How atoms produce light http://www.wbateman.demon.co.uk/asa2sums/sum3.1B/topic3.1B.htm
What is Light? • LIGHT is a form of energy • Light can be considered as a bunch of individual light “packets” called PHOTONS • Each packet has its own set of properties (wavelength, etc) • A bunch of these packets traveling together is like a ray of 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. • 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?
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. • 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?
What kind of spectrum? • Look at the overhead lights again. Did anything change? • Are photons of ALL colors being given off by these lights? • This is called a brightline spectrum!
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 middle school 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 a bundle of energy. • We see that bundle as a photon!
Observing Elements • You will use the spectroscopes to see what photons are given off by the elements used in the flame test. • There will be some “background” light in your spectra. • Only focus on the bright lines! http://njsas.org/projects/atoms/spectral_lines/1/
Atomic Spectra/Flame Tests • When we heat up an element, its electrons gain and lose energy while it is being heated. • The photons released with each energy loss travel together as rays of light that blend into a specific color. • This light can be passed through a prism so you can see the individual colors of each type of photon.
Sodium Sodium Emission Lines 400 500 600 700 Wavelength in nanometers http://www.800mainstreet.com/spect/emission-flame-exp.html
Analyze the spectrum • How many different types of photons are in the visible range for a sodium atom emission? • Two! Both are yellow, but have different wavelengths, so are different yellows.
Copper http://en.wikipedia.org/wiki/Flame_test Copper Emission Lines 400 500 600 700 Wavelength in nanometers http://webmineral.com/help/FlameTest.shtml
Analyze the spectrum • How many different types of photons are in the visible range for a copper atom emission? • Eight! A purple one, a blue one, and several wavelengths of greens and yellows.
The individual photon colors emitted by the electrons in any atom form the “atomic emission spectrum” Also called “brightline spectrum” http://library.thinkquest.org/19662/low/eng/model-bohr.html
Conclusion • Atoms only emit photons of specific energies • WHY??
All about… light
LIGHT • A form of energy! • Travels in waves • Wave properties are all related • All light is part of the electromagnetic spectrum (like energy from the sun)
Wave properties • Speed • Wavelength • Frequency • Energy http://web.chemistry.gatech.edu/~williams/bCourse_Information/6582/problem_sets/waves/index.html
Speed • Light travels at the speed of light (duh!) • The speed of light = 2.998 x 108 m/s • The symbol “c” stands for the speed of light • c = 2.998 x 108 m/s • All light waves will have the same speed, so speed is a constant
Wave Equations • Without the energy component c = λ • c = 2.998 x 108 m/s • λ (lambda) = wavelength in meters • (nu) = frequency in 1/s or s-1 or Hz
Wave Equations • With the energy component E = h or E = hc/λ • E = energy in Joules • h =Planck’s constant= 6.636x10-34 J·s
Quantum • A specific quantity of light • Bohr said that when energy is added to atoms, the electrons gain a “quantum” of energy to move to a higher level. • When electrons relax back to their normal state, they emit a quantum of energy to go back to the lowest level.
Quantum…photon • Photon is just the name for a quantum 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.
Quantum…photon • The emitted photon is just a “piece” of light. • It has a specific energy value, so it has a specific wavelength, frequency and color • If you can measure the wavelength of the photon, you can calculate its energy.
Example • The photon released by a certain electron transition has an energy of 4.56x10-19 J. Calculate the wavelength and frequency of this light. Is it in the visible range? • E=h so =E/h • (4.56x10-19 J)/(6.626x10-34J·s) • = 6.88x1014Hz • c=λ so λ=c/ • (2.998x108m/s)/(6.88x1014Hz) • = 4.36x10-7m = yes in the visible range
Gas Discharge Tubes • Another way to give energy to the atom is using electricity http://www.physics.lsa.umich.edu/demolab/graphics2/7b10_10.jpg
Gives a spectrum just like that of a flame… Figure 5.12 in your textbook
Colors…energy? • Once you read the wavelength from your spectroscope scale, you can calculate the energy the electrons had to lose in order to release that color of photon.
Ta-daa • This is why scientists can calculate the energy values of the levels within an atom even though they can’t see them! • Please complete the packet, “Analysis of Spectral Lines”