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9_Luminescence_Fluorescence.html
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<p class="head">Luminescence and Fluorescence</p>
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<p>Before an <a class="black" href="https://cvertan.github.io/physics4dh.github.io/5_Atom.html" target="frameterms">atom</a> emits a <a class="black" href="https://cvertan.github.io/physics4dh.github.io/2_Photon.html" target="frameterms">photon</a>, an <a class="black" href="https://cvertan.github.io/physics4dh.github.io/5_Atom.html" target="frameterms">electron</a> in the atom must first be excited up to higher <a class="black" href="https://cvertan.github.io/physics4dh.github.io/6_Energy%20levels.html" target="frameterms">energy level</a>. An electron can become <a class="black" href="https://cvertan.github.io/physics4dh.github.io/6_Energy%20levels.html" target="frameterms">excited</a> if extra energy is given. <a class="def">Luminescence</a> is <a class="black" href="https://cvertan.github.io/physics4dh.github.io/8_Emission.html" target="frameterms">spontaneous emission</a> of the photon, when an atom is excited by an external energy source other than heat. </p>
<p> There are many different types of luminescence differ from each other in the way how excited atom was produced: </p>
<p>- as a result of absorption of photon – <b>fluorescence</b>; </p>
<p>- as a result of a chemical reaction – <b>chemiluminescence</b>; </p>
<p>- as a result of an electric current passed through a substance – <b>electroluminescence</b>; etc. </p>
<p><a class="def">Fluorescence</a> is a type of luminescence involving electron excitation by <a class="black" href="https://cvertan.github.io/physics4dh.github.io/7_Absorption.html" target="frameterms">absorption</a> of a photon. Video shows the process of fluorescence: at first, the electron is excited absorbing the photon, then falls to the ground state releasing the photon of another wavelength. </p>
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<source src="Videos/Fluorescence.mp4" type="video/mp4"> </video>
<p>Being in the excited state the electron can lose some <a class="black" href="https://cvertan.github.io/physics4dh.github.io/20_Energy.html" target="frameterms">energy</a> (in the form of heat) and therefore the emitted photon has lower energy, than the absorbed photon. As far as photon energy is related to its wavelength and frequency, according to <a class="black" href="https://cvertan.github.io/physics4dh.github.io/3_Photon%20energy.html" target="frameterms">Planck–Einstein relation</a>, the emitted photon has a longer <a class="black" href="https://cvertan.github.io/physics4dh.github.io/15_Wave.html" target="frameterms">wavelength</a> (lower <a class="black" href="https://cvertan.github.io/physics4dh.github.io/15_Wave.html" target="frameterms">frequency</a>), than the absorbed photon. A perceptible example of fluorescence occurs when the absorbed radiation is in the <a class="black" href="https://cvertan.github.io/physics4dh.github.io/11_ultraviolet.html" target="frameterms">ultraviolet region</a> of the <a class="black" href="https://cvertan.github.io/physics4dh.github.io/13_EM_spectrum.html" target="frameterms">electromagnetic spectrum</a> (invisible to the human eye), while the emitted light is in the <a class="black" href="https://cvertan.github.io/physics4dh.github.io/18_visible_light.html" target="frameterms">visible region</a>. As an example, Figure demonstrates green fluorescent security strip in a US twenty-dollar bill under <a class="black" href="https://cvertan.github.io/physics4dh.github.io/11_ultraviolet.html" target="frameterms">UV light</a>. </p>
<p><a class="black" href="https://cvertan.github.io/physics4dh.github.io/8_Emission.html" target="frameterms">Lifetime</a> of an electron in exited level is very short (about 10<sup>-3</sup>-10<sup>-9</sup> s), wherefore, for the human eyes, emission of light occurs immediately after absorption of the light, so that under illumination of the object by light of one color, one can see the object glowing with light of another color. As an example, the Figure shows fluorescent minerals emitting visible light when exposed to ultraviolet. </p>
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<img src="Figures/L1_fig.jpg" width="600" height="400">
<div class="desc">Fluorescent security strip in a US twenty dollar bill under UV light ©<a href="https://en.wikipedia.org/wiki/Fluorescence" target="_blank">Wikipedia</a></div>
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<img src="Figures/L2_fig.jpg" width="600" height="400">
<div class="desc">Fluorescent minerals emit visible light when exposed to ultraviolet ©<a href="https://en.wikipedia.org/wiki/Fluorescence" target="_blank">Wikipedia</a> </div>
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<p class="important">More on the subject: <a href="" target="_blank">www.youtube.com</a>, <a href="https://www.britannica.com/science/luminescence" target="_blank">www.britannica.com </a></p>
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