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The Quality of Galileo's Lenses

3.1 - Optical Analysis of 17th Century Lenses

Interference fringes produced with a soap bubble Galileo's broken objective lens without the frame (IMSS, inv. 2429) Scheme of the instrument for measuring spectral transmittance Transmittance curves for 17th century lenses

Since light consists of waves, interference is a physical phenomenon that appears, among other things, as the fringes of colour visible in a soap bubble. In this case the fringes reveal the shape of the inter-space between the inner and outer surface of the bubble. With appropriate laboratory instrumentation, the phenomenon of interference can be utilised to analyse how far each of the two surfaces of a lens deviates from the ideal spherical shape. We can detect, for instance, the optimum shape of Galileo's broken objective lens.

But the quality of a lens also depends on the excellence of the material of which it is made. Studying the composition of the glass provides information on its provenance and its production procedure. Some of the elements present in the glass can be detected by measuring the so-called "transmittance" of a lens in relation to the colour of the light traversing it. The transmittance curve in relation to colour reveals, in fact, the maximum and minimum transmissions of light in the presence of some elements, both contaminating ones (such as iron) and those utilised by the glassmakers to eliminate traces of colouration (such as manganese).

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