Light IIIThe Atom & Spectra

February 12, 2012

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Test 1

Average: 65

Takeaway Message:• YOU NEED TO STUDY MORE

• you need to come to class EVERY DAY (TPS questions will NOT be posted on the website)

• almost NOBODY is coming to office hours, if you do not understand something, come ask us

• several of the questions were same/similar to those in your lecture tutorials and think-pair-share questions, make sure you understand these!!! go to office hours, look over questions later, discuss them with classmates outside of class. In short, you will have to STUDY OUTSIDE OF CLASS

• future tests will be just as hard (if not harder)

• There will be opportunity for extra credit, but you will have to be in class to get it

Recap: Blackbody CurveThe peak of the

blackbody curve tells you temperature

The overall height of the blackbody curve tells you Luminosity

Luminosity depends on Temperature and

Size

So by observing a star’s energy output as a function of wavelength we can determine its temperature What else can we find out about stars by looking at their light?composition (i.e. what its made of): how?

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An atom consists of a small, dense

nucleus (containing protons and neutrons)

surrounded by electrons Model

Proposed by Niels Bohr

1913

Bohr model of Atom

A nucleus is about 10-15 m in size and the first electron orbits out at 10-10 m

from the center of the atom – The size of the electron

orbit is 100,000 times greater than

the size of the nucleus

Atoms are mostly empty space

So if a nucleus the size of an orange

(10 cm) was located at the center of the football field,

where would the electron be?

End Zone?Grandstands?On Campus?In Tucson?

If the electron’s orbit is 100,000

times bigger than the nucleus then the electron would be 10,000 m or 6.21 miles away from the center of the Football Field!

Still in Tucson, up in the foothills shopping

at La Encantada!!

Nucleus

Nucleus contains Protons and Neutrons

“Ground” state is lowest energy state

other states are called “excited” states

Energy levels become more closely spaced further from the nucleus

Nucleus

Electrons orbit the nucleus on energy levels

Electron

These energy levels are fixed and discrete

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

energy level

Nucleus

Emission

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

Absorption

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E = hf =hcλ

Hydrogen

Each chemical element produces its own unique set of spectral lines when it is excited

demo

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We will study three types of spectra!!!

Continuous Spectrum

Hot/Dense Energy Source prism

Emission Line Spectrum

prismHot low density cloud of Gas

Absorption Line SpectrumCooler low density cloud of Gas

Hot/Dense Energy Source prism

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.

*Note: although I have only drawn visible light, this is for *all* forms of light

We will study three types of spectra!!!

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.

The type of spectrum given off depends on the objects involved

We will study three types of spectra!!!

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.

The type of spectrum given off depends on the objects involved

We will study three types of spectra!!!

Absorption Line SpectrumCooler low density cloud of Gas

Hot/Dense Energy Source prism

Atoms & Spectra• Spectra occur because atoms

absorb and emit photons with only certain wavelengths

• Electrons have discrete energy states

Atom & Spectra

• This is the spectrum of the Sun

• Dark features are absorption lines

• Tell us about composition of the Sun

• The Sun has a hot dense core surrounded by a lower density atmosphere

Lecture Tutorial: Light and Atoms-65-69 AND Analyzing Spectra-71-74

• 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

another group.

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.

The type of spectrum given off depends on the objects involved

All stars produce dark line

absorption spectra

What can we learn by analyzing starlight?

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

Tutorial (homework): Types of Spectra – 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

another group.