Chapter 30 Emission of Light (Sources of Light) Lecture 35

Lecture 35

Chapter 30 Emission of Light (Sources of Light)

Production of Light • Incandescent (hot) source (Stars; light bulb) – Give off continuous spectrum (all frequencies) of EM waves (electrons vibrate randomly) – Inefficient source of visual light unless temperature very high – most radiation is IR (heat) • Electrons moving between energy states in atoms, molecules, solids – Fluorescent light – Light-emitting diode (LED) – Laser Incandescence Incandescence • The frequency at which the most radiation is emitted by a hot body is proportional to its Kelvin temperature

fmax ∝ T • Radiation curve of brightness versus frequency for emitted light.

Spectrum of Incandescent Source Light Emission from Atomic Transitions • Light emission can be understood in terms of the “planetary” model of the atom. • Just as each element is characterized by the number of electrons that occupy the orbits or shells surrounding its atomic nucleus, each element also possesses its own characteristic pattern of electron shells, or electron energy states. • These states are found only at certain definite energies. We sometime call these allowed states electron quantum states, or Bohr orbits.

Electron States in Atom – Bohr Orbits Light Emission from Electron Transition • Electrons excited to higher energy quantum state by heating or passage of electric current; drop back

Photon Frequency & Energy

• Emission of photon as electron de-excites

• The frequency of an emitted photon ~ energy- level difference in de-exciting: E = hf

© 2010 Pearson Education, Inc. Photon Energy & Frequency • The photon (quantum wave-packet of light) has an energy = difference in electron state energies • The photon frequency is related to its energy by the Planck relation: E = hf where E = energy (in Joules); f = frequency (Hz); h = Planck’s constant = 6.63 x 10-34 J/Hz • For example, transitions to the 2nd Bohr orbit in Hydrogen from higher orbits give photons with frequencies in the visible range

Excitation CHECK YOURSELF

Which has less energy per photon?

A. Red light B. Green light C. Blue light D. All have the same.

Which has less energy per photon?

A. Red light B. Green light C. Blue light D. All have the same.

Explanation: In accord with E ~ f, the lowest-frequency light has the lowest energy per photon.

Excitation CHECK YOURSELF

Excitation is the process in which

A. electrons are boosted to higher energy levels in an atom. B. atoms are charged with light energy. C. atoms are made to shake, rattle, and roll. D. None of the above.

Excitation is the process in which

A. electrons are boosted to higher energy levels in an atom. B. atoms are charged with light energy. C. atoms are made to shake, rattle, and roll. D. None of the above.

Emission Spectra Spectroscope • Arrangement of slit, focusing lenses, and prism or diffraction grating to see emission spectrum of light from glowing element

• Each component of color is focused at a definite position according to frequency.

Emission Spectra Spectral lines emitted are characteristic of element

Most of what we know about atoms is gained by investigating the

A. masses of elements. B. electric charge of elements. C. periodic table of the elements. D. light they emit.

Emission Spectra CHECK YOUR ANSWER

Most of what we know about atoms is gained by investigating the A. masses of elements. B. electric charge of elements. C. periodic table of the elements. D. light they emit.

Explanation: The spectra of light emitted by atoms are © 2010 Pearson Education,considered Inc. to be the fingerprints of Absorption Spectra Absorption spectra • Atoms in a gas absorb light of the same frequency they emit.

• A spectroscope can detect “dark” lines in an otherwise continuous spectrum.

Absorption Spectrum of Hydrogen Fluorescence

Fluorescence • Many materials excited by ultraviolet light emit visible light upon de-excitation.

Fluorescence

• Many substances undergo excitation when illuminated with ultraviolet light.

© 2010 Pearson Education, Inc. Fluorescence • This excitation and de-excitation process is like leaping up a small staircase in a single bound, and • then descending one or two steps at a time rather than leaping all the way down in a single bound. • Photons of lower frequencies are emitted. UV light going in comes out a visible light,

Fluorescence Fluorescent lamps

• UV emitted by excited gas strikes phosphor material that emits white light.

