged.
Protons - positively charged.
Like charges repel, unlike charges attract.
Electric field - extends outward from a
charged particle.
• The electric field extends outward in a wave if
the particle is moving - we can learn about
the particle from afar from studying this wave.
Electromagnetic Waves
• Electromagnetic waves are caused by changing electric and
magnetic fields (magnetic fields accompany magnetized objects,
just like electric fields accompany charged objects).
• We learn about distant stars from their electromagnetic waves
(radiation).
• Since the speed of light is finite and constant, when we say an
object is 3 LY away (where 1 LY is the distance light travels in a
year), we are looking at light that left the star 3 years ago.
Photons (light-waves) are emitted
from an atom when an electron
moves from a higher energy level
to a lower energy level.
Nucleus
Photons can also be absorbed by
an atom when an electron moves
from a lower energy level to a
higher energy level.
Nucleus
Each chemical element produces its
own unique set of spectral lines when it
is excited
Tutorial: Light and Atoms – p. 63
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer, ask me
or another group.
If an electron in an atom moves
from an orbit with an energy of 5
to an orbit with an energy of 10,
A. a photon of energy 5 is emitted
B. a photon of energy 15 is emitted.
C. a photon of energy 5 is absorbed.
D. a photon of energy 15 is absorbed.
E. None of the above
Which of these shows the atom emitting
the greatest amount of light?
A
B
e-
ee-
C
E
D
e-
e-
e-
There are three types of spectra.
prism
Hot/Dense Energy Source
Continuous Spectrum
prism
Hot low density cloud of Gas
Emission Line Spectrum
prism
Hot/Dense Energy Source
Cooler low density cloud of Gas
Absorption Line Spectrum
Tutorial: Types of Spectra – p. 61
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer, ask me
or another group.
All stars
produce dark
line absorption
spectra.
So what do we learn from
light?
The Blackbody Spectrum
• Intensity - amount of strength of radiation at any point in space.
• Blackbody - an object that absorbs all radiation falling upon it.
• Blackbody curve - describes the distribution of re-emitted
radiation.
• Blackbody objects absorb radiation and then re-emit that
radiation if it is too remain in a steady state (does not increase
or decrease in temperature).
• Wien’s Law - wavelength of peak emission  1 / temperature
• Stefan’s Law - total energy radiated per second  temperature4
Temperature
• We can use blackbody
curves as thermometers
to determine the
temperatures of distant
objects (thanks to
Wien’s law - which
relates the wavelength
of peak emission to the
temperature of the
object).
Spectroscopy
• Spectral lines are indicators of chemical
composition. As such, they can be used
to identify the chemical composition of
any body that emits (or absorbs) light.
• This is how we know the composition of
the sun and other stars.
The Doppler Effect
• (Apparent wavelength / True wavelength) = (True frequency /
Apparent frequency) = 1 + (Recession velocity / Wave speed)
• Objects moving toward us are blueshifted (higher frequency)
and objects moving away from us are redshifted (lower
frequency).
• By determining the amount of shift in spectral lines, we can
determine an object’s recessional velocity (in general how fast
it’s moving away from us, or sometimes how fast it’s moving
towards us).
• The recessional velocity (this applies only to objects moving
away from us) relates directly to the object’s distance from us,
giving us a new way to measure distance.
Real Life Examples of Doppler
Effect
• Doppler Radar (for weather)
• Airplane radar system
• Submarine radar system
– Ok, anything with radar
• Radar gun, used by Law Enforcement
Officers…
Doppler Effect
• When something which is giving off light
moves towards or away from you, the
wavelength of the emitted light is changed or
shifted.
V=0
Doppler Effect
• “Along the line of sight” means the Doppler
Effect happens only if the object which is
emitting light is moving towards you or
away from you.
– An object moving “side to side” or
perpendicular, relative to your line of
sight, will not experience a Doppler Effect.
Astronomy Application
V=0
Lecture Tutorial: Doppler Shift (p. 73)
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one
another. Take time to understand it now!!!!
• Come to a consensus answer you both agree on and
write complete thoughts into your LT.
• If you get stuck or are not sure of your answer, ask me
or another group.
The Doppler Effect causes
light from a source moving
away to:
A.
B.
C.
D.
E.
be shifted to shorter wavelengths.
be shifted to longer wavelengths.
change in velocity.
Both a and c above
Both b and c above
You observe two spectra (shown below) that are redshifted
relative to that of a stationary source of light. Which of the
following statements best describes how the sources of light
that produced the two spectra were moving?
BLUE
RED
Spectrum A
Spectrum B
A.
B.
C.
D.
Source A is moving faster than source B.
Source B is moving faster than source A.
Both sources are moving with the same speed.
It is impossible to tell from looking at these
spectra.
Spectral Line Analysis
• The composition of an object is determined by matching its
spectral lines with the laboratory spectra of known atoms and
molecules.
• The temperature of an object emitting a continuous spectrum
can be measured by matching the overall distribution of
radiation with a blackbody curve.
• The (line-of-sight) velocity of an object is measured by
determining the Doppler shift of its spectral lines.
• An object’s rotation rate can be determined by measuring the
broadening (smearing out) of its spectral lines.
• The pressure of the gas in the emitting region of an object can
be measured by its tendency to broaden spectral lines.
• The magnetic field of an object can be inferred from a
characteristic splitting it produces in many spectral lines when a
single line divides into two (known as the Zeeman effect).