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The Quality of Galileo's Lenses

3.2 - Analysing Lenses with the XRF Technique

The photoelectric effect Energy curve of Galileo's broken lens The photoelectric effect

Not all of the components of glass can be detected by measuring transmittance. To verify the presence of some of them - including those of optical importance, such as lead - other analyses must be conducted. The X-ray fluorescence technique (XRF) detects the elements present in a material through the so-called "photoelectric effect".

The material to be analysed is irradiated with a flow of X-rays. When a ray strikes an atom, it displaces an electron close to the nucleus (ionization). Instantaneously, another electron fills the gap created, passing from a higher energy level to a lower one. In the transition, the electron liberates the difference in energy under the form of another X-ray, which is however characteristic of the particular atomic element that has been struck. The X-rays produced in this way can be recognised and counted with a special detector.

The elements present in the irradiated zone can be identified by examining the energy curves of the X-rays emitted. In this way it has been discovered that the glass used for telescope lenses in the early 17 th century was tendentially lead-free and that, moreover, it did not differ greatly from the glass used in other products.

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