If you put a clear incandescent light bulb in a glass of water, place the pair in a microwave oven, and turn the oven on, the bulb will emit light.
1. First, the filament of the bulb will begin to glow red, orange, yellow, white, or even blue-white. The microwaves are heating the filament. How?
Answer: The microwaves are driving electric charges back and forth through the wires and filaments and this alternating current heats the filament
Why: The filament is thin and cannot tolerate large currents without becoming very hot. The electric fields in the microwaves push charges back and forth through the filament and this moving charge is enough of a current to heat the filament to incandescence.
2. The color of the hot filament is limited to a particular class of colors: red, orange, or yellow. Why can't the hot filament appear green?
Answer: The spectrum of thermal radiation is too broad to appear green, regardless of the filament temperature.
Why: Green light falls in a relatively narrow range of the visible spectrum. While there are discharge or fluorescent lamps that can emit green light, a hot object emits too broad a spectrum of light to appear green.
3. After a few seconds, the gas in the bulb will also begin to emit light. You may even begin to see sparks emerge from sharp corners of the filament support wires inside the bulb and these sparks are what initiate the glow of the gas. Why do the sharp metal corners inside the bulb initiate sparks when the microwave oven is on?
Answer: Charge accumulations on the sharp corners lead to corona discharges and allow charge to enter the gas.
Why: To get the gas glowing in the microwaves, something must inject free electric charges into that gas. This injection can occur because of corona discharge at a sharp point or because of thermionic emission from the heated filament. Charge accumulation at a sharp point occurs easily in the microwave oven because the microwave electric fields push charges around in the metal wires and those charges often pile up in one place or another.
4. The gas inside the bulb becomes a bit warm, but the light the gas emits isn't caused by the gas's temperature. What determines the colors of light emitted by this glowing gas?
Answer: The colors are characteristic of the structures of the atoms and molecules in the gas.
Why: Once charges begin to move through the gas inside the bulb, that gas begins to emit light characteristic of its constiuent atoms and molecules. The impacts of the charges on those atoms and molecules cause those atoms and molecules to become electronically excited and they emit light through radiative transitions between electron orbitals.
A blacksmith can judge the temperature of a piece of hot iron by looking at the light it emits.
5. As the iron becomes hotter in the furnace, how do the wavelengths of light emitted by the hot iron change?
Answer: They become shorter and shorter, on average.
Answer: The hotter the iron becomes, the faster its charged particles vibrate back and forth and the shorter the wavelengths of light become. The fraction of bluish light (short wavelengths) increases while the fraction of redish light (long wavelengths) decreases.
6. When the blacksmith cools a red hot piece of iron in a bucket of water, the piece stops glowing and appears black. What has happened to the iron's thermal radiation?
Answer: The thermal radiation emitted by the cool iron is all in the infrared and cannot be seen by eye.
Why: All objects emit thermal radiation, but below about 400 C, you can't see it with your eyes. It's all in the infrared, at wavelengths too long for eyes to detect.
7. As the piece of iron was approaching the bucket of water, the blacksmith could see its reflection in the surface of the water. Why does light reflect from the surface of the water and not from some layer an inch below that surface?
Answer: Light reflects when it experiences an abrupt change in speed. Such a change occurs when the light tries to enter water from air, but not when light simply moves through water.
Why: Light slows as it enters the water's surface, causing about a 4% reflection. That's why the blacksmith sees an image of the iron in the water. But when light passes from one part of the water to another part of the water, its speed remains constant and it doesn't reflect.
8. Light from the blacksmith's white-hot furnace passes through a cut crystal vase and produces a spray of rainbow-like patterns on the wall. What process allows the cut crystal to separate the colors of light from the furnace?
Answer: The speed at which light travels through the crystal depends on the wavelength of that light. Because of this dispersion, light refracting as it passes into and out of the crystal facets bends differently according to its wavelength and the different wavelengths (colors) of light become separated from one another.
Why: Lead crystal is a type of glass that has considerable dispersion. It slows blue light much more than red light, so that blue light bends much more during refraction than does red light. The different wavelengths of light thus follow different paths through the crystal vase and emerge in different directions from its facets.