Physical chemistry of light

1
Physical chemistry of light

• OBJECTIVES
• Calculate the wavelength, frequency, or energy of
  light, given two of these values.

2
Physical chemistry of light

• OBJECTIVES
• Explain the origin of the atomic emission
  spectrum of an element.

3
Light

• The study of light led to the development of the
  quantum mechanical model.
• Light is a kind of electromagnetic radiation.
• Electromagnetic radiation includes many kinds of
  waves
• All move at 3.00 x 108 m/s c

4
Parts of a wave
Origin
5
Parts of Wave

• Origin - the base line of the energy.
• Crest - high point on a wave
• Trough - Low point on a wave
• Amplitude - distance from origin to crest
• Wavelength - distance from crest to crest
• Wavelength is abbreviated by the Greek letter
  lambda l

6
Frequency

• The number of waves that pass a given point per
  second.
• Units cycles/sec or hertz (hz or sec-1)
• Abbreviated by Greek letter nu n
• c ln

7
Frequency and wavelength

• Are inversely related
• As one goes up the other goes down.
• Different frequencies of light are different
  colors of light.
• There is a wide variety of frequencies
• The whole range is called a spectrum,

8
Radiowaves
Microwaves
Infrared .
Ultra-violet
X-Rays
GammaRays
Long Wavelength
Short Wavelength
Visible Light
9
Prism

• White light is made up of all the colors of the
  visible spectrum.
• Passing it through a prism separates it.

10
If the light is not white

• By heating a gas with electricity we can get it
  to give off colors.
• Passing this light through a prism does something
  different.

11
Atomic Spectrum

• Each element gives off its own characteristic
  colors.
• Can be used to identify the atom.
• How we know what stars are made of.

12

• These are called discontinuous spectra, or line
  spectra
• unique to each element.
• These are emission spectra
• The light is emitted given off

13
Light is a Particle

• Energy is quantized.
• Light is energy
• Light must be quantized
• These smallest pieces of light are called
  photons.
• Photoelectric effect?
• Energy frequency directly related.

14
Energy and frequency

• E h x ?
• E is the energy of the photon
• ? is the frequency
• h is Plancks constant
• h 6.6262 x 10 -34 Joules x sec.
• joule is the metric unit of Energy

15

• 2 equations so far
• c ??
• E h?

16
Examples

• What is the wavelength of blue light with a
  frequency of 8.3 x 1015 Hz?
• What is the frequency of red light with a
  wavelength of 4.2 x 10-5 m?
• What is the energy of a photon of each of the
  above?

17
Explanation of atomic spectra

• When we write electron configurations, we are
  writing the lowest energy.
• The energy level, and where the electron starts
  from, is called its ground state- the lowest
  energy level.

18
Changing the energy

• Lets look at a hydrogen atom

19
Changing the energy

• Heat or electricity or light can move the
  electron up energy levels (excited)

20
Changing the energy

• As the electron falls back to ground state, it
  gives the energy back as light

21
Changing the energy

• May fall down in steps
• Each with a different energy

22



23
Ultraviolet
Visible
Infrared

• Further they fall, more energy, higher frequency.
• This is simplified
• the orbitals also have different energies inside
  energy levels
• All the electrons can move around.

24
What is light?

• Light is a particle - it comes in chunks.
• Light is a wave- we can measure its wavelength
  and it behaves as a wave
• If we combine Emc2 , c??, E 1/2 mv2 and E
  h?
• We can get ? h/mv
• called de Broglies equation
• Calculates the wavelength of a particle.

25
Sample problem

• What is the approximate mass of a particle having
  a wavelength of 10-7 meters, and a speed of 1
  m/s?
• Use ? h/mv
• 6.6 x 10-27
• (Note 1 J N x m 1 N 1 kg x m/s2

26
Matter is a Wave

• Does not apply to large objects
• Things bigger than an atom
• A baseball has a wavelength of about 10-32 m
  when moving 30 m/s
• An electron at the same speed has a wavelength of
  10-3 cm
• Big enough to measure.

27
The physics of the very small

• Quantum mechanics explains how the very small
  behaves.
• Classic physics is what you get when you add up
  the effects of millions of packages.
• Quantum mechanics is based on probability

28
Heisenberg Uncertainty Principle

• -It is impossible to know exactly the location
  and velocity of a particle.
• The better we know one, the less we know the
  other.
• Measuring changes the properties.
• Instead, analyze interactions with other particles

29
More obvious with the very small

• To measure where a electron is, we use light.
• But the light moves the electron
• And hitting the electron changes the frequency of
  the light.

30
Before
After
Photon changes wavelength
Photon
Electron changes velocity
Moving Electron