# Coarse diffraction grating

##### Class Practical

A diffraction pattern showing bands of colour produced from white light.

#### Apparatus and materials

Retort stand, boss, and clamp

Light source, compact

Power supply, low voltage, variable, to supply 8A at 12V

#### Health & Safety and Technical notes

The lamp should have a good straight filament.

Each student pair will need a coarse diffraction grating.

The diffraction grating should not be blazed. The laboratory should be darkened. If you don’t have a compact light source (quartz iodine lamp) use a 48W 12V lamp.

#### Procedure

a Mount the compact light source at the end of the laboratory and connect it to the power supply, set at 12 V.

b Ask students to hold the grating near to the eye and to look through it at the distant light source.

#### Teaching notes

1 Talk students through the observation. They look at the white-hot filament of a lamp with a grating held close to the eye. They should see a central white line where waves of all colours go straight through the grating. Out to each side, they should see a bright band. This is where light arrives from adjacent slits with one wavelength path difference. Since the light is white, each bright fringe is spread into a wide spectrum.

Looking further out to each side, students may see a still wider, but fainter spectrum. This corresponds to the next bright fringe out from the centre (two wavelengths’ path difference).

2 For a diffraction grating d sin A = n (wavelength), where A = angle at which the light appears, n is the diffraction order (1,2, ...), d = spacing between slits.

3 Many teachers progress from single slit diffraction to double slit diffraction (Young’s fringes), treating the double slit case as two overlapping single slit diffraction patterns.

You can progress through three, four, five, six, etc. slits to the diffraction grating. The use of two distinct equations can mislead students, so they think of double slit interference and gratings patterns as quite different phenomena. Show them the similarities.

Young’s fringes
n (wavelength) = distance between slits x fringe separation / distance from slits to screen

n is the number of the fringe

Diffraction grating
n (wavelength) = distance between slits x sin (angle at which the light appears)

n is the diffraction order (1, 2,3,..)

4 ALTERNATIVE: This video, from the National STEM Centre eLibrary, shows how to produce a diffraction pattern using a laser source and a thin, straight wire.

This experiment was safety-checked in February 2006