Measurement of the wavelength of monochromatic light

Apparatus

Sodium lamp, spectrometer and diffraction grating (300 lines per mm).

Procedure

• Adjust the eyepiece of the telescope so that the crosswires are sharply focused.

• Focus the telescope for parallel light using a distant object. There should be no parallax between the image seen in the telescope and the crosswires seen through the eyepiece.

• Place the sodium lamp in front of the collimator.

• Level the turntable of the spectrometer if necessary.

• Looking through the telescope, focus the collimator lens and adjust the width of the slit until a clear narrow image is seen.

• Place the diffraction grating on the turntable at right angles to the beam.

• Move the telescope to the right until the cross wires are centred on the first bright image. Take the reading θ₁ from the scale on the turntable. (To see the scale more easily shine a lamp on it and use a magnifying lens.)

• Move the telescope back through the centre and then to the first bright image on the left.

• Take the reading θ₂ from the scale.

• Calculate θ using θ = (θ₁ - θ₂)/2

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Measurement of the wavelength of monochromatic light

• Calculate the distance $d$ between the slits using $d = \frac{1}{N}$ where $N$ is the number of lines per metre on the grating.

• Calculate the wavelength $\lambda$ using $\lambda = d \sin \theta$.

• Repeat this for different orders ($n$) and get an average value for the wavelength.

Results

$n$	$\theta_n/°$	$\theta_1/°$	$\theta = \frac{\theta_n - \theta_1}{2}/°$	$\lambda/m$

Average $\lambda$ =