↳ Grades K-5: Math and Computer Science.↳ Grades K-5: Life, Earth, and Social Sciences.Active Forums (Make all new posts here).It's important not to get this equation mixed up with the inverse square law, which describes how light intensity changes with distance from the source. The purpose of this equation is to convert resistance (what you are actually measuring) to lux (the unit of light measurement). If you bought a different photoresistor, you would need to look at its datasheet to get the slope and intercept of the line. To be clear, the numbers 375.81 and 1.084 are specific to this photoresistor. But it would also be valid to write the equation as So the slope of the line is negative 1.084, we can write that as 1/R^1.084 instead of R^(-1.084). Trying to put that in words: x to a negative number is equal to one over x to that number. The last part is also a bit of algebra your grandson may or may not have seen yet: The y-intercept of the line is 375.81, and the slope of the line is -1.084. Where a is the y-intercept of the line and k is the slope of the line (for more information and some examples, see this page ). The equation for a straight line on a log-log plot is Again, depending on what grade your grandson is in, he might not have encountered logarithms in math class yet. A plot with logarithmic scales for both axes is called a log-log plot for short. However, if you look at the graph on the datasheet you will see that it has a logarithmic scale on both axes (e.g. The equation is derived from the graph labeled "Typical Resistance vs Variable Illumination" on the photoresistor's datasheet:ĭepending on what grade your grandson is in, he is probably familiar with the equation for a line (usually expressed as y = m*x + b) on a graph with linear axes. Here's the more detailed explanation from our email: In case you didn't see it yet, there is a very brief explanation of this equation on the "Make it Your Own" tab of the project: Pasting the full reply here so others can see it as well. Radiance is also sometimes called intensity, especially by astronomers and astrophysicists, and in heat transfer.We sent the following message as a reply to your email - please let us know if that email didn't go through (if so, we apologize). This can cause confusion in optics, where intensity can mean any of radiant intensity, luminous intensity or irradiance, depending on the background of the person using the term. In photometry and radiometry intensity has a different meaning: it is the luminous or radiant power per unit solid angle. The intensity should then be defined as the magnitude of the Poynting vector. ![]() For example, an evanescent wave may have a finite electrical amplitude while not transferring any power. The treatment above does not hold for arbitrary electromagnetic fields. I = c n ε 0 2 | E | 2, įor non-monochromatic waves, the intensity contributions of different spectral components can simply be added. This is an example of the inverse-square law.Īpplying the law of conservation of energy, if the net power emanating is constant, If a point source is radiating energy in all directions (producing a spherical wave), and no energy is absorbed or scattered by the medium, then the intensity decreases in proportion to the distance from the object squared. ![]() For example, the intensity of an electromagnetic wave is proportional to the square of the wave's electric field amplitude. The intensity of a wave is proportional to the square of its amplitude. The resulting vector has the units of power divided by area (i.e., surface power density). Intensity can be found by taking the energy density (energy per unit volume) at a point in space and multiplying it by the velocity at which the energy is moving. The word "intensity" as used here is not synonymous with " strength", " amplitude", " magnitude", or " level", as it sometimes is in colloquial speech. For example, one could calculate the intensity of the kinetic energy carried by drops of water from a garden sprinkler. Intensity can be applied to other circumstances where energy is transferred. ![]() Intensity is used most frequently with waves such as acoustic waves ( sound) or electromagnetic waves such as light or radio waves, in which case the average power transfer over one period of the wave is used. In the SI system, it has units watts per square metre (W/m 2), or kg⋅ s −3 in base units. In physics, the intensity or flux of radiant energy is the power transferred per unit area, where the area is measured on the plane perpendicular to the direction of propagation of the energy. For other uses, see Intensity (disambiguation).
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