How atoms

produce

light

http://www.wbateman.demon.co.uk/asa2sums/sum3.1B/topic3.1B.htm

LIGHT

• A generic term for electromagnetic radiation (like energy from the sun)

• Small packets of energy that travel as waves…these packets are called PHOTONS

• Wave properties for a given photon are all related to each other

Wave properties• Since light is a form of energy, we know that it

travels as a wave. Waves have a set of properties that are related to each other.

•  

• Wavelength- The distance between two equal

parts of a wave• Frequency – The # of waves that pass a point

each second• Speed – How fast a wave travels

(all light travels at the same speed)

WAVE A WAVE B

Questions:

• Which of the two light samples above has a longer wavelength?

• If both light samples are travelling by you at the same speed, which will have more waves pass by you in one minute?

• From your answer to # 2, which light has the higher frequency?

• Are wavelength and frequency related directly or inversely?

WAVE A WAVE B

Electromagnetic Spectrum

• Which has longer waves, gamma rays or microwaves?

• Which has shorter waves, orange light or green light?

• Which has shorter waves, violet light or ultraviolet light?

• Which has longer waves, red light or infrared light?

• In the spectrum modeled above, which type of radiation do you think

has the highest energy?

• Based on your answers above, are energy and wavelength directly

or inversely proportional?

• Are energy and frequency directly or inversely proportional?

• Which has higher energy, orange light or green light?

• Which has higher energy, red light or infrared light?

• Which has higher energy, violet light or ultraviolet light?

Spectroscope

• A simple spectroscope has a flat prism that separates light so we can see the individual colors.

http://asd-www.larc.nasa.gov/edu_act/simple_spec.html

“White Light”

• Aim the vertical slit toward the incandescent light source (the sun)

• You will see the light through the slit.

• Without moving the spectroscope, drift your eyes to the right until you see the numbers on the scale.

• What do you see over the numbers?

Pass out Spectroscopes and colored pecils

Continuous Spectrum= all colorsThere are no “blank spots” in the spectrum!

http://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter3.html

Why continuous spectrum?

• A solid is heated…all of its atoms/molecules and their parts move really fast

• Energy is given off as the atoms constantly vibrate.

• Photons of all colors can be emitted.• (given off)• All colors blend into “white light”

Another type of spectrum

• Aim the vertical slit at the overhead lights in the room.

• How does this look different from the incandescent light?

• Do the colors that show up always show up in the same place?

Brightline Spectrum

When only certain photons are observed, it means that only light packets of a particular type are being emitted!• Each photon has a specific energy

value…• …so only certain energy exchanges are

happening within the heated substance.• …so there must only be certain ways of

changing the energy in the substance!

How?• This can be explained by the movement of

electrons!• We know from the Bohr model that atoms have

“layers” of electrons called energy levels.• Each energy level has electrons with a certain

amount of energy in them that matches the level.

• When the electrons change levels, they have to gain or lose energy to do so.

• Each time they lose energy, they emit (release) a bundle of energy.

• We see that bundle as a photon!

Line Spectrums• When heated, the electrons in an

atom are excited. Electrons “jump” to higher energy levels.

• When they go back down they release energy (photons) as light.

• This can be recorded as a line spectrum.

Line Spectrums• Different elements emit

different wavelengths of light because each element has a different electron arrangement.

• Each element has a different pattern of wavelengths and a different line spectrum.

Photons and Atoms• Photon is just the name for a small piece

of light

• Electron Transition – when an electron moves from one level to another

– When an electron transitions to a higher energy level, a photon is absorbed.

– When an electron transitions to a lower energy level, a photon is emitted.

• Techniques like spectroscopy have allowed us to discover new elements like Caesium and Rubidium.

• Helium was discovered by looking at the line spectrum of the sun during an eclipse!

Conclusion

• Atoms only emit photons of specific energies

• That is why we can use atomic emissions spectra to identify elements in an unknown sample.