General Chemistry
Behavioral Sciences
Lab Techniques

Electromagnetic Spectrum

In this module, we’ll cover the electromagnetic spectrum in its entirety, providing you with the foundational knowledge you need to understand and remember the different types of electromagnetic radiation. We’ll then focus on how this information is relevant to the MCAT, teaching you strategies to use this knowledge effectively in the exam.

Understanding Energy, Frequency, and Wavelength

Before diving into the spectrum itself, it’s important to understand the core principles that define the behavior of electromagnetic waves.

  • Energy ((E)): This is the capacity to do work. In the context of photons, which are particles of light, energy can be calculated using the equation ( E = hf ), where:
  • (h) is Planck’s constant \((6.626 x 10^{-34} J·s)\)
  • (f) is the frequency of the wave.

This means that as the frequency of a wave increases so does its energy. So microwaves which have relatively low frequencies when compared to X-rays would be much lower in energy. Since frequency and wavelength are inversely related as the wavelength of light increases that light’s energy decreases.

This relationship between energy and wavelength can also be described by the equation \( E = \frac{hc}{\lambda} \), where (c) is the speed of light in a vacuum (3.00 x 108 m/s). This equation highlights that energy is inversely proportional to wavelength: shorter wavelengths have higher energy and vice versa.

The Full Electromagnetic Spectrum

With an understanding of the fundamental relationships, we can explore the full electromagnetic spectrum, from the longest to the shortest wavelengths, and then figure out what this means in terms of energy and frequency from radio waves to gamma rays.

To help remember the order of the waves we will use the following mnemonic: “Raging Martians Invaded Venus Using X-ray Guns”, standing for Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma.

  • Spectrum Range and Characteristics:
  • Radio Waves: These have the longest wavelength and hence the lowest frequency and energy.
  • Microwaves: Shorter wavelength than radio waves, these are used in communication technologies and kitchen appliances.
  • Infrared (IR): Just below the visible spectrum, infrared waves are primarily felt as heat.
  • Visible Light: This is the range visible to the human eye, from red to violet.
  • Ultraviolet (UV): Beyond violet, UV rays have shorter wavelengths and are energetic enough to cause skin damage.
  • X-rays: With even shorter wavelengths, X-rays penetrate most substances and are used extensively in medical imaging.
  • Gamma Rays: These have the shortest wavelengths, highest frequencies, and are the most energetic waves in the spectrum, produced by nuclear reactions and radioactive decay.

The Visible Spectrum

The visible spectrum is the most familiar to us, as it’s what we can see. It’s vital to know the order and the range rather than the exact wavelengths of each color as you can usually infer the right wavelength in AAMC questions.

The visible spectrum ranges from 750 nm (red) to 380 nm (violet) and the order that can be remembered using the mnemonic ROY G BIV is Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Since both frequency and energy are inversely related to wavelength red light will have the smallest frequency and the lowest energy while violet light will have the highest frequency and the highest energy out of the visible spectrum.

Application to UV and Infrared

The knowledge of the visible spectrum cutoffs is essential for deducing the relative wavelengths and energies of light outside this range, such as UV and infrared. Since both bookend the visible spectrum, with UV having shorter wavelengths and higher energy, and IR having longer wavelengths and lower energy. Remembering that UV light gives us sunburn can help you recall that it has more energy than visible light, hence a shorter wavelength.

  • UV Light: With wavelengths shorter than 380 nm, UV radiation is high in energy and can cause harm to biological tissues.
  • Infrared Light: Wavelengths longer than 750 nm fall into the infrared range, where they are typically lower in energy and perceived as heat.

MCAT Application

The MCAT often tests your understanding of the electromagnetic spectrum in the context of relative changes. You might be asked to compare wavelengths, frequencies, or energy levels of different types of radiation. Or be asked to identify the wavelength of UV or infrared light, which you can do by simply knowing the cutoff for the visible spectrum. With this in mind let’s dive into some practice problems to test what you know and get a feel for how the AAMC asks these questions.