![]() The light electronic transitions in atoms produces may not be in the visual part of the electromagnetic spectrum, but for atoms that are neutral or have lost only one or two electrons (yes, ‘atomic spectra’ refers to the line spectrum of ions too!), most lines are in the UV, visual, or near infrared. Of course, for an extragalactic object – a quasar, perhaps – you need more than one line to make a certain identification … because the universe is expanding (and so you don’t know how much just one line may have been redshifted). Note the sharp decrease in the intensity of radiation emitted at wavelengths below 400 nm, which constituted the ultraviolet catastrophe.As the atomic electron energy levels are unique to each element, the lines in a spectrum (emission or absorption) can be used to identify the elements present in the source (a star, say) or gas between the source and us (e.g. The white light spectrum shown for an object at 6000 K closely approximates the spectrum of light emitted by the sun. At high temperatures, all wavelengths of visible light are emitted with approximately equal intensities. As the temperature of the object increases, the maximum intensity shifts to shorter wavelengths, successively resulting in orange, yellow, and finally white light. At relatively low temperatures, most radiation is emitted at wavelengths longer than 700 nm, which is in the infrared portion of the spectrum. ![]() : Relationship between the temperature of an object and the spectrum of blackbody radiation it emits. There was no agreement between theory and experiment in the ultraviolet region of the blackbody spectrum. Since the intensity actually drops to zero at short wavelengths, the Rayleigh-Jeans result was called the ultraviolet catastrophe (Figure 1.2.1ĭashed line). ![]() A theory developed by Rayleigh and Jeans predicted that the intensity should go to infinity at short wavelengths. Attempts to explain or calculate this spectral distribution from classical theory were complete failures. One experimental phenomenon that could not be adequately explained by classical physics was blackbody radiation (Figure 1.2.1 Soon, however, scientists began to look more closely at a few inconvenient phenomena that could not be explained by the theories available at the time. Thus matter and energy were considered distinct and unrelated phenomena. ![]() The universe appeared to be a simple and orderly place, containing matter, which consisted of particles that had mass and whose location and motion could be accurately described, and electromagnetic radiation, which was viewed as having no mass and whose exact position in space could not be fixed. They could calculate the motions of material objects using Newton’s laws of classical mechanics, and they could describe the properties of radiant energy using mathematical relationships known as Maxwell’s equations, developed in 1873 by James Clerk Maxwell, a Scottish physicist. To understand how energy is quantized in blackbody radiationīy the late 19th century, many physicists thought their discipline was well on the way to explaining most natural phenomena. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |