Question

Performance A spectroscope is a device used to produce and analyze spectra. Construct a simple spectroscope, and determine the absorption spectra of several elemental gases. (Your teacher will provide you with the gas discharge tubes containing samples of different gases.)

Short answer

Build a cardboard spectroscope with a CD, view gas spectra through a slit, and compare to known spectra.

Step-by-step solution

Gather Materials

Obtain a small cardboard box, a CD or DVD, aluminum foil, a craft knife, tape, and the gas discharge tubes provided.

Prepare the Viewing Slit

Cut a small, narrow slit on one side of the cardboard box using the craft knife. Cover the slit with a piece of aluminum foil and cut a thin opening in the foil to create a narrow slit.

Insert the CD/DVD

On the opposite side of the box, cut a hole just big enough to fit the CD/DVD. Insert the disc with the shiny side facing inward.

Secure the Spectroscope

Tape the CD/DVD securely inside the box and ensure the viewing slit and the CD/DVD hole are aligned properly for diffraction. Close the box, leaving only the slit and CD/DVD exposed.

Set Up the Gas Discharge Tubes

Place the gas discharge tube in front of the slit and turn it on. Make sure the spectral lines are projected onto the CD/DVD.

Observe Spectrum

Look through the viewing slit of your spectroscope to see the diffraction pattern of the light emitted from the gas.

Record Observations

Note the positions and colors of the lines you see. Each gas will produce a unique set of spectral lines.

Compare to Known Spectra

Compare the observed spectra with known absorption spectra to identify each gas.

Key concepts

Absorption Spectra

Absorption spectra are a fundamental concept in spectroscopy. When light passes through a gas, certain wavelengths are absorbed by the gas' atoms. This results in dark lines in the spectrum, known as absorption lines, each corresponding to a specific wavelength. These lines act as the gas' 'fingerprint' because each element absorbs light at different wavelengths.

To observe absorption spectra, a spectroscope is used. With our DIY spectroscope, light from gas discharge tubes passes through a slit, creating a narrow beam. This beam then hits a CD/DVD, which acts as a diffraction grating, separating the light into different wavelengths. By observing these wavelengths, we can determine which elements are present in the gas.

Elemental Gases

Elemental gases, like hydrogen, helium, and neon, are often studied using spectroscopy. Each elemental gas emits light at specific wavelengths when energized in a gas discharge tube.

For example:

• Hydrogen emits red, blue-green, violet, and deep violet lines.
• Helium shows bright yellow, green, blue and violet emissions.
• Neon produces strong red, orange, and yellow lines.

When we use a spectroscope to observe these gases, we see their unique absorption spectra, which helps in identifying them. This is crucial in many fields such as astronomy, where scientists identify the elements in stars by analyzing their absorption spectra.

Diffraction Pattern

The diffraction pattern in a spectroscope is what allows us to see the different wavelengths of light. When light passes through the slit and hits the CD/DVD, it bends around the edges causing diffraction. The CD/DVD has closely spaced tracks that act as a diffraction grating, spreading out the light into its component colors or wavelengths.

This pattern is observed as a series of colored lines on the screen that we can see through the viewing slit. Each color corresponds to a specific wavelength of light. By analyzing these lines and their positions, we gain insights into the composition of the light source, such as the gas in the discharge tube. The unique diffraction pattern for each gas enables us to distinguish between different elements.

Gas Discharge Tubes

Gas discharge tubes are essential for studying absorption spectra. They contain a specific gas and have electrodes at each end. When an electric current passes through the gas, it excites the gas atoms, causing them to emit light at characteristic wavelengths.

To set up the gas discharge tubes:

• Place the tube in front of the spectroscope slit.
• Turn on the tube to energize the gas.
• Ensure the light passes through the slit and hits the CD/DVD inside the spectroscope, creating a visible spectrum.

The light emitted by the gas in the tube is diffracted by the CD/DVD, producing a spectrum. Observing this spectrum through the viewing slit helps us identify the gas based on its unique set of spectral lines.