September 7, 2001

Chapter 2--

A large section on Light--

• Let's look at this today

Element of the day--

• Copper (and Ancient Bronze Age)

• Due by 5 p.m. on Monday
• bonus for attending and keeping up
• details on the Web page click here for details
• can collect in class on Monday
• e-mail it to me (link on the Web page)

During class, we started with the demonstrations.

A brief description of the demos is available ... click here for the demos

Light-- A form of energy

• called electromagnetic radiation
• only small part is visible light
• Need a numerical scale to classify light
  • actually an electrical field or disturbance
  • oscillates -- is periodic (repeats)
  • distance between peaks is called the wavelength
  • we'll classify light by its wavelength

Starting at the short end we have

• gamma rays (10^-12 meters, for example)
  • atom is about 10^-10 meters for comparison
  • 0.000 000 000 1 meters
  • we need notation for such tiny numbers
• X-rays (in the 10^-10 region)
• Ultraviolet Light
  • (from about 10^-9 to about 10^-7 meters)
• Visible Light (a very tiny slice)
  • 400-800 nm (nm= 10^-9 meters)
  • 400 nm = 0.000004 meters = 4 x 10^-7 m
    • 450 is deep blue or violet
    • 400 is blue
    • 500 is green
    • 550 is yellow
    • 600 is orange
    • 700-750 is red
• Infrared Region (from red to
  • about 10^-3 meters)
• Microwaves (typically 1 mm wavelength)
• Radio Waves (up to many meters long)

Different regions do different things

• X-rays pass through matter (like skin and muscle) fairly easily
  • cast shadows for dense material like bone
  • along the way, X-rays can do damage to the molecules
    • can disrupt DNA, lead to mutations
• Ultraviolet Light
  • Often stopped by matter like water,
    • even gases
  • The light energy gets taken up by molecules
  • Can lead to breaking up molecules, ripping off electrons (ions)
  • Short wavelength are more powerful
  • Causes sunburn,
    • degradation of wood and paint
    • Sun Blocker= molecules that absorb
    • the UV before it reaches the skin

• Visible Light
  • Can cause some chemical reactions
  • (Otherwise, we couldn't see or take photographs)
  • Generally much less destructive
  • Often the reactions are reversible

• Infrared Region
  • Very little damage to molecules
  • can make molecules vibrate a bit (useful)
    • energy mostly ends up as heat
• Microwaves
  • Sometimes absorbed and end up as heat
  • Microwave ovens target water in food

• Albert Einstein (and others) found light appears to react as tiny packages
• Energy of a packet varies as 1/ wavelength
  • red light is thus weaker than blue
  • UV is stronger than visible
  • X-rays are stronger than UV light
• Each packet (quantum) is absorbed and pumps energy into a single molecule

1. we can make molecules do things by getting them to take on this energy

• in photography, light causes a chemical reaction
  • AgCl ----light--> silver metal
  • (black spot on the film)
• in photosynthesis, plant stores energy
  • from light, then transfers it into
  • manufacture of sugars...
• photodynamic therapy
  • some medical techniques use light to
  • destroy tumors (more later)

• the color of light that is absorbed tells us about the molecules that did absorb the light
• chemists can find lots of information from a spectrum
• can often measure concentrations by measuring how much light is absorbedat specific wavelength,
  • so only that kind of molecule is being measured

• molecules designed for specific energy
• sun-block, paints, UV absorbers

• by reactions that provide exactly that
• much energy for one photon

Looking at a spectrum

• Looking-- must only consider visible light but principles are same for UV, IR.

• wavelengths are represented
• A rainbow-- light is separated by color
• rainbow relies on tiny droplets of water
• a glass prism also works
• objects with lines spaced closely together
  • closely = size of light wavelength
  • (too small to see directly)
    • CD-ROM is common example
    • thin oil film also works
    • laboratory grating

• light from a slide projector
• glass grating
• screen
  • should see separate regions of
    • violet-blue-green-yellow-orange-red

Absorption of light?

color of solution--

• really color of the light that gets through
• what gets through is what isn't absorbed
  • complementary colors
  • e.g., remove green-- see color purple

Instruments--

• more likely to measure brightness electronically
• display on a graph
• often plot intensity or loss (absorbance)

Ozone--

• Oxygen's other form-- O₃
• It is reactive and toxic (not our concern here)
• Ozone absorbs light in the Ultraviolet

• Eventually, energy ends up as heat

• UV can be used to kill germs and other living beings
  • plants would be seriously damaged
  • skin would blister
  • UV damage to DNA causes skin cancer
• Complete loss of ozone would probably wipe out life as we know it
• Modest decrease in ozone lets more UV light through
  • even now, sunlight exposure increases risk of skin cancer
  • small ozone changes could make matters worse

• appears as "ozone holes"
• regions with less ozone than normal
• regions of decreasing ozone

Why?

• Could be change in way ozone forms
  • something might interfere with normal formation of ozone
• Could be that ozone reacts with something else

Culprit--

• Chlorinated Fluoro Carbons or Freons
• CH₄ = methane
  • CH₃F is monofluoro methane
  • CCl₂F₂ is dichrolor difluoro methane
    • (Freon 2-2)
• these fluoronated hydrocarbons also absorb UV light (not a problem)
• they produce free F atoms (reactive)
• this actually attacks and eliminates ozone

• widely used in refrigerators and A/C
• used to form bubbles in plastic foams
• used in some fire extinguishers
• used as propellent in spray cans
  • (hair spray, paint)
  • (won't burn, condenses and fills can)

• Always expenses to change
• To develop alternative materials
  • might not be as good; costly changeover
• To build plants for new materials