the interaction of light and matter (or decoding...

The Interaction of Light and Matter(or Decoding Starlight)

Guest Lecturer: Alison Coil

In astronomy, all we have is light!

Can’t do direct experiments like in other sciences.

Have to know how to decode light, like a fingerprint.

Need to understand how light is created and what it can tell us!

Full spectrum of light = electromagnetic spectrum

Light from each part of the spectrum provides us with different information.

A

B

C

D

If you pass white light through a prism, it separates into its

component colors.

ROY G B I V

spectrum

long wavelengths

short wavelengths

Photograph of a Star Cluster

Spectra of a Star Cluster

Which object is hotter, an object that is emitting mainly red light or mainly blue light?

increasing temperatureA B C

Which object is hotter, an object that is emitting mainly red light or mainly blue light?

increasing temperature

Energy output from a star

Stefan-Boltzmann lawEnergy depends on temperature (T4)

Energy = constant * T4

Hot stars have greater energy than cool starsA little higher temperature means a lot more energy!

2x the temperature = ? x the energy

The hotter an object, the more energy/radiation it emits!

(some #)

Filter

Detector

81

blue 4600 A 81color wavelength amount of light

Filter

Detector

85

blue 4600 A 81green 5300 A 85

Filter

Detector

83

blue 4600 A 81green 5300 A 85yellow 5800 A 83

Filter

Detector

78

blue 4600 A 81green 5300 A 85yellow 5800 A 83orange 6100 A 78

Filter

Detector

70

UV IR

“Spectral Curve” - a graph of an object’s energy output versus wavelength. All stars have the same shape curve, called a black body curve. The PEAK of this curve is related to the star’s temperature.Wavelength

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All heated objects have this same shape curve! Stars, people, heated wire. Peak for stars is UV (hot stars) to IR (cool stars), peak for people is longer IR (cooler than stars).

The WAVELENGTH of the PEAK of this curve tells us about the object’s TEMPERATURE and COLOR.

UV IR

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Wavelength

Hot objects emit light that PEAKS at short wavelengths (blue).

Cool objects emit light that PEAKS at long wavelengths (red)

increasing temperature

Wien’s law

λpeak = (3 x 10-3) / Tkelvin(wavelength unit here is meters)

• The higher the object’s temperature, the shorter the wavelength of the peak for the light emitted by the object.

Relates the temperature of an object to the wavelength of the peak in the black body curve.

Which is the longest wavelength?

Which is the highest energy?

A

B

C

D

What is the wavelength of the PEAK of this “Blackbody” curve

What color is our 5800K Sun?

The Sun emits all wavelengths of electromagnetic radiation (light); however, the wavelengths of light it emits most intensely are in the green/yellow part of the spectrum.

What color does the Sun appear?WHITE!!

A star, like the Sun, which peaks in the middle of the visible part of the spectrum (green/yellow light) will appear WHITE to the human eye because it is giving off nearly equal amounts of all the visible colors of light.

Our Sun

What if the Sun became hotter?

Our Sun

What if the Sun became cooler?

1. Which object gives off the greatest amount of Blue light?

2. Which object gives off the greatest amount of Red light?

3. Which object would appear Red?

4. Which object would have the lowest temperature?

AB

C

What is this a picture of ?

Find the hottest star(s), how do you know ?

electron

Accelerating charges produce light – electromagnetic radiation!

But, where does light actually come from?

An atom consists of

a small, dense

nucleus (containing protons and neutrons)

surrounded by electrons

- Model

NucleusAtoms have

discrete energy levels.

Atoms can only have specific amounts of energy.

The lowest energy level is the ground state.

Photons (light-waves) are emitted from an atom when an electron moves from a higher energy level to a lower

energy level

Nucleus

Photons (light-waves) can also be absorbed by an atom when an

electron moves from a lower energy level to a higher energy level

Nucleus

Only photons with the exact right energy (wavelength) will be

absorbed!

Lowest energy level is the ground state.

It takes energy for an atom to go to a higher energy level

(absorb energy from a photon or collide with another particle).

An atom releases energy when an electron moves to a

lower energy level (emits a photon).

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 emittedB. a photon of energy 15 is emitted.C. a photon of energy 5 is absorbed.D. a photon of energy 15 is absorbed.

more energy less energy

Which of these shows the atom emitting the greatest amount of

light?

e-

A e-

B

e-

C e-

D

Which of these would cause a “Violet” or high energy absorption line?

A.B.C.D. None of the above

Each chemical element produces its own unique set of spectral lines

when it is excited

We can measure on Earth what the lines are for each element!

We will study three types of spectra!!!

Continuous Spectrum

Hot/Dense Energy Source

prism

Emission Line Spectrum

Hot low density cloud of Gas

Absorption Line SpectrumCooler low density cloud of Gas

Hot/Dense Energy Source

The type of spectrum given off depends on the objects involved

Law #1 – The excited atoms within a hot dense object give off light of all colors (wavelengths) and produce a continuous spectrum -- a complete rainbow of colors (range of wavelengths) without any spectral lines.


Continuous Spectrum

Hot/Dense Energy Source prism

Law #2 – The excited atoms within a hot cloud of gas give off only particular colors (wavelengths) of light and produce an emission line spectrum - a series of bright spectral lines against a dark background.



Emission Line Spectrum

prismHot low density cloud of Gas

Law #3 – When the light from a hot dense object passes through a cool cloud of gas, the atoms within the cloud can absorb particular colors (wavelengths) of light and produce a absorption line spectrum - a series of dark spectral lines among the colors of the rainbow.


What physical situation makes this spectrum?

Law #3 – When the light from a hot dense object passes through a cool cloud of gas, the atoms within the cloud can absorb particular colors (wavelengths) of light and produce a absorption line spectrum - a series of dark spectral lines among the colors of the rainbow.


What physical situation does a star like the Sun present?

A hot dense core surrounded by a low density outer atmosphere

The Sun’s Spectrum

All stars produce dark

line absorption

spectra

What can we learn by analyzing starlight?

• A star’s temperature• A star’s chemical composition

More on starlight next time!

Read section 6-2 on The Interaction of Light

and Matter(fills in many details)

(Recycle your voting cards!)

Work with the person next to you to answer these questions (write down your answers):

1. If a star existed that was only a hot dense core and did NOT have a low density atmosphere around it, what type of spectrum would you see from that star?

2. If you put a telescope above the Earth’s atmosphere and took a measurement of the Sun’s spectrum, what would it look like?

3. Will an atom emit light if all of the atom’s electrons are in the ground state?

4. There are dark lines in the absorption spectrum that represent missing light. What happened to this light that is missing?

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