![]() Radiant energy is characterized by its ability to transfer energy from one object to another. ![]() Radiant energy encompasses a broad spectrum of electromagnetic radiation, including solar energy, gamma rays, radio waves, X-rays, and visible light. ![]() It is a type of kinetic energy associated with the motion of electromagnetic waves. If photons of light with energy less than E o strike a metal surface, no single photon has enough energy to eject an electron, so no electrons are emitted regardless of the intensity of the light.Radiant energy refers to the electromagnetic energy that is propagated through space in the form of transverse waves. Einstein postulated that each metal has a particular electrostatic attraction for its electrons that must be overcome before an electron can be emitted from its surface ( E o = hν o). The key feature of Einstein’s hypothesis was the assumption that radiant energy arrives at the metal surface in particles that we now call photons. The energy of violet light is above the threshold frequency, so the number of emitted photons is proportional to the light’s intensity.Īlbert Einstein (1879–1955 Nobel Prize in Physics, 1921) quickly realized that Planck’s hypothesis about the quantization of radiant energy could also explain the photoelectric effect. (c) In contrast to predictions using classical physics, no electrons are emitted when photons of light with energy less than E o, such as red light, strike the cathode. If the incoming light is interrupted by, for example, a passing person, the current drops to zero. When light strikes the metal cathode, electrons are emitted and attracted to the anode, resulting in a flow of electrical current. (b) A photocell that uses the photoelectric effect, similar to those found in automatic door openers. As things turned out, Planck’s hypothesis was the seed from which modern physics grew.įigure 2.2.3 The Photoelectric Effect (a) Irradiating a metal surface with photons of sufficiently high energy causes electrons to be ejected from the metal. In time, a theory might be developed to explain that law. If quantization were observed for a large number of different phenomena, then quantization would become a law. Initially, his hypothesis explained only one set of experimental data-blackbody radiation. ![]() (you may need to install JAVA to run the applets).Īt the time he proposed his radical hypothesis, Planck could not explain why energies should be quantized. You can get a feel for this by clicking on the black body applet from PHeT below. The result is a maximum in the plot of intensity of emitted radiation versus wavelength, as shown in Figure 2.2.2, and a shift in the position of the maximum to lower wavelength (higher frequency) with increasing temperature. At any temperature, however, it is simply more probable for an object to lose energy by emitting a large number of lower-energy quanta than a single very high-energy quantum that corresponds to ultraviolet radiation. As the temperature of an object increases, there is an increased probability of emitting radiation with higher frequencies, corresponding to higher-energy quanta. We can understand Planck’s explanation of the ultraviolet catastrophe qualitatively as follows: At low temperatures, radiation with only relatively low frequencies is emitted, corresponding to low-energy quanta. By assuming that energy can be emitted by an object only in integral multiples of hν, Planck devised an equation that fit the experimental data shown in Figure 2.2.2. \]Īs the frequency of electromagnetic radiation increases, the magnitude of the associated quantum of radiant energy increases.
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