Mr. Harwood

Updated: May 2015
Course: SNC2D1
Unit: Optice

Lesson 2: Title: Production of Light

Apparatus needed: fluorescent tube, nichrome wire or heater wire would be nice to show incandescence.

Preliminaries: Show the two power points from Mr. Holmes. (How light is produced, Emissions of light)

Lesson:

How is visible light made?

  • incandescent (light due to heat)  hot stove, fires, sun, incandescent bulb
  • excitation of electrons  neon signs, fluorescent lights, LEDs, laser
  • other: bioluminescence, triboluminescence, sonoluminescence, Cherenkov radiation

(see for more info)

Students need to learn and be able to explain: incandescence, fluorescence (below).
Luminescence means producing light without any heat.
They also need to know what the terms bioluminescence, chemiluminescence, and phosphorescence mean.

What happens at an atomic level?

At normal temperatures electrons in atoms are in their ground state. If the atom gains energy from electricity or a collision or an incoming photon, an electron will jump to a higher energy level. The atom is now in an excited state. Almost immediately, the electron drops down to the ground state and gets rid of the extra energy by emitting light, a single photon comes out of the atom.

The bigger the difference in energy levels, the more energy the photon has.


Example:Hydrogen atom
Electrons which jump down to the first energy level (with only 2 electrons in it) produce UV light.

Next, electrons which jump down to n=2 make visible light. All other electron transitions make IR light.

Example: a He-Ne laser emits red light. This is due to a transition between the 3s2 level and the 2p4 level.

This means that, since each element has slightly different energy levels (when it’s a gas), each atom will emit slightly different colours. We can use this to identify elements.

Optional: demonstrate spectra using gas discharge tubes to illustrate energy levels

How a fluorescent light works.

 draw diagram

  • high voltage needed (200-300 V)
  • this makes electrons hit mercury atoms in the tube
  • these atoms get excited
  • when the electrons drop back to the ground state UV light is emitted
  • the UV light hits a phosphor coating on the inside of the tube
  • the phosphors absorb the UV light and emit visible white light

What are advantages and disadvantages of incandescent and fluorescent light?

Incandescent bulbs

-advantages: warm colour, heat produced. More heat than light (good for heat lamps), cheaper

-disadvantages: most of the energy goes into heat.

-

Fluorescent:

-advantages: use less electricity, last longer

-disadvantages: needs mercury (toxic), often a sterile white colour, sometimes flickers.

How are other forms of EM radiation produce?

 Any time that a charge particle (electrons, protons) accelerates it emits EM radiation.

Type of Radiation /

Device

/ Motion of charged particles / Uses
Radio wave / antenna / oscillating electrons in a long wire or antenna
Microwave / magnetron or klystron / bunches of electrons oscillate in a wavy circular pattern
Infra Red / heat / molecules and atoms move, rotate, and vibrate.
Small electron transitions
Visible / (see above) / Medium electron transitions in atoms
Ultraviolet / mercury vapour lamps / Large electron transitions in atoms
X-ray / X-ray tubes / High energy electron hits copper atom and knocks inner electron out. Outer electron drops down to inner level
Gamma ray / Radioactive material, cosmic rays / photon emitted from excited nucleus

Seat/Homework: p 407 #1,2. p410 #1,2
Define fluorescence, phosphorescence, luminescence,