Lecture 2: Thermal Radiation- Nailing the Planck Curve

"It isn't the heat, its the humidity."
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Date: January 26, 1995
Reading Assignment: pp. 62-78

Description : overview of the EM spectra, thermal radiation, and the Doppler shift

Objectives

be able to list the 5 major components of the electromagnetic spectra

be able to distinguish explain the relationships between energy, wavelength, and frequency for EM radiation

be able to apply the ideas of energy, wavelength and frequency to the 5 major components of electromagnetic energy

understand that the atmosphere blocks most of the electromagnetic wavelengths that arrive from space

be able to understand the differences between energy, energy flux, and intensity

be able to apply the Wien's law to calculate the wavelength of peak intensity of a perfect thermal radiator

be able to apply Stephan's law to determine the energy flux of a perfect thermal radiation

Lecture Outline

Slide # 1: Thermal Radiation

Nailing the Planck Curve

Slide # 2: The Universe- A Quick Inventory

What is the Universe like?

What is it made of?

What types of objects are in it?

Slide # 3: The Universe - composition

everything that we can observe

contains about 100 billion galaxies

made mostly of stars and hydrogen gas

Slide # 4: The Universe - size and age

about 15 billion light years to the edge

Slide # 5: TodayÕs Lecture Objectives

identify the 5 regions of the EM spectra

distinguish between energy, wavelength and frequency in EM radiation

apply these ideas to the EM spectra

understand how atmospheric opacity blocks most of the EM spectra

apply WienÕs law

apply StephanÕs law

apply the Photoelectric effect

Slide # 6: Visible Light (GRAPHICS) Slide # 7: Cycles of Space and Time (GRAPHICS)

Wavelength, frequency, and velocity are related

Slide # 8: Cycles of Space and Time (GRAPHICS)

velocities of waves are usually constant

frequency and wavelength are inversely proportional

Slide # 9: Wavelengths of Visible Light

Red light = 7 x 10-5 cm

0.00007 cm

Blue light = 4 x 10-5 cm

0.00004 cm

Slide # 10: Red Light vs Blue Light

Red has a longer wavelength

Red has a lower frequency

Remember: Longer wavelengths = lower frequencies

Slide # 11: Electromagnetic Waves

wavelengths > 7 x 10-5 cm are invisible to humans

wavelengths < 4 x 10-5 cm are invisible to humans

longer and shorter wavelengths are also electromagnetic radiation

Slide # 12: The Electromagnetic Spectrum

Radio

Infrared

Visible

Ultraviolet

X-ray

Gamma-ray

Slide # 13: Radio

Wavelengths > 1 mm

Frequencies 0 to 300 Giga-Hz (3x1011Hz)

Typical Examples

AM/FM Radios

TV signals

Slide # 14: Infrared

Wavelengths between 1mm and 7 x 10-5 cm

Frequencies between 3 x 1011 Hz and 4 x 1014 Hz

Typical Examples

Heating devices

stove burners

space heaters

TV remote controls

Slide # 15: Visible

Wavelengths between 7 x 10-5 cm and 3 x 10-5 cm

Frequencies between 4 x 1014 Hz and 1 x 1015 Hz

Typical Examples

All that you see

Slide # 16: Ultraviolet

Wavelengths between 3 x 10-5 cm and 3 x 10-7 cm

Frequencies between 1 x 1015 Hz and 1 x 1017 Hz

Typical Examples

Sunlight which causes sunburn and suntan

Slide # 17: X-rays

Wavelength between 3 x 10-7 cm and 3 x 10-10 cm

Frequencies between 1 x 1017 Hz and 1 x 1020 Hz

Typical Example

Medical X-ray Machines

Slide # 18: Gamma-Rays

Wavelengths < than 3 x 10-10 cm

Frequencies > 1 x 1020 Hz

Typical Examples

Nuclear reactions

Nuclear bombs

Slide # 19: Atmospheric Blockage

some wavelengths are blocked by the atmosphere

Slide # 20: Opacity

the extent which a wavelength is blocked

opaque is the opposite of transparent

Slide # 21: Atmospheric Opacity- Why does it happen?

