Download Chem 115 - Waves, Radiation and Spectroscopy (lecture 16) 3/31

Chem 115 - Waves, Radiation and Spectroscopy (lecture 16)
3/31/09 (DL)
Waves
A wave is a manifestation of kinetic energy of the particles of a medium, if the wave is matter.
Particles in the medium move up and down with a certain frequency.
Waves of light are similar. They always move at a constant rate, c, called the speed of light. (seen in E =
mc2)
Important parameters to describe waves:
Wavelength - λ length in meters of a cycle
Amplitude maximum distance a particle gets from the center
Frequency ν (Greek letter “nu”) or f (starting letter of frequency) = number of cycles up and down
(cycles/second or hertz Hz)
Speed = frequency * wavelength
or c = λν
*Picture toilet paper with a sine wave drawn on it passing behind a view through a narrow opening in
curtains on stage. The speed that the toilet paper is being walked along behind the curtain is the speed of
light, c, traveling (propagating) in meters per second.
When the wavelength of a wave is shorter the frequency increases.
When the wavelength is longer the frequency decreases.
Light and Electromagnetic Waves
c = 3.00 x 10^8 - speed of light in a vacuum
therefore c = v λ (v is frequency) (λ is wavelength)
An oscillating electric field generates a magnetic field.
An oscillating magnetic field generates an electrical field.
The electromagnetic spectrum is divided up based on frequency.
The visible region of light is from about 400 to 750 nm.
Wavelength increases as frequency decreases, and vice versa (it is an inverse relation because c is
constant)
Ultraviolet light is more dangerous because it has higher energy.
Infrared is associated with lower energy.
Note: energy follows frequency
How light waves differ from each other
Adjectives used in chemistry to describe wavelength are longer and shorter
Adjectives used to refer to frequency are high and low
Leaves appear green because green is the light least absorbed by chlorophyll, thus green light reflects off
the leaf and enters your eye which detects the leaf as green then.
Calculation of Light Properties
If the wavelength is in nm, convert it to standard SI units so that the meters will cancel with m/s in the
speed of light.
Black body radiation is light emitted by the fact that an object is hot
The color has to do with the temperature (lava glows, heating element glows, etc.)
Planck’s Genius
Planck proposed that understanding radiation can be mathematically described by oscillating objects
An object contains oscillators with various individual frequencies, ν (Greek letter “nu”)
h is plancks constant
En = nhν
n = 1, 2, 3, 4, ...
Light can be described by waves and they can have particular energies
Light also has particle behavior with quantized amounts of energy
Wave-particle duality of light and matter
Light has no mass yet they can be thought of as particles. Particles of light are called “photons”.
Electrons have mass (so they are particles) yet can be thought of as waves.
Photoelectric effect
If you shine light of a particular wavelength onto metal you can get the
electrons to come off of the metal if the light has high enough frequency (thus energy). The minimum
threshold energy where electrons begin to come off the metal is called the “work function”.
Photoelectric effect is an example of a property of light that can be explained only by particle behavior
Photons are particles of light.
Diffraction is an example of a property of light that can only be explained only by wave behavior
We observed how diffraction gratings work. m=0 peak is directly across. m=1 peaks are the first ones out
on both sides (less bright). m=2 peaks are next (less bright than m=1), etc.
We used diffraction gratings to separate out the different wavelengths of light given off by hydrogen
emitting photons, and by mercury emitting photons. These are called the spectra of the elements. They
were different from each other. At the next lecture, we will begin to study the model that explains why this
happens.