Color and Absorption Spectroscopy

Ozone molecules absorb ultraviolet light. This is radiation in a frequency too high (wavelength too short) for us to detect with our eyes. We can detect and distinguish electromagnetic radiation between about 400 to 700 nm. Below is the picture representing the electromagnetic spectrum that you saw in the last lecture.

Spectrometers can accurately distinguish and quantify radiation in the ultraviolet, visible, and infrared regions of the spectrum.

You know that visible light is composed of a range of frequencies. The frequency of the radiation is proportional to its energy and the wavelength of the radiation is inversely proportional to the energy. Red is the lowest energy visible light and violet is the highest.

A solid object has color depending on the light it reflects. If it absorbs light in the red and yellow region of the spectrum, it will have a blue color.

Here is an example. Chlorophyll, the pigment that makes plants green, absorbs light in the red end of the spectrum and light in the blue end of the spectrum. A green leaf is green to us because the middle band of visible light is not absorbed and is instead reflected into our eyes.

Our eyes have 3 types of specialized cells, called cone cells. Each type of cone cell is sensitive to a range of frequencies. Below right is a graph of the wavelengths of light absorbed by each of these cells.

When a cone cell absorbs light in its range, it sends an electrical signal to the brain. The intensity of the signals from each of these 3 types of cells tells us the color of the light coming in. Each person may have cone cells that are more or less sensitive so our perception of color is not precise.

Instruments such as UV-visible spectrometers are precise and highly reproducible. They can also detect and quantify electromagnetic radiation with frequencies higher and lower than the human eye can perceive.

In a spectrometer, a beam of radiation is split into two. One beam passes through the sample and the other goes straight to a detector.

The detector compares the sample and the reference beam to produce its signal.

Professor Patricia Shapley, University of Illinois, 2012