By Robert M. Hazen, Ph.D., George Mason University
The fundamental discoveries by Niels Bohr and others regarding the nature of atoms and light-matter interactions led to an arsenal of spectroscopic techniques and the invention of laser technologies. Read on to learn more about these inventions and how they help in improving people’s lives.
The study of light-matter interactions in great detail is called spectroscopy. It is used in astronomy, biology, chemistry, and environmental science.
A spectrum or spectrograph is a graphical record of light intensity, usually on the vertical scale, versus wavelength on the horizontal scale. Sometimes, instead of wavelength, frequency will be seen on one of these spectrographs.
Measuring the Intensity of Light
Spectra are measured with a device called a spectroscope that measures the intensity of light and separates light into different wavelengths. In a spectroscope, light enters one end and there is a lens that focuses that light. At the central portion where there is a bend in the tube, there is either a prism or something called a diffraction grading that is a very fine set of lines inscribed on a piece of glass that splits the light into rainbow colors.
For example, when using the spectroscope to look at the glowing hydrogen gas, or a neon bulb, bright lines of different wavelengths that have been spread out by the spectroscope can be seen.
Learn more about atoms.
Different Kinds of Spectra
There are several different kinds of spectra that are important in research. Glowing objects such as incandescent light bulbs, fires, and stars produce an emission spectrum. These objects emit different wavelengths of different intensities.
The incandescent light bulbs produce a very continuous smooth curve of intensities—more intense in the infrared radiation in the red end, less intense in the blue end. Glowing gases produce line spectra whereas a laser produces just a single, sharp, bright line.
Applications of Spectroscopy
One of the very first applications of spectroscopy was the study of flame spectra. Each chemical element has its own distinct set of lines that correspond to those electron levels. So if an element is heated up, it’s going to glow in its own characteristic way. And many new elements were discovered using emission spectra in the 1870s and 1880s.
These studies had a profound consequence of demonstrating that stars are made of the same chemical elements that occur on the Earth. For example, the Sun is made primarily of hydrogen and helium. Helium was actually first discovered as an element in the sun and later identified on Earth.
If light passes through a transparent medium, the result is an absorption spectrum. Many solids absorb some wavelengths of light preferentially to other wavelengths. Red rubies absorb a lot of the green light, while green emeralds absorb a lot of the red light. Hence, the complementary color comes through the gemstone. Absorption Spectra are used in all sorts of technological ways. For example, they reveal pollutants in the atmosphere and are very critical for environmental monitoring.
Spectra can be obtained by reflecting light off the surface of an object. This is called reflectance spectroscopy, though the spectra are not as clean or as precise this way. Reflectance spectroscopy is of immense importance in astronomy because it helps orbit a satellite above a planet or a moon. It also shows the composition of the surface of the celestial bodies. For example, the possibility of water at the poles of the Moon was discovered by reflectance spectroscopy.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Laser is an acronym for ‘light amplification by stimulated emission of radiation’.
There are three principle components of a laser. Firstly, a collection of atoms that have one dominant quantum jump, which is the characteristic color of the laser. Secondly, a source of energy to stimulate these atoms and get them into an excited state. Thirdly, a pair of parallel mirrors to align the resultant light into a single pencil-thin beam.
Workings of a Laser Beam
The collection of atoms to start with can be a gas, a liquid, or a crystal. The common red lasers used for lecture pointers use a tiny ruby crystal. The red wavelength corresponds exactly to the energy of the quantum jump in that ruby crystal—specifically, in the chromium atoms which are a small fraction of the atoms in the ruby crystal.
Electrical energy is needed to pump the atoms into their excited state. At first, just a few electrons drop down and produce that red wavelength. But then the red photons start cascading. And they pulse and stimulate other electrons into cascading in the same way.
Eventually, there is a crescendo of more and more, a cascade of photons, all of the same wavelength. And then there are two mirrors that align the photons that are bouncing back and forth, but one of those mirrors is slightly transparent. It may be 95 percent reflective, but five percent of the photons are allowed to pass through, and that’s the laser beam that comes shooting out of the laser.
Learn more about the quantum world.
Differences between Laser Light and a Flashlight
There are three major differences between laser light and a flashlight. A flashlight emits white light of all the different wavelengths whereas a laser light emits a single wavelength of red radiation.
The second difference being a flashlight emits a cone of light, illuminating a fairly wide area. Laser light, on the other hand, focuses on a single spot. In fact, laser light is so focused that a laser could shine from the surface of the Earth to the Moon and actually bounce back. It will only irradiate a small area on the surface of the Moon. So, lasers can be used to measure distances over very long distances such as the Earth-to-the-Moon distance.
And a third subtle difference is that a flashlight emits electromagnetic waves that are all overlapping each other. But in a laser, these waves are called coherent waves. They line up exactly so that every wave is superimposed on top of every other one.
Coherent waves have practical applications such as in the design of holograms and conveying information because the wave of laser light is controlled. These special qualities of lasers find hundreds of applications in science, medicine, and industry.
Common Questions about Spectroscopy and Laser
Spectroscopy provides a direct probe of the atomic-scale interaction because it measures both the wavelength and intensity of electromagnetic radiation.
A spectroscope measures the intensity of light and separates light into different wavelengths. There are various specialized techniques to measure all parts of the electromagnetic spectrum.
The study of flame spectra helps in the understanding of the stars and other astronomical objects. It also helped demonstrate that stars are made of chemical elements that occur on Earth.