Wien's Law
All objects emit electromagnetic waves. The emitted light has a range of wavelengths, but one wavelength is brightest.
- Hotter objects emit more light.
- The hotter the object is, the shorter that peak wavelength becomes.
An interactive form of this graph is available at http://phet.colorado.edu/new/simulations/sims.php?sim=Blackbody_Spectrum
Getting a little more mathematical, the temperature (measured from absolute zero) is inversely proportional to the peak wavelength. This relationship between the temperature and the peak wavelength is called Wien's Law.
, where T is the temperature in Kelvin.
Recall that on the Kelvin scale, the "degrees" are the same size as Celsius degrees, but are counted from absolute zero, -273°C. So to convert a Celsius temperature to a Kelvin temperature, just add 273.
Example What is the peak wavelength of light emitted by the Sun's surface, at a temperature of 5800K?
Activities & Practice
to do as you read
1. What is the wavelength of the peak radiation you emit? (Hint: Human body temperature is 37°C.)
2. Old-style incandescent lightbulbs glow because a thin wire, the "filament" is heated to high temperature by an electric current running through it. These lightbulbs typically emit their brightest light at about 1000 nm, in the "near" infrared. What is the temperature of the lightbulb filament?
Additional Problems and Activities
3. The Earth’s average global temperature is about 15°C.
(a) What is the Earth’s average temperature in Kelvin?
(b) At what wavelength does the Earth glow most brightly?
4. How hot does an object have to be, so that the wavelength of its peak emission is 1.0 µm?