once again we have a question coming to the inbox of the Quantum Tunnel blog. If you are interested in asking a question, please feel free to get in touch using this page. We have once again a question by a very avid reader, let’s take a look:
Dear Quantum Tunnel,
Could you please explain how Fresnel lenses work? I am asking after listening to Dr Carlos Macías-Romero talking in one of the Quantum Tunnel podcasts. Thanks a lot.
Hello yet again Pablo, thanks a lot for your question. Well, I assume that you are familiar with the idea of a lens and that you may even wear a pair of spectacles or know someone who does and so you know that you can correct, among other things, the focal point and thus read your favourite blog (the Quantum Tunnel site of course!) with trouble.
Well, have you ever had a chance to go and see a lighthouse close enough? But not just the building, the actual place where the light is beamed out to see? If so you may have seen the lenses they use. If not, take a look the image here:
You can see how the lens is made out of various concentric layers of material and the design allows us to construct lenses that otherwise would be way to thick and therefore heavier. A lighthouse requires a light beam that uses a large aperture but a short focal length and a Fresnel lens offers exactly that without the need of a really thick lens. Fresnel lenses are named after the French physicist Augustin-Jean Fresnel.
Another example of Fresnel lenses are flat magnifying glasses such as the one shown below, you can see that they are effectively flat and no need to use one such as those used by Sherlock Holmes…
The design of a Fresnel lens allows it to capture more oblique light from a light source. Remember that a lens works by refracting (bending) the light and the way in which the “layering” in the Fresnel lens helps with the refraction needed. See the diagram below:
A couple of other uses for these lenses are in overhead projectors and the headlights of cars. So next time you attend or give a lecture or drive at night, think of Monsieur Fresnel.