PreAP Chemistry 2013-2014
UNIT 3 NOTES
Standard 8: Understand the electromagnetic spectrum & the mathematical relationships between energy, frequency, and wavelength of light.
Standard 9: Calculate the wavelength, frequency, & energy of light using Planck’s constant and the speed of light
LIGHT AND ATOMIC SPECTRA (Textbook Ch 5 pp 136-145)
Much of what we know about the atom has been learned through experiments with light; thus, you need to know some fundamental concepts of light in order to understand the structure of the atom, especially the placement of the electrons.
The speed of light in a vacuum is the most accurately-known constant in the universe. You should memorize the value for the speed of light because we will be using it extensively.
The speed of light in a vacuum is 3.0 X 108meters per second.
Light actually has a "dual" nature (or split personality) because there are instances in which it behaves like a wave and other times when it behaves like a particle and sometimes as both. When we consider light as a wave, we must be able to describe its wavelength,its frequency and its velocity.
(1) wavelength is represented by the Greek letter lambda (λ) which looks like an upside down "y". It is the length between corresponding parts of adjacent crests and can be expressed in ANY units of length you choose (feet, inches, meters, kilometers, miles) but normally we choose meters or nanometers as the most common units for wavelength.
(2) frequency is represented by the Greek letter nu (ν) which looks like a “v”. Frequency is the number of wave crests which pass a given point in 1 second. Its units are therefore (most commonly) cycles/second or sometimes simply /sec. The unit "Hertz" (abbreviated Hz) is also used as a label for frequency, although this label seems to be gradually going out of existence.
Example 3-8Do you think a longer or a shorter wavelength would have the greater
frequency?
Example 3-9What mathematical relationship can you draw about wavelength and
frequency?
VISIBLE light is considered to have a wavelength of between 700 nm and 400
nm. The longest visible light is red (about 700 nm) and the shortest visible wavelength is 400 nm (violet). The colors of visible light in order from longest to shortest spell out ROY G BIV. The only thing which makes one color of light different from another is its wavelength (and therefore its frequency). All light travels at the same velocity.
The following are APPROXIMATIONS of the wavelengths of each of the colors:
Red 700 - 650 nm
Orange 649 - 580 nm
Yellow 579 - 575 nm
Green 574 - 490 nm
Blue 489 - 455 nm
Indigo 454 - 425 nm
Violet 424 - 400 nm
However, light is much more that just what you can see. The entire range of all light energy is known as the electromagnetic spectrum and you should be very familiar with it.
When light behaves as a wave, we can use the mathematical relationship that wavelength and frequency vary inversely by expressing it in an equation.
c = λνc = speed of light
This means that if you know the wavelength, you can find the frequency and vice versa since the speed of light is constant. Using the equation is easy, BUT YOU MUST BE CAREFUL OF THE UNITS YOU USE. If you use the speed of light in as 3.0 X 108 METERS/sec, be sure that the wavelength is entered in METERS.
Example 3-10If the wavelength of light is known to be 550 nm, what is its frequency?
Example 3-11If the frequency of light is known to be 9.45 X 1014 Hz, is the light visible? If not, is it UV or IR?
When light behaves as an individual particle instead of a wave, it is called a photon (not proton). When we consider photons of light we are interested in how much energy they have. The unit of energy which we will learn is the Joule (abbreviated J), and the equation which will allow us to calculate the energy of one photon is
E1 photon = hν(h = Planck's Constant which will be given to you)
Planck’s Constant is 6.63 X 10-34J-sec.
Frequently the two equations are used in tandem (piggyback)--that is, the answer you get in one equation will then be filled into the second equation.
Example 3-13What is the energy of a photon of light whose frequency is 7.85 X 1015 Hz?
(b) Is this light visible?
Example 3-14If light has a wavelength of 550 nm, what is the energy of one photon of this light?
Example 3-15. If one photon of light is known to have energy of 3.33 X 10-19 J, is it
visible?
Please notice that the equation says that the energy of ONE photon is = to Planck's Constant times frequency. BE CAREFUL WHEN THE PROBLEM GIVES YOU TOTAL ENERGY—you cannot put total energy directly into this equation.
Example 3-16The total energy of light from a distant star is found to be 2.3 X 10-17 J. If it is known that the light has a wavelength of 477 nm, how many photons of light have been detected by the telescope?
Example 3-17 If it is known that 500 photons of light have a total energy of 1.87 X 10-16 J, would you expect the light to be visible? What color is it if visible and if not, is it IR or UV?
Answers to Practice Problems:
1
PreAP Chemistry 2013-2014
Example 3-8
Shorter wavelength has a greater frequency.
Example 3-9
Wavelength and frequency are inversely proportional. As one goes up, the other goes down.
Example 3-10
f = c/ = 3 x 108m/sec (550 nm x 10-9 m/nm)= 5.5 x 1014 /sec
Example 3-11
= c/ν = 3 x 108m/s 9.45 x 1014 Hz = 3.17 x 10-7 m ÷ 10-9 m/nm) = 317 nm
Not visible, UV
Example 3-13
E = hν = (6.63 x 10-34 J sec)(7.85 x 1015 /sec) = 5.20 x 10-18 J
= c/ν = 3 x 108m/s 7.85 x 1015 /sec = 3.8 x 10-8m ÷ 10-9 m/nm)= 38 nm
Not visible
Example 3-14
f = c/ = 3 x 108m/sec 550 x 10-9m = 5.5 x 1014 /sec
E = hν = (6.63 x 10-34 Jsec)(5.5 x 1014 /sec) = 3.6 x 10-19 J
Or you can combine the two equations: E = hc/
Example 3-15
ν= E/h = 3.33 x 10-19 J 6.63 x 10-34 Jsec = 5.02 x 1014 Hz
= c/ν = 3 x 108m/sec 5.02 x 1014 /sec = 5.98 x 10-7m = 598 nm
Visible, Orange
Or you can combine the two equations: = ch/E
Example 3-16
E1photon= hc/ = (6.63 x 10-34 Js)(3 x 108 m/s) (477 x 10-9m) = 4.17 x 10-19 J
#photons = EtotalE1photon = 2.3 x 10-17 J 4.17 x 10-19 J = 55 photons
Example 3-17
E1photon = Etotal #photons = 1.87 x 10-16 J 500 = 3.74 x 10-19 J
= ch/E = (3 x 108 m/s)(6.63 x 10-34 Js) (3.74 x 10-19 J) = 5.32 x 10-7m = 532 nm (GREEN)
ADDITIONAL RESOURCES