Six Chapter 7

Black Body Radiation

• There have been two major discoveries in physics made in the early 20^th century
  • Relativity
  • Quantum theory
• These resulted in significant changes to our view of reality.
• Relativity – Discovered by Einstein.
• Quantum theory – developed over 30 years by many scientists.
• Max Planck was a physicist who made significant contributions to Quantum Theory as a result of trying to explain Black Body radiation.
• Black body – an ideal emitter or absorber of radiation.
  • Absorbs all light striking it, but can emit light.
  • Reflects no light
  • Doesn’t really exist, but objects can approximate these characteristics.
• Black body radiation – the radiation emitted from a black body. This radiation is electromagnetic radiation.
• During Planck’s time (end of 19^th century) researchers were observing the spectrum of light emitted by hot solids and gases.
• For hot solids, they noted that as the temperature increased, different types of light were radiated
  • Infrared was first radiated.
  • Dull red as well as infrared was next emitted.
  • Next was orange-red, dull red and infrared.
  • With continued increase in temperature yellow, orange-red, dull red and infrared.
  • Finally the object became white hot. It emitted all the colors of the visible spectrum as well as infrared.
• Generally
  • The higher the temperature, the greater the number of frequencies of light emitted (The whiter the glow)
  • Relative brightness of colors radiated depends upon the temperature
  • As temperature increases, the spectrum emitted retained lower frequencies, but added higher frequency radiations.
  • A graph of relative intensity of emitted radiation versus frequency (wavelength) for different temperatures looks like these.

• Maxwell’s theory explained radiation from objects as resulting from oscillations of electrons in atoms with the frequency increasing with temperature.
• Maxwell’s theory also predicted that the intensity of radiation (high points in the curves on the graphs above) from Black Bodies should continue to increase with frequency (dashed lines in the diagrams).
• The region of divergence between prediction and actual observations is in the ultra violet region of the electromagnetic spectrum.
• Ultraviolet catastrophe – the difference between the classical predicted curve and the actual curve for black body emission.
• Max Planck (Plonk) explained this ultraviolet catastrophe and the shape of these graphs by using the quantum model and statistical methods. He suggested:
  • That vibrating molecules in a heated material can only vibrate with a specific quantity of energy (a quantum idea).
  • When energy was radiated, it was not emitted continuously but in bundles or packets called quanta (a quantum idea).
  • The energy emitted was a whole number times the minimum energy emitted when the molecules first radiate (a quantum idea).
  • The energy of a quanta is proportional to the frequency.

E = hf

• h = 6.63 X 10^-34 Js
• f = frequency
• E = energy of a quanta
• The equation E = hf tells us that red (lower frequency) has low energy and ultraviolet (higher frequency) has higher energy.
• Quantized values is not a new concept
  • Dalton’s idea of atoms is quantization
  • J.J. Thomson showed that charge is quantized.
• Planck was saying that energy is quantized.
• Planck’s theory is revolutionary because:
  • It challenged the wave model of light by suggesting light does not transmit energy in a continuous way, but in small packets or bundles.
  • It challenged Newton’s physics by saying that objects cannot vibrate with any random energy, because energy is restricted to certain values.
• Planck was skeptical about his theory and hoped that a better theory would be developed.
  • His hypothesis became the foundation of a new era in physics.

Applications of Black Body Radiation

• Greenhouse Effect - Is the absorption of short wavelength radiations which are re-radiated as long wavelength radiations. These long wavelength radiations are then trapped by some process.
• A greenhouse operates this way
  • Short wavelength electromagnetic radiation enters through the windows.
  • Short wavelength electromagnetic radiation is absorbed by the soil inside the greenhouse.
  • The soil radiates long wavelength electromagnetic radiation.
  • The long wavelength electromagnetic radiation is not able to pass through the windows and so it is trapped inside the greenhouse.
  • The reason the light is emitted at longer wavelengths is because the earth is at lower temperature than the sun. The left hand graph above, shows us that the high point in the curves is at longer wavelengths when the temperature is lower. This high point represents the most common frequency of electromagnetic radiation emitted from an object of given temperature.
  • In the Earth's atmosphere pollutants such as carbon dioxide remains transparent to incoming short wavelength radiation, but not to the outgoing long wavelength radiation. Therefore the energy of this long wavelength radiation gets absorbed and the atmosphere heats up
  • In reality, in order for the Earth to sustain life, there must be some greenhouse effect. If there was no greenhouse effect, the Earth would be a frozen ball of ice.
  • The concerns related to global warming are that we are increasing the carbon dioxide in the atmosphere, thereby increasing the greenhouse effect, and heating up the planet unnaturally.

Crooke's Radiometer

• Used to detect electromagnetic waves.

• The vanes are dark on one side and silvered on the other
• The dark side of the vanes absorb the electromagnetic radiation and the silvered side reflect the electromagnetic radiation. The quanta of light transfer their momentum to the vanes on one side of the vanes but not the other. The result is that the vanes turn on the pivot point.
• The speed of rotation is a measure of the amount of electromagnetic radiation present.
• The most common detector of infrared radiation is photographic film or a television camera which is sensitive to infrared.
• Can be used to detect areas of heat loss from a house.
• Heat images of the human body indicate areas of infection or locations of tumors
• Infrared satellite photoes show
  • type of crops being grown
  • population densities of urban areas
  • distribution of acid rain
  • locations of military installations.

January 19, 2014