Spectroscopes

 

Light from a Historical Perspective

 

Everyone has probably noticed one or more rainbows projected onto a table or wall from some glass that was placed in the sun. What is the origin? Where did they come from? Why are they always in the same order with red on one side and blue on the other? Only a few hundred years ago, these simple observations were the source of great interest for many scientists.

 

Everyone has probably noticed one or more rainbows projected onto a table or wall from some glass that was placed in the sun. What is the origin? Where did they come from? Why are they always in the same order with red on one side and blue on the other?

 

Only a few hundred years ago, these simple observations were the source of great interest for many scientists. At a time when it was  commonly believed that light was made up of tiny particles called corpuscles, Christian Huygens (1629-1695) , who had been studying the behavior of water waves, made a conceptual leap and transformed the scientific community by using models that described the behavior of water waves to explain the reflection and refraction of light. Light was not necessarily a particle as previously thought - it behaved like waves of water.

 

Later, many scientists, such as Augustin Fresnel (Fresnel lens) and Thomas Young (1773-1829) contributed experimental evidence to support the wave theory of light. Young's experiments, especially those dealing with the diffraction of light conclusively demonstrated that light must travel in waves.

 

This model of light worked well until problems arose at the turn of the century those dealing with the diffraction of light conclusively demonstrated that light must travel in waves.

 

This model of light worked well until problems arose at the turn of the century  when scientists tried to explain spectra made by burning chemical salts in a flame. These spectra were different; they did not stretch continuously from red to blue as the spectra of ordinary daylight does. They had several bright lines and many large dark spaces in between.

 

The tide turned again, however, in 1921. A.H. Compton demonstrated that light possessed momentum, a very particle like quality.

 

Today, our current theory now embraces both its wave and particle aspects. Light is an electromagnetic wave, that travels in small particle like packets called photons. Each photon travels at the same speed: 3x10⁸ m/s, the speed of light. The energy of a photon is determined by its frequency or color. The higher the frequency, the more energy it has. The higher the frequency, the bluer the light, the lower the frequency the redder the light.

 

Some are familiar with ROY B. BIV. The letters stand for the colors in the rainbow with Red, Orange, Yellow, Green, Blue, Indigo, and Violet.  These colors are listed in order of increasing energy (decreasing wavelength) and comprise the visible spectrum.

 

The electromagnetic spectrum does not stop there, however. It continues beyond the visible into higher energies with ultra violet, x-rays, and gamma rays. It also extends below into lower energies with infra red, and radio waves.

 

Where Does Light Come From?

 

Light (electromagnetic radiation) is created when an electron moves from a higher energy level (or orbit) to a lower energy level. The photon of light that is emitted has an energy that corresponds exactly to the difference in energy between the two orbits.

 

Diffraction Gratings

 

The diffraction grating is the heart of the spectroscope. It is a thin film of plastic with thousands of very closely spaced lines etched into its surface, about 5000 lines per inch for the spectroscope we will be using.

 

This grating uses a combination of diffraction to bend the light waves as they pass by the lines etched on its surface, and interference to constructively add colors at certain locations and destructively eliminate the colors at other locations.

 

What we end up with after a beam of light is passed through a diffraction grating is a separation of the colors (wavelengths) of which the incoming light beam was composed.

 

Using the Spectroscope

 

It should be held so that the small end with the square hole is toward you. The wider end has a narrow slit (which lets light enter the spectroscope) and a wide window with a numbered scale.

 

Look through the eyepiece of the spectroscope (the square hole in the small end which holds the diffraction grating and point the slit at an incandescent light bulb. The numbered scale should be on  the left of the slit as you look through the eyepiece.

 

You should find a continuous spectrum located on the left side of the bright "white" slit. Be sure the slit is pointed directly at the light source for the best and brightest spectrum.

 

Each number on the scale indicates the wavelength of light in angstroms when multiplied by 1000. In other words, a reading of 5 on the scale is equivalent to 5000 angstroms. 1 angstrom equals 1x10^-10 meters.

 

Most solids that are heated to glowing, like the filament of a light bulb, will produce a smooth distribution of colors called a continuous spectrum.

 

Types of Spectra

 

Continuous Spectra

The source is usually a luminous solid or liquid, such as the flowing filament of a lamp.

 

Emission Spectra

Sometimes called bright line spectra, the source is a glowing gas. This gas emits photons with very specific energies (frequencies, wavelengths) that are characteristic of the chemical element of which the gas is composed.

 

Absorption Spectra

Sometimes called dark line spectra, it is generated b having a continuous spectra (white light) pass through a cooler gas located between the source of the continuous spectrum and the observer. The cooler gas absorbs those wavelengths that it would normally emit if it were the glowing source.

 

The dark line spectra has the same spectral fingerprint that the cooler gas would if it were emitting a bright line spectra. you can think of the absorption spectra as the photographic negative of the bright line spectra.

 

Chemical Fingerprints

 

Because each element has a different atomic structure, the electrons that orbit the nucleus emit photons of light at frequencies that are very characteristic of that particular atom. It can be said that the bright line emission or dark line absorption spectra is a chemical fingerprint for the particular element in question.

 

Internet References for Typical Spectra

 

http://astro.u-strasbg.fr/~koppen/discharge/

http://www.colorado.edu/physics/2000/quantumzone/index.html