Physics / 10 min read / 14 August 2021
Many of you, who wear spectacles, might’ve experienced seeing objects as just colored blobs with no defined boundary. Sometimes two or more objects in far might seem like one combined blurry object. You might’ve also seen that while driving at night, when you see a headlight coming from far you see 1 bright spot, but as it comes closer, we see it being separated into two spots. The answer to all of these is Resolution.
It is governed by a more basic phenomenon called diffraction, which arises due to the wave nature of light. And there is a Diffraction Limit to every optical device we use. Increasing the resolving power has been the biggest challenge in measurements involving any optical device, be it the electron microscope or the Hubble telescope.
The limits of resolving power in these instruments can be given by something called as Rayleigh Criterion. According to which the power mainly dependent on two factors: the aperture or the diameter of the device, for example the pupil in human eyes, and the wavelength of incident light. And for any two objects to be resolvable by an optical device, there minimum angular separation should be θ=1.22 λ⁄D .
And to increase the resolution this angle must be decreased. For the same you might’ve seen aperture mirrors having diameter as large as 10 meters in telescopes. Bigger problems start when we wish to observe atoms, so to do that we would need wavelength of the order or lesser than the size of atoms; to tackle the same we started using electron beam as the incident wave, which typically has a wavelength 100,000 times smaller than the photons of visible light, consequently, increasing the resolution by the same factor theoretically (though in practice it just increases the resolution by 100 times). This breakthrough in microscopy caused a string of discoveries of subcellular structures that were impossible to resolve with the light microscope. More methods are being developed to increase the resolution, for example recently a technique known as Fluorescence microscopy involving labelling molecules has even allowed researchers to distinguish subcellular structures as small as 10-20 nm across.
In one of the more recent papers by The Optical Society, researchers now propose a new solution to tackle the Diffraction Limit of the telescopes. They say that any independent photon we detect from a random star, we can narrow the pathway of the photon to the area of the telescope’s aperture (as they travel in straight lines). Now by Heisenberg’s Uncertainty Principle as we know the path of the photon precisely, the uncertainty in its momentum will be high. This limits its resolution. However, they suggest that as this is due to only independent photons, but if we amplify these photons making more coherent photons (photons vibrating in same phase) we can actually overcome the Diffraction limit.
There are many more such researches and this is a hot topic of research. So be on a lookout for more such inventions!