Kevin McCaig, age 16, of Kingston, Ontario, Canada, for his question:

WHAT IS A SPECTROGRAPH?

We know that when white light is passed through a prism, the band of colors which can be seen is called a spectrum. White light is a mixture of violet, blue, green, yellow, orange and red light. Each of these colors has a different wave length.

A spectrograph is an instrument which separates a mixture into its parts and records the amount of each. An optical spectrograph separates white light into the individual colors that make it up. The colors are made to fall on a photographic plate.    

The amount of darkening of the plate at one particular place gives information about the amount of light of one particular color that was in the original white light. The position on the plate shows the wavelength of that particular one relative to the wave lengths of the other colors.

A mass spectrograph separates equally charged particles in a gas according to the mass of each particle. The main element of a spectrograph is a prism which spreads out the light that enters it into its various colors.

Spectrographs can analyze and record not only visible light but also ultraviolet, which has wave lengths of less than 400 angstroms and infrared, which has wave lengths greater than 6,000 angstroms.

By analyzing the light emitted from atoms and molecules when they are heated or excited in some other way, the spectrograph can give information about the energies of vibration and rotation of the atoms or molecules

In a mass spectrograph, the substance to be analyzed is first ionized. The ions then are injected into a magnetic field which continuously deflects the ions around a circular path. The path will depend on the ratio of the charge of the ion to its momentum.

The greater the mass, the smaller the curvature of the path. Therefore, ions of the same charge and velocity but having different masses will strike different places along a photographic plate.

A spectroscope is the instrument used for the study of various kinds of spectra. It is a device which uses a prism to separate visible light into the various wave lengths which make it up.

Colored bands of light, when viewed through lenses and compared on the instrument, give information about the material giving off 'the light.