An atom that absorbs a photon can then emit one

A. only at the same energy. B. of any energy depending on the situation. C. only at a higher energy. D. only at the same or lower energy.

Fluorescence CHECK YOUR ANSWER

An atom that absorbs a photon can then emit one

A. only at the same energy. B. of any energy depending on the situation. C. only at a higher energy. D. only at the same or lower energy.

• When excited, certain crystals as well as some large organic molecules remain in a state of excitation for a prolonged period of time. • Unlike what occurs in fluorescent materials, their electrons are boosted into higher orbits and some become “stuck.” • As a result, there is a time delay between the processes of excitation and de-excitation. • Materials that exhibit this peculiar property are said to have phosphorescence.

Phosphorescence • Atoms or molecules in these materials are excited by incident visible light. • Rather than de-exciting immediately, as fluorescent materials do, many of the atoms remain in a metastable state — a prolonged state of excitation— sometimes as long as several hours, although most de-excite rather quickly. • If the source of excitation is removed—for instance, if the lights are put out— an afterglow occurs while millions of atoms spontaneously undergo gradual de- excitation. • Many living creatures—from bacteria to fireflies and larger animals, such as jellyfish—chemically excite molecules in their bodies that emit light. Such living things are bioluminescent. © 2010 Pearson Education, Inc. Lamps Incandescent lamp • A glass enclosure with a filament of tungsten, through which an electric current is passed. • The hot filament emits a continuous spectrum, mostly in the infrared, with smaller visible part. • The glass enclosure prevents oxygen in air from reaching the filament, to prevent burning up by oxidation. • Argon gas with a small amount of a halogen is added to slow the evaporation of tungsten. • The efficiency of an incandescent bulb is 10%

Fluorescent lamps

• UV emitted by excited gas strikes phosphor material that emits white light. • About 40% efficient for visible light.

• Miniaturize a fluorescent tube, wrap it into a coil, and outfit it with the same kind of plug a common incandescent lamp has, and you have a compact fluorescent lamp (CFL). • CFLs are more efficient than incandescent lamps, putting out about 4 times more light for the same power input. • A downside to the CFL is its mercury content, which poses environmental disposal problems.

LED Lamps Light-emitting diodes (LEDs) • Monochromatic photons emitted when electrons change energy states in a solid • Low voltage source powers LED. • Very efficient at producing visible light -- about seven times better than incandescent light bulb. • Light is monochromatic, but can get white LED by combining RGB LEDs or by using blue LED + phosphorescent material

© 2010 Pearson Education, Inc. Lamp Efficiency • Incandescent (hot) source (Stars; light bulb) – Only about 10% power used gives visual light – most radiation is IR (heat) – 10 lumens/Watt • Atomic transition sources – Fluorescent light – 45-50 lumens/Watt – Light-emitting diode (LED)–70 lumens/Watt

Lamp Efficiency CHECK YOURSELF

Which lamp is more efficient for emitting visible light?

A. Incandescent lamp B. Fluorescent lamp C. Both the same for the same wattage D. None of the above.

Which lamp is more efficient for emitting visible light?

A. Incandescent lamp B. Fluorescent lamp C. Both the same for the same wattage D. None of the above.

Lasers & Coherent Light Most light sources give incoherent light

• Incoherent light (many frequencies and out of phase)

• Monochromatic light out of phase

Lasers Lasers can produce light that is monochromatic AND coherent

• Coherent light of identical frequencies in phase

• A device that produces a beam of coherent light • Many types and many ranges of light • Not a source of energy (as is sometimes thought)

KeyKey PointsPoints ofof LectureLecture 3535 • Types of light sources • Incandescent source • Light emission from atomic transitions • Emission Spectra • Absorption Spectra • Fluorescence • Phosphorescence • Lamps and Lamp Efficiency • Lasers

z Before Monday Nov. 29, finish Hewitt Chap. 29. z Homework #24 due by 11:00 PM Monday Nov. 29