Reflection and Absorption

Objects may also emit electromagnetic radiation by absorbing or reflecting electromagnetic radiation from other sources. For instance, imagine a picnic table sitting in the sun. The sun is producing electromagnetic radiation as a result of its temperature, as is the table. The sun is so hot that it is producing electromagnetic radiation in the visible range, whereas the table is only producing electromagnetic radiation in the infrared. You can’t see the electromagnetic radiation produced as a result of the table’s temperature, because you can’t see infrared. But you can see the table because it is reflecting electromagnetic radiation from the sun.

Whether an object absorbs or reflects electromagnetic radiation from another source depends on the elements that make up the object. Again, each element has a very particular number and distribution of electrons, and only very specific amounts of energy can cause that distribution to change. Electromagnetic radiation of a particular frequency has a specific amount of energy; that amount of energy has to be exactly the right amount to charge a given atom. Therefore, if the electromagnetic radiation is the wrong frequency, it won’t charge the atom. The electromagnetic radiation will instead bounce off the atom, changing neither the atom nor the photon. This is known as reflection.

When the frequency of the electromagnetic radiation is exactly the right amount of energy to charge the atom, the atom uses that energy to get charged. This process is known as absorption. Eventually, the atom will become uncharged and release the exact amount that it absorbed. This is known as emission.

An element can only ever absorb very specific frequencies—the frequencies that are the correct amount of energy to charge the atom. Once the atom absorbs that energy, it can only ever emit the same amount. Therefore, when the atom emits electromagnetic energy, it’s always the exact same frequency that it absorbed.

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Electromagnetic Radiation | Electromagnetic Frequency | Light | Generating Electromagnetic Radiation | Sources of Electromagnetic Radiation | Reflection and Absorption | Atomic Spectroscopy | Molecular Spectrscopy