PHYSICS 20A Lecture Notes - Lecture 10: James Clerk Maxwell, Electric Field, Photon
Physics 20A – Lecture 10 – Blackbody Radiation
Light
• Electromagnetic radiation that carries energy
• Why do hot objects emit light?
o Light is a wave of electric and magnetic fields
▪ Electric charges create fields around them that attract or repel other charges
▪ Magnets create fields around them that deflect the path of charged particles
▪ Wiggling a charged particle sets up oscillations in the electric field around it
▪ A changing electric field creates a magnetic field and vice versa
− They propagate as a wave
▪ Scottish physicist James Clerk Maxwell
− One of the four forces of nature is electromagnetism
− First to say electricity, magnetism, and light are related
➢ All aspects of the same underlying set of physical laws
− 1862 – calculated the speed of an electromagnetic wave = very close to c
o Hot objects radiate
▪ Molecules in hot gas scatter and vibrate rapidly
▪ That motion creates electromagnetic waves i.e. light
▪ Photons and molecules in equilibrium with each other → black-body spectrum
▪ What radiates like a black-body?
− Hot embers
− Hot lava
− The sun and stars
− Light bulbs
▪ Black-body – the simplest possible object that gives off radiation
− Absorbs all radiation
− Nothing reflected or transmitted
− Atomic motions within the object cause it to emit energy in a way that depends
only on its temperature, not on chemical composition or anything else
− Emit light in a distribution of wavelengths
▪ Wien’s law – the peak of the distribution λmax shifts to smaller wavelengths as the
temperature of the body increases
− λmax = 3000Å[(104K)/T]
− Stefan-Boltzmann law – energy per second emitted by black-body increases with
temperature
➢ ET4 – Hotter objects emit more energy per unit time
− Ex. Star’s surface temp = 6000 K, what wavelength will its spectrum peak (λmax)?
➢ λmax = 3000Å[(104K)/6000K]
= 3×103Å[(104K)/(6×103K)]
= (3/6)×103Å(104/103)
= 1/2×103Å(104-3)
= 1/2×103Å(10)
= 1/2×104Å
= 5×103Å
= 5,000Å
− What’s your spectrum?
➢ Person’s T = 98F ≈ 300K
➢ λmax = 3000Å[(104K)/300K]
= 3000Å(33.33…)
= 100,000Å (infrared light; heat vision?)
➢ Visible is around 4000 to 7000Å
➢ We don’t see the light we emit
➢ But we do see the light reflected off us and each other from the sun and
other light sources
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