It’s depressing as you get older and your vision seems to get worse with every passing year. But in reality the human vision is pretty poor anyway. You might be able to see a passing ship on the horizon but you won’t be able to read the name on the side at any distance. Our eyes have pretty useless resolution so images get blurred the further the distance you are. Of course we can use binoculars to magnify an image but why can’t we see this stuff with the naked eye?
The reason is basically down to the size of our pupils also known as their apertures. Our eyes need to be open wide enough to let through enough light to make sure that the sensors in the eyes start working. However the more open the aperture is the more the light entering the pupil will blur. The reason is that all the light arriving into our eyes comes from a variety of directions, again the wider open the more directions the light will come from.
Most of the light waves come in parallel and form a bright clear central spot. But the width of this ’clear spot’ is blurred at the edges by light waves arriving in different phases and hence cancelling each other out. The result is that the edges have a series of light and dark alternating bands but looks very much blurred. The width of the clear spot is actually what regulates the finest level of detail that our eyes are able to pick up.
Fraunhofer diffraction describes the blurred images that will be observed when the light rays arrive on a lens or aperture like the eye in parallel routes. It is also known as Far-field diffraction and is easiest seen when we pass a light source from a long way away (perhaps the Sun) through a lens. It doesn’t matter if the lens is in our eyes or a telescope the effect is the same. The diffraction effect will also smear the final image for a camera, eye or telescope. The limit of clarity is imposed when the light is passed through any sort of optical lens. To learn more about science subjects like this, make a habit of watching the best science programmes. I’d particularly recommend some of the superb ones being shown on the BBC recently, you can watch on the Iplayer.
If you’re outside the UK you’ll need to change your IP address to view the Iplayer – try this site for downloading from the BBC – http://www.iplayerabroad.com/2017/04/28/download-bbc-iplayer-videos-abroad/ It’s quite easy to do, routing your web connection through a British server takes minutes and gives you full access to all the wonderful BBC Iplayer programmes.