Photons and Emission Spectra

Need help preparing for the General Chemistry section of the MCAT? MedSchoolCoach expert, Ken Tao, will teach everything you need to know about Photons and Emission Spectra for Electronic Structure. Watch this video to get all the MCAT study tips you need to do well on this section of the exam!
While we mentioned photons briefly in the context of the Bohr model, it is important to understand in more depth the nature of what a photon is, and, most importantly, how to calculate their energy levels. The first equation to be familiar with is one you may have seen before, E = hf, where E stands for the energy of a photon h is Planck's constant (6.6x10-34 J/s), and f is the frequency of the photon. In physics the variable v often is used for frequency instead of f, so do not be surprised if that comes up on the MCAT. From Plank’s equation, we can observe a direct relationship between the energy of a photon and its frequency, a relationship that holds true for all types of electromagnetic waves. Remember that since the speed of light in a vacuum (c) is equal to the product of the wavelength (λ) and frequency of the wave, Plank’s equation can also be rewritten as E = hc/λ, showing an inverse relationship between wavelength and energy.
Electromagnetic Spectrum
The MCAT will expect us to be familiar with the scale and progression of the electromagnetic spectrum, as well as where and how visible light and color fit into that. Electromagnetic waves progress from radio waves, the lowest energy, lowest frequency, highest wavelength portion of the electromagnetic spectrum, up through increasingly higher energy manifestations, moving in order from microwaves, to infrared waves, visible light, ultraviolet waves, X-rays, and gamma rays.
While it is not necessary to memorize the specific wavelengths of any particular type of wave, the MCAT will expect you to know that visible light falls in the range of 400 nm (violet) to 700 nm (red). Based on that, we can also predict the approximate wavelength range of infrared light, microwaves, and radio waves (above 700 nm) as well as the range of wavelengths for ultraviolet light, X-rays, and gamma rays (below 400 nm).
Continuous and Line Spectra
Light can be shown as both a continuous spectrum and line spectrum. A continuous spectrum is light that contains all wavelengths of light, i.e. white light. This can be shown by refracting white light through a prism, which will produce a rainbow like pattern of all visible light, which again we call the continuous spectrum.
This is in contrast to a line spectrum, which contains light of only certain, discrete wavelengths. It will therefore be discontinuous, not containing all wavelengths of light. For the MCAT, we should be familiar with both absorption line spectra and emission line spectra.
An absorption line spectrum is formed after light has been shone on an absorbent material. Some wavelengths of light will be absorbed by the absorbent sample, with the rest reflecting back and passing through a prism, being separated out into its component wavelengths. An absorption line spectrum, then, will show us the wavelengths of light that have not been absorbed, and can allow us my elimination to determine what wavelengths of light were absorbed. An absorption line spectrum will look similar to a continuous spectrum, except for the presence of dark bands indicating the absence of the wavelengths of light absorbed by the sample.
Emission line spectra, in many ways, are the opposite of absorption line spectra. Whereas, in absorption line spectra we began with a sample and its ground state which then absorbed certain wavelengths of light, in emission line spectra we will start with a sample in its excited state, which will spontaneously relax and emit photons. The photons emitted by the excited material will be dispersed by a prism, sorting out into their respective wavelengths, generating the emission spectrum. Like in the absorption spectrum, dark bands will represent the absence of light, with bands of color reflecting which wavelengths of light were emitted by the excited material. Note the almost inverse nature between the two spectra: as we discussed in previous lessons, the energy required to excite a sample will be the same as the energy released when that sample relaxes.
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