Maker's Name: 
Adam Hilger, London
Where made: 
42 × 68 × 60 cm

This spectrometer was made in circa 1920 by the Adam Hilger company. It consists of a cast-iron stand with two brass arms. When in use, light travels through one of the brass arms of the device (the collimator) and enters the prism located at the centre of the device. Here, the different wavelengths of light are refracted and can be viewed through the second arm (the telescope). The collimator houses an adjustable slit, able to reduce the viewing area to a small rectangle. This improves the accuracy of the device and reduces eyestrain.

Unlike the standard prism spectrometer, the telescope cannot be rotated and the base on which the prism sits rotates instead. The shape of the special prism ensures that the final deviation of light rays that pass through the prism is always 90° and rotating the prism allows different wavelengths to be viewed. The turntable on which the prism sits can be rotated by turning a screw with an attached calibrated drum. The user of the device may rotate the turntable until they can observe light through the telescope. At this point, they can read the wavelength of the light from the drum. Alternatively, the spectra can be recorded by a plate camera.

Spectrometry is the science of analysing the interaction of matter and EM radiation. This spectrometer is used to analyse the wavelengths of photons emitted by various chemical sources. Atoms of different chemical compounds emit light when they shed excess energy through electron orbitals. For example, sodium lamps have a clear spectral emission at wavelengths of 588.9 and 589.6 nanometres. Other elements may produce bright emissions over a wide range of visible wavelengths. Due to their clarity, sodium lamps are often used with devices such as these for calibration purposes. For this device the user rotates the prism until one of the two sodium spectral lines are observed and the wavelength can be reset on the drum.

The analysis of the emission spectrum is a useful tool for scientists when determining the elements present in materials of unknown composition. A famous use of this spectral analysis was the discovery of the Fraunhofer lines. In 1812 German scientist Joseph von Fraunhofer identified that there were dark bands present in sunlight - that is, there were certain wavelengths of light that were seemingly not being emitted. He identified over 570 bands in total. It was later found that these lines were caused by elements in the Sun’s atmosphere absorbing these wavelengths. Understanding why atoms absorbed and emitted light only at certain wavelengths required the development of Quantum Mechanics.

This item is part of the UQ Physics Museum ‘2015, International Year of Light’ Tour
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