Interactive Lecture Demonstration: Diffraction spectra of hydrogen, helium, and mercury

by Erees Queen Macabebe and Ivan Culaba
Department of Physics, Ateneo de Manila University

Spectral tubes for hydrogen, helium and mercury. The diffraction grating has 600 lines per mm.

Objective

Show that different wavelengths of light diffract at different angles.

Description of the Set-Up

This demonstration requires diffraction gratings, spectral tubes containing different gases and power supply for the tubes.

The spectral tubes contain different gases: hydrogen, helium and mercury. The diffraction grating used has a specification of 600 lines per mm

Demonstration

Attach a spectral tube to the power supply and observe the spectral lines emitted by the gas using the diffraction grating. Is the light emitted monochromatic? Is the spectrum continuous or is it composed of discrete lines?

Do the same for the other spectral tubes. Compare the emission spectra of the different gases.

Precaution

The power supply for the spectral tubes has a high voltage output. Do not touch its electrodes.

Hydrogen gas in a spectral tube

Helium gas in a spectral tube

Mercury gas in a spectral tube

Diffraction spectra of hydrogen

Diffraction spectra of helium

Diffraction spectra of mercury

Interactive Lecture Demonstration: Multiple slit diffraction

by Erees Queen Macabebe and Ivan Culaba
Physics Department, Ateneo de Manila University

Laser pointer and diffraction grating mounted on an optical bench

Objective

Show diffraction of light through multiple slits.

Description of the Set-Up

The set-up is composed of an optical bench, a laser Pointer, diffraction gratings and a white screen.

The laser pointer and the diffraction grating are mounted on the optical bench

Demonstration

Best results are attained when this activity is done in a dark room.

Point the LASER towards the screen. Switch it ON and place the diffraction with the least number of lines per mm on the path of the laser beam. A series of red bright spots are observed on the screen.

What will happen to the distance between the bright spots if the grating is moved closer to the screen?

Repeat the activity using a grating having greater number of lines per mm. How will the decrease in slit distance affect the distance between the bright spots found on the screen?

Precaution

Be extra careful in handling the laser. Never point the laser beam at anybody.

Fringe pattern produced using the 100 lines per mm diffraction grating

Fringe pattern produced using the 300 lines per mm diffraction grating