If you ever took a class in practical physics that included a lab, then you remember those little diffraction gratings that could be used as a substitute for a prism to observe the spectrum of light. The gratings exhibited the properties of prisms, but without the bulk. If you did not happen to have a physics lab, then you may remember those flat Fresnel (pronounced ‘freh-nel’) fish-eye lenses that were popular in the back windows of vans and motor homes. Again, the properties of a larger, bulkier lens made in a compact format.
Image: (L. Koechlin et al/OMP) – a metallic prototype of a diffraction lens – click on the image for a larger view.
Now, it appears that a diffraction grating is being considered as a lightweight, and even more accurate, optical device for focusing light without the usual mirrors and lenses. That will be a boon for the future of space and even terrestrial telescopes.
This excerpt from New Scientist explains how that is possible.
A proposed space telescope would focus light primarily with a patterned sheet of metal rather than a large mirror or lens. The telescope would have amazingly sharp vision and could spot Earth-size planets around other stars, its backers say.
Telescopes used for astronomy normally focus light using a curved mirror. But there are limits to how large a mirror can be sent into space. That’s because of the tremendous cost of lofting heavy objects into orbit, as well as the limited cargo capacity of launch rockets.
But a team of scientists is experimenting with a completely different approach for focusing light. It does not require a large primary mirror or lens, though it does use a smaller secondary mirror and lens.
The technique takes advantage of the wave nature of light, which causes light to bend around an object’s edge. The same phenomenon, called diffraction, is the reason you can hear sounds coming from around the corner of a building.
This means that light can be focused into an image simply by passing it through a certain pattern of holes carved in an opaque sheet. Such patterned sheets have long been used for focusing laser beams, but have so far not been used for astronomy. They are called Fresnel zone plates, after the French physicist Augustin-Jean Fresnel, who studied diffraction in the 1800s.
