Einstein was able to use Plancks quantization hypothesis to explain the photoelectric effect. Radiant intensity, a radiometric quantity measured in watts per steradian (W/sr) Luminous intensity, a photometric quantity measured in lumens per steradian (lm/sr), or candela (cd) Irradiance, a radiometric quantity, measured in watts per square meter (W/m 2). The relationship given by Planck's radiation law, given below, shows that with increasing temperature, the total radiated energy of a body increases and the peak of the emitted spectrum shifts to shorter wavelengths. Einstein recognized that this could be explained by Plancks equation where one could define a 'particle of light' or photon as having a fundamental energy of h. Several measures of light are commonly known as intensity. While Planck originally regarded the hypothesis of dividing energy into increments as a mathematical artifice, introduced merely to get the correct answer, other physicists including Albert Einstein built on his work, and Planck's insight is now recognized to be of fundamental importance to quantum theory.Įvery physical body spontaneously and continuously emits electromagnetic radiation and the spectral radiance of a body, B ν, describes the spectral emissive power per unit area, per unit solid angle and per unit frequency for particular radiation frequencies. In 1900, German physicist Max Planck heuristically derived a formula for the observed spectrum by assuming that a hypothetical electrically charged oscillator in a cavity that contained black-body radiation could only change its energy in a minimal increment, E, that was proportional to the frequency of its associated electromagnetic wave. Īt the end of the 19th century, physicists were unable to explain why the observed spectrum of black-body radiation, which by then had been accurately measured, diverged significantly at higher frequencies from that predicted by existing theories. In physics, Planck's law (also Planck radiation law : 1305 ) describes the spectral density of electromagnetic radiation emitted by a black body in thermal equilibrium at a given temperature T, when there is no net flow of matter or energy between the body and its environment. The classical (black) curve diverges from observed intensity at high frequencies (short wavelengths). Shown here are a family of curves for different temperatures. From the equation above, a doubling of the light intensity (X2) causes a 18 mV rise in V OC. Planck's law accurately describes black-body radiation. Therefore, under concentration, V oc increases logarithmically with light intensity, as shown in the equation below where X is the concentration of sunlight. If you cut a 1 square meter large hole in the sphere, 1 lumen of light would come out of that hole. Not to be confused with Planck relation or Planck's principle. The candle produces light intensity of 1 candela and emits light uniformly in all directions.
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