Every object sitting around you emits some sort of radiation, though you wouldn't consider them radioactive. That's because the radiation emitted is in the form of electromagnetic waves. If the object is relatively cool, the radiation is usually infrared and not visible. However, if the object is heated up, it will start to emit electromagnetic waves in the visible spectrum. The metal in the image is an example—when it is heated, it glows red. Objects at even higher temperatures will glow white or blue.

To study this phenomenon, scientists studied the ideal situation where all incoming radiation was absorbed. This type of body is called a blackbody. To observe what happens to a blackbody, scientists look at a black hollow object that has one small hole. The blackbody traps the incoming radiation, the radiation is for the most part fully absorbed. Any radiation escaping from the small opening has bounced around so much that the emitted radiation is the same wavelength of the walls of the object.

Using classical physics methods, scientists predicted that light would be emitted as soon as it had absorbed enough energy. Since shorter wavelengths provided higher energy, the light that the blackbody radiated should go to infinity. At lower wavelengths, the energy that was emitted would approach zero. At these lower wavelengths, the match to the classical predictions was pretty good; however, at the longer wavelengths, the experiment showed that almost no energy was emitted by the blackbody. The experimental results did not match what classical physics predicted.

As you can see, the predictions completely fell apart in the ultraviolet region. Because of this anomaly, scientists call this the ultraviolet catastrophe.