Light bulbs, Electric Ranges, and Stars: Thermal Radiation Explained

Posted: August 19, 2013 in Science & Technology

Ever wonder what happens when you turn on your electric range?  Well, it’s like this: electric ranges are made from nichrome, which possesses the important property of high electrical resistance.  When you flip the switch, your alternating current (AC) mains voltage is placed across it, causing electrons to slosh back and forth through the burner 60 times a second.  The high resistance of the nichrome is a complicated topic of solid-state physics and has to do with the electronic band structure of the atoms, but you can think of it like this: the atoms in the burner block the flow of electrons.  Some of the electrons breeze right through, but others collide with the atomic nuclei at full speed, transferring some of their energy.  The electrons don’t have nearly enough energy to actually dislodge one of these atoms from their place in the material, but they do have enough energy to make the atoms vibrate.

Well, the magnitude of the vibrations of the atoms in a material is exactly what we mean when we talk about temperature.  Run a current through a piece of nichrome, and it gets hot.  Touch it, and that vibrational energy will transfer to your finger.  Unfortunately, the molecules of human flesh don’t handle that much kinetic energy very well.  The proteins will denature, cell walls fly apart, etc etc and it’s just generally a mess.

But what we’re interested in today isn’t how the burner feels to the touch — it’s how the burner feels from a distance.  Hold your hand over the burner, and you’ll feel heat.  A lot of what you feel is high-temperature air expanding upward from the burner.  But now hold your hand to the side of or underneath the burner.  You’ll still feel some expanding air, but now most of the heat you feel is being transferred through a different mechanism: electromagnetic radiation in the infrared frequency range.

You see, electromagnetic theory tells us that a charged particle (electron or proton) generates an electric field at all times, and a magnetic field while it is moving.  Relativistic electromagnetic theory tells us that when a charged particle accelerates, it produces field distortions that that actually radiate energy into the surrounding space.  So what happens when a charged particle is vibrating back and forth rapidly?  It produces electromagnetic waves at the frequency of its oscillation.

The atoms in the nichrome of the electric burner are effectively electrically neutral — each atom has an equal number of protons and electrons.  However, the electrons passing through the material are transferring their kinetic energy only to the nuclei.  Electrons can deflect each other through their electric fields, but a direct electron-electron collision has never been observed.  In fact, we’re not even really sure how small the electron really is — it’s effectively pointlike.  So the effect of the current is to cause the nuclei to begin vibrating, and as they do so, they drag their electron clouds with them.  The net effect is that the positive and negative charges within each atom become displaced from each other, forming an electric dipole.  The dipole isn’t static — the positive and negative ends are moving away from each other, reaching their limit, moving toward each other, overlapping, and moving away from each other again many times a second.  As they do so, they emit electromagnetic waves at the frequency of their oscillation.  In the case of the nichrome burner, the atoms are vibrating trillions of times a second and emitting high-energy infrared radiation.

This radiation travels through space at the speed of light and strikes your hand.  Now the reverse process happens.  As the electromagnetic waves pass into your flesh, they excite oscillatory motion in the dipole moments of the atoms and molecules there.  Your hand becomes hot, and you feel that heat.

( As an aside, this is similar to how microwave ovens work.  Microwaves are longer and lower frequency than infrared, so the mechanism is slightly different.  Instead of instigating dipole oscillation inside molecules, microwave ovens set up standing waves within a resonant cavity.  Then molecules with permanent dipole moments, like, water, must rotate to align themselves with the field as it rapidly oscillates, producing vibration. )

But infrared isn’t the only type of electromagnetic radiation coming out of that stovetop.  There’s also the kind you can see.  As the burner heats up, it gets redder and brighter.  This is electromagnetic radiation in the visual wavelength range, only slightly higher energy than infrared.  The thing about waves in general is that they can only drive oscillators at frequencies close to the natural resonant frequency of the oscillator.  Visible electromagnetic waves are too high in frequency to drive a dipole oscillation within an atom, but they’re high enough in frequency to be absorbed and scattered by electrons.  That’s how your eye detects them — electrons in your retina absorb the light, setting off one of several chemical reactions (depending on the incident wavelengths) that send signals to your brain along the optic nerve.

Though these two types of radiation are distinguishable on a certain qualitative basis, really, they’re both just electromagnetic radiation in two arbitrarily defined wavelength ranges.  The electric stovetop emits radiation along a continuous spectrum, with a power distribution determined by Planck’s Law.  I won’t give the formula, because I’m trying to keep this treatment mathless, but here’s a picture of it, courtesy of Wikipedia:



As you can see, the peak wavelength is determined by the temperature.

Tungsten filament light bulbs work the same way as electric ranges.  They get hotter and so emit more of their radiation in the visible range, but still produce a large amount of infrared, which is why incandescent bulbs are such a dangerous household appliance.  Since the invention of the incandescent filament, the lighting industry has moved on to other technologies, such as fluorescent bulbs, and now Light Emitting Diodes.  LEDs display a phenomenon known as electroluminescence, which is far more efficient than incandescence at producing visible light.  But that’s a topic for another article.  If you’d like to know more about LED lighting, visit  If you’d like to buy some LED lighting fixtures, visit

  1. mistaben says:

    Very nice, Morgan. I had a vivid illustration of this phenomenon when I was first learning about it in college. My basement apartment had a motion-sensitive light fixture above the door, and once when I was backing out my car the light turned off right as I looked at it. More accurately, I saw that the bulb took a finite amount of time to go out; it was a quick dimming rather than a sudden winking out.

    I had a sudden and vivid mental image like the one above, but animated over the time it took the light bulb to turn off. In my mind’s eye I saw the power distribution of the bulb shift smoothly to the right so that the portion of it in the visible range quickly shrank down to almost nothing. It was very cool to see Planck’s Law in action. What’s astonishing to me is that Planck’s radical insight that finally allowed for a theory that actually matched reality (cf. the black curve labeled “Classical Theory (5000 K)” on the above image) also ushered in the entire revolution that is 20th century physics.

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