# Pre-lab 8: Spectroscopy

## The Electromagnetic Spectrum

White light is a mixture of colors, which we conventionally divide into six major hues—red, orange, yellow, green, blue, and violet. As shown in Figure 2.7, we can separate a beam of white light into a rainbow of these basic colors—called a spectrum (plural: spectra)—by passing it through a prism. This experiment was first reported by Isaac Newton more than 300 years ago. In principle, the original beam of white light could be recovered by passing the spectrum through a second prism to recombine the colored beams.

 Figure 2.7 Visible Spectrum When passed through a prism, white light splits into its component colors, spanning red to violet in the visible part of the electromagnetic spectrum. The slit narrows the beam of radiation. The image on the screen is just a series of different-colored images of the slit. Human eyes are insensitive to radiation of wavelength shorter than 400 nm or longer than 700 nm.

## The Components of Visible Light

What determines the color of a beam of light? The answer is its frequency (or, equivalently, its wavelength)—we see different colors because our eyes react differently to electromagnetic waves of different frequencies. Red light has a frequency of roughly 4.3 x 1014 Hz corresponding to a wavelength of about 7.0 x 10-7 m. Violet light, at the other end of the visible range, has nearly double the frequency—7.5 x 1014 Hz —and (since the speed of light is the same in either case) just over half the wavelength—4.0 x 10-7 m. The other colors we see have frequencies and wavelengths intermediate between these two extremes.

Scientists often use a unit called the nanometer (nm) when describing the wavelength of light (see Appendix 2). There are 109 nanometers in one meter. An older unit called the angstrom (1 Å = 10-10 m = 0.1 nm) is also widely used by many astronomers and atomic physicists, although the nanometer is now preferred. Thus, the visible spectrum covers the wavelength range from 400 to 700 nm (4000 to 7000 Å). The radiation to which our eyes are most sensitive has a wavelength near the middle of this range, at about 550 nm (5500 Å), in the yellow-green region of the spectrum.