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AP Chem



Unit 3

3.11 Spectroscopy and the Electromagnetic Spectrum

2 min readaugust 6, 2020


Dalia Savy

Kanya Shah


Dylan Black

What Is Light?💡

Scientists began to understand light and the way light works by analyzing light that was emitted or absorbed by substances.
The light we see with our eyes👀, visible light, is one type of electromagnetic radiation; because electromagnetic radiation carries through space, it is also known as radiant energy.
Light is carried in the form of photons, a type of quantum particle that acts as a "force carrying particle" for electromagnetic energy. For example, when you turn on a flashlight🔦, trillions of photons shoot out, creating a beam of light that illuminates a dark area. The same concept is applied in lasers and in many incredibly important measurements involving concentrations of solutions, as we'll see in Unit 3.13.
😲Fun fact! The word LASER is actually an acronym for Light Amplification by Stimulated Emission of Radiation ⚡
We'll get a deeper understanding of what light really is and its role in the quantum world in Unit 3.12.


Wavelength and the Electromagnetic Spectrum

When we discuss light, we typically refer to its wavelength and its frequency. These two terms can help us describe how light will act. Light is an interesting quantum idea because of the fact that it acts both as a particle (the photon) and as a wave. This is called particle-wave duality.
When thinking of a wave, it is useful to visualize it as a sine wave, oscillating back and forth periodically. Wavelength(λ) is just that - the length of one period of the wave. This can be described either as the peak to peak distance, or the zero to zero distance.
Frequency(ν) describes the number of waves that pass a fixed place in a given amount of time and is measured in per-seconds (s^-1) or Hertz (Hz). Frequency and wavelength have an inverse relationship. Essentially, a high wavelength implies a small frequency and vice versa.

Image Courtesy of Florida State College

The Electromagnetic Spectrum🌈

This is further explained in the electromagnetic spectrum which ranges from very short gamma rays to very long radio waves. A key trend to note is that electromagnetic radiation can be characterized by a wavelength; the shorter the wavelength, the higher the frequency. 
Spectroscopy is the study of the interaction of radiant energy and matter. The electromagnetic spectrum diagram is shown below to use a reference.

Image Courtesy of Khan Academy

There are a few things to notice about the electromagnetic (EM) spectrum:
  • It is broken up into sections based on wavelength from ten-thousandths of nanometers long to hundreds of meters long.
  • Gamma rays have the shortest wavelength and thus the highest frequency
  • Radio waves have the longest wavelength and thus the lowest frequency
  • On the visible light spectrum: purple💜 has the shortest wavelength while red❤️ has the longest wavelength

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