gases absorb and scatter light

the amount of absorption depends on

the type of molecule

the wavelength of light

Slide # 22: Atmospheric Opacity - Some Examples

Water and Oxygen absorb some radio frequencies

Carbon Dioxide absorbs some infrared frequencies

the Ozone layer blocks most Ultraviolet, X-ray and Gamma-rays

Slide # 23: Constellation Corner (GRAPHICS)

The Constellation de Jour

Slide # 24: Ursa Minor (GRAPHICS)

Fairfax - Feb 1 - 10pm - NNE - 6.0

Slide # 25: Ursa Minor (GRAPHICS)

Fairfax - Feb 1 - 10pm - NNE - 6.0

Slide # 26: Ursa Minor (GRAPHICS)

Fairfax - Feb 1 - 10pm - NNE - 5.0

Slide # 27: Ursa Minor (GRAPHICS)

Fairfax - Feb 1 - 10pm - NNE - 4.0

Slide # 28: Ursa Minor (GRAPHICS)

Fairfax - Feb 1 - 10pm - NE

Slide # 29: Intensity

the strength of radiation

how bright something appears

proportional to number of photons per second per unit area seen by an observer

Slide # 30: Temperature

a measure of an objectÕs heat

Kelvin scale = Celsius Scale + 273

Nothing is below zero Kelvin

Slide # 31: Thermal Radiation

Intensity is only related to temperature

Independent of the type of material

Slide # 32: Thermal Radiation Spectra

The Planck Curve is one type of spectra

curve of a perfect thermal emitter

Also known as the Òblack-body curveÓ

Slide # 33: A Planck Curve (GRAPHICS)

the signature of thermal emission

Slide # 34: The Planck Curve (GRAPHICS)

The Shape of the Curve is Independent of

Temperature

Composition

Slide # 35: The Planck Curve (GRAPHICS)

The position of the peak and the area under the curve depend on Temperature

Slide # 36: The Planck Curve

Two laws describe the Planck Curve

WienÕs Law

StephanÕs Law

Slide # 37: WienÕs Law (GRAPHICS)

Describes the wavelength of the Peak of the Planck Curve

Slide # 38: WienÕs Law

Peak Wavelength in cm

T in Kelvin

Slide # 39: WienÕs Law (GRAPHICS)

changing temperature changes the peak wavelength

Slide # 40: WienÕs Law

If you can measure the Peak Wavelength, you can determine the Temperature

The peak wavelength is determined using spectroscopy

Slide # 41: StephanÕs Law

Describes the flux from a black-body

Flux is the

amount of energy

per second

per square centimeter

A measure of how bright a Surface appears

Slide # 42: StephanÕs Law (GRAPHICS)

energy radiated depends on the fourth power of Temperature

Slide # 43: StephanÕs Law (GRAPHICS)

sigma is

Stephan-Boltzmann constant

5.67 x 10-5 erg/s/cm2/K4

Slide # 44: StephanÕs Law (GRAPHICS) Slide # 45: Applications of the Planck Curve

The Sun is approximately a black-body

temperature is about 6000K

Planets emit most of their infrared energy as black-bodies

peak of the spectrum tells us the planetÕs average temperature

Slide # 46: Planck Curves

one way to tell the temperature and flux of distant objects

Slide # 47: Units Review

Hertz (Hz) = cycles per second

Angstroms = 10-10 meters = 10-8 cm

Ergs - unit of energy

very small unit

100 watt light bulb = 109 erg/second

Slide # 48: Thermal Radiation vs Photoelectric Effect

StephanÕs and WienÕs laws are for thermal radiators

the Photoelectric effect applies to individual photons

Slide # 49: The Photoelectric Effect

Blue Light

wavelength = 4000 Angstroms

frequency = 7.5 x 1011 Hz

Energy/photon =5.0 x 10-15 ergs

Red Light

wavelength = 7000 Angstroms

frequency = 4.3 x 1011 Hz

Energy/photon = 2.8 x 10-15 ergs

Slide # 50: PlanckÕs Constant - h

6.63 x 10-27 erg seconds

relates frequency in Hz to energy in ergs

only applies to individual photons

Slide # 51: Photoelectric Effect

Discovered by Albert Einstein

Einstein receive Nobel Prize for this work (1921)