The diffraction of electromagnetic radiation is of particular interest to astronomers and is very much a mixed blessing.

On the one hand diffraction gratings can be used to examine the spectrum of an object; on the other hand, diffraction limits the resolution of a telescope, depending on its aperture.

Like the double slit experiment we explored in Part 1, a diffraction grating uses electromagnetic radiation to examine the absorption and emission spectra of stars, nebulae and galaxies.

​​​​​​​In place of one or two slits allowing the spreading of light waves into the paths of their neighbours with the resulting interference pattern, thousands of extremely narrow rules per centimetre are etched into glass or polished metal, creating transmission or reflection gratings, respectively. The nature of line spectra of astronomical bodies, whether they are absorption lines (like the Fraunhofer lines in the light from the Sun) or emission lines from a star-forming region of the Milky Way or other galaxies, give astronomers a wealth of information about the elements found in a star or galaxy, or the velocity, size and density of gas clouds, and so on.

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​​​​​​​Diffraction gratings are effective because the great number of slits will result in well-defined fringes of constructive interference separated by relatively wide dark regions where the phase difference between many paths of light causes the waves to cancel each other. However the destructive interference doesn't destroy the energy, so most of the light goes into the bright fringes - further adding to their clarity.

However, as mentioned previously, the resolution of a telescope, the minimum diameter in an astronomical image of a point source of light, is limited by the aperture of the instrument. So, for a star, the so-called Airy disc (named after George Bidell Airy), the bright central spot of light surrounded by a series of rings (which is caused by the diffraction of light entering the telescope) can be decreased in size by increasing the size of the telescope's lens or mirror. This is why radio telescope dishes are so enormous. In the same way, diffraction becomes more noticeable when the wavelength of the medium is comparable to the diffracting aperture; because radio waves have such long wavelengths (in the order of metres) the dish diameters have to be commensurately large to mitigate the loss of resolution. Incidentally, the resolving power of a telescope is set by the so-called Rayleigh criterion (after Lord Rayleigh) which says that two neighbouring stellar images can be distinguished if the edge of the central maximum of one falls within the first dark fringe of its neighbour.

At the other end of the scale, diffraction is similarly important in microscopy; the resolution of microscopes is limited due to the diffraction of photons by fine sample details. The de Broglie wavelength of electrons (depending on their energy) being up to several orders of magnitude shorter than light waves means that, when diffracted, they have a lower scattering angle. So, consequently, transmission electron microscopes can “see” much finer detail than traditionally illuminated instruments.

By the early 20th century, it was known that X-rays are a form of electromagnetic radiation, and in 1912 Max Von Laue was the first person to experiment with the diffraction of X-rays by crystals successfully. The mantle of this discipline was taken up by father and son team William and Laurence Bragg, who went on to refine the methods by which one can determine the structure of a lattice crystal by observing the diffraction pattern produced by X-ray scattering. The spaces between the planes in the crystal lattice represent the rulings, or slits, in a diffraction grating, though now with depth as well as width to be considered. Laurence Bragg, in particular, gave us Bragg's law which can be used to determine the spacing between the planes in a crystal, or if that is already known, the wavelength of the X-rays.

At the same time as James Watson and Francis Crick were attempting (somewhat flamboyantly, it could be said) to create a model of the structure of DNA in the Cavendish laboratory of Cambridge University, the brilliant X-ray crystallographer, Rosalind Franklin, was diligently working in King's College, London, making X-ray scattering images of DNA to ascertain its structure. Now whether or not it was meant to be thus, in what with hindsight appears to be at best rude and at worst an appalling act of academic skulduggery, Franklin's mentor, Maurice Wilkins, without her permission, gave Crick and Watson her best image to date: a diffraction pattern, which combined with the knowledge that DNA bases come in pairs, allowed the Cambridge duo to publish the double helix model. Sadly Franklin’s more cautiously scrupulous scientific technique (an approach which was possibly misogynistically interpreted, at the time, as stubbornness) meant that she was pipped at the post in this epoch-making discovery! 

With time, Rosalind Franklin's contribution to the discovery of the double helix has been addressed, but sadly came too late as she passed away at a tragically young age.

The story of diffraction goes on, and will continue as long as physicists delve ever deeper into the mysteries of quantum mechanics!

Photographs/gifs

• Diffraction spikes by NASA, ESA, and H. Richer (University of British Columbia) - HubbleSite: gallery, release., Public Domain, https://commons.wikimedia.org/w/index.php?curid=1185170
• Reflection grating by Primefac at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=54658478
• Solar spectrum Fraunhofer lines by Fraunhofer_lines.jpg: nl:Gebruiker:MaureenVSpectrum-sRGB.svg: Phrood~commonswikiFraunhofer_lines_DE.svg: *Fraunhofer_lines.jpg: Saperaud 19:26, 5. Jul. 2005derivative work: Cepheiden (talk)derivative work: Cepheiden (talk) - Fraunhofer_lines.jpgSpectrum-sRGB.svgFraunhofer_lines_DE.svg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=7003857
• Pollen grains by Dartmouth College Electron Microscope Facility - Source and public domain notice at Dartmouth College Electron Microscope Facility ([1], [2]), Public Domain, https://commons.wikimedia.org/w/index.php?curid=24407
• DNA by Zephyris, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=6285050