Spectroscopy Lab

Images & portions of this lab have been provide by Dr. Walt Volland, Bellevue Community College

Spectroscopy is the analysis of light spectra and the way in which light interacts with matter. When light is analyzed it is commonly separated into its component colors. The light source is directed on a slit and the "beam" of light is separated using a prism or grating.

The reason that the images are lines is that the light from the lamp is focused on a narrow slit. The illustration shows the separation of a light beam into its component colors.

You can see the specific colors emitted by the light source. A white light (Tungsten lamp or white fluorescent  lamps) source will give a spectrum like the one shown above. This is an example of a continuous spectrum, also know as the visible spectrum of colors.
Each color has a characteristic wavelength. The wavelength is the distance between the beginning and end of a complete cycle of the light wave or the distance from one crest to the next. The crest is the tallest part of a wave and the trough is the lowest point of a wave. All colors of light travel at the same speed, 300,000 kilometers/ second. The animation shows how a prism separates photons of red light from photons of blue light. The photons of different colors fall in different positions on the color spectrum. The position is determined by the wavelength.
Blue light has shorter wavelength in the range of 400 nm (4000 Ångstrom or 0.00000004 m)
Red light has longer wavelength and is lower in energy than blue light. The wavelength of red light corresponds to the range of 700 to 600 nanometers, (7000 Ångstrom or 0.0000007 meters).

3 Types of Spectra

1. Continuous Spectrum- produced by a glowing solid, liquid, or gas under certain conditions. This spectrum consists of a continuous set of emission lines side by side, with no gaps, and appearing as a smooth transition of all colors from red to violet.

2. Dark-Line Spectrum / Absorption Spectrum- produced when a cooler gas lies between the observer and an object emitting a continuous spectrum. The cooler gas absorbs specific wavelengths of radiation passing through it. This spectrum appears as a continuous spectrum of all colors with a number of gaps or dark lines throughout it.

3.  Bright-Line Spectrum / Emission Spectrum- produced by a glowing gas which radiates energy at specific wavelengths characteristic of the element or elements composing the gas. This spectrum consists of a number of bright lines against a dark background. Different elements produce different spectra. These different spectra are called the atomic spectra and are unique enough to be thought of as a finger print of the element. See Part 2 below.

 

Emission Spectroscopy - Identification of Elements
Part 1 Flame tests for known elements

Click on each element name below to view its flame test. 
(place answers on lab sheet)

Metal ion

Unknowns

Flame color

Identity of metal ion based on flame test

barium

Unknown 1

click for flame test

____________

__________

calcium

Unknown 2

click for flame test

____________

__________

sodium

 

rubidium

potassium

lithium

 Part 2 Emission line spectra for selected elements

Element

Emission

Emission spectrum

Sodium

click here to view emission tube

click here to view emission spectrum

Neon

click here to view emission tube

click here to view emission spectrum

Mercury

click here to view emission tube

click here to view emission spectrum

Helium

click here to view emission tube

click here to view emission spectrum