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Chemistry LibreTexts

1.1: Light

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    75267
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    If we are going to understand how light and matter interact, we need to know what light is. Isaac Newton favored a corpuscular theory of light, but his theory could not satisfactorily explain results such as those of Thomas Young's double slit experiment. Indeed Young's wave theory quickly overtook Newton's theory and would be the reigning paradigm for understanding light until the dawn of quantum mechanics in the early 20th century.

    Light is electromagnetic radiation

    Specifically, light is an oscillating electric and magnetic field, in which the fields are perpendicular to each other and the direction of propagation.

    554px-Light-wave.svg.png800px-Onde_electromagnétique.png

    \(c=\lambda\nu\)

    The above equation relates the speed of light, \(c\), to the wavelength, \(\lambda\), and frequency, \(\nu\), of light. Thus, if one knows the frequency, one can find the wavelength, and vice-versa. The various types of electromagnetic radiation are shown in the figure below.

    787px-EM_spectrum.svg.png

    Light is made of particles called photons

    While the above picture of light is all well and good, there were a few outstanding "issues" in physics at the end of the 19th and beginning of the 20th century. One of those issues was the photoelectric effect, in which light striking a metal surface ejects electrons from the metal. Play around with the photoelectric effect simulation available at PhET; you should find that while the number of electrons ejected depends on the intensity of the light, the energy of those electrons depends on the wavelength of the light. If the wavelength is too large (the frequency too low) for the particular metal, no electrons will be ejected at all. Indeed there is a characteristic threshold frequency for each metal below which there are no electrons ejected regardless of the intensity of the incident light (well, until you throw in so much energy you turn the metal into a plasma, but that is cheating, that is not the photoelectric effect).

    Albert Einstein won the Nobel Prize in Physics (1921) in part for his explanation of the photoelectric effect. While the wave picture of light is good for many things, sometimes, like when trying to understand the photoelectric effect, it is more useful to think of light as being made of particles. These particles, we call them photons, each carry a certain amount of energy, namely \[E=h\nu\] The energy of a photon, \(E\), is equal to Planck's constant, \(h\), (a number Max Planck found in his explanation of blackbody radiation, another of those minor issues physics was dealing with back then, and one we'll be seeing a lot of in this course) times the frequency, \(\nu\).

    We can now look back at the electromagnetic spectrum above and realize that it is also ordered by energy; gamma photons each carry more energy than ultraviolet photons and so on down the line.


    1.1: Light is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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