Day 9 - Shambles Advent

Blog in a Teacup by Dr Helen Czerski


Every day until Christmas Dr Helen Czerski will be finding some cool, hidden science in pics of people’s every day lives. To get involved, tweet your pic to @helenczerski and @cosmicshambles with the hashtag #ShamblesAdvent

This photo was sent by @SERATONINSTEVE , and it’s a lovely example of something that we often take for granted in photos with point-like bright objects in them – radiating lines coming from the bright point.  They’re really common in photos of the night sky, but the number and angle of the lines can vary quite a bit.  We can also occasionally see them ourselves, when looking at street lamps or stars.  But you will never be able to photograph the star-shape that you see, because it’s not generated at the star – it’s not “real”.  It’s an artefact caused by the way light gets in to your eye.   

They’re known as diffraction spikes.  The reason they surround very bright points is that they’re very faint compared with the original object, and so they only stand out enough to be seen when they’re caused by a point of light that is super-bright compared with its surroundings.  Here, we can see that the three bright objects – Moon, Venus and streetlamp – are over-exposed, so there’s plenty of light going spare.   The first thing to notice is that the three crosses all line up perfectly, because they’re all caused by the same thing.   But what is that thing?

Light gets into a camera or into our eye through a very small hole: the aperture.  The aperture effectively selects the light that is going to form the image, and prevents light that isn’t relevant getting to the sensor.   So you have to have an aperture.  You would think that the obvious shape is a circle, but changing the size of a circle is hard.  So camera manufacturers choose a shape that is easier to expand – a hexagon or square, for example.  And this is where the lines come from.  The top and bottom of a hexagon are parallel lines, forming a slit that the light has to pass through.  But it’s a narrow slit so the light diffracts, spreading out in the direction perpendicular to the slit.  So if you’ve got a hexagon that’s flat on the top and bottom, you’ll get lines stretching upwards and downwards.  The lines are made of a tiny fraction of the original light, but they’re still there.  And you can tell from the photo that the aperture is a hexagon because there will be three pairs of lines, one for each pair of sides. 

In this case, I think that the camera has a diamond-shaped aperture, because that’s what will produce the squished cross shape that we see here. 

But what about our eyes?  We don’t see these lines all the time, and there’s some debate about what causes them when they do pop up.  The most likely culprit seems to be a stray eyelash, because light will also diffract around a long thin obstacle.   But there must be a small straight-sided obstacle in the light path, because the telltale lines give it away.

It seems that this is why we draw stars with lines radiating out of them – this pattern is common when we photograph stars and it’s much prettier than tiny dots of light, so we choose that representation.  All the pointed Christmas stars that you see this season were inspired by diffraction, either in a lens or in the human eye.  Never say that physics can’t make things more beautiful!

Read the rest of the open windows here

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Dr Helen Czerski is a physicist, first and foremost, but she’s acquired a few other labels along the way: oceanographer, presenter, author and bubble enthusiast. A regular on The Cosmic Shambles Network, she has also presented a number of acclaimed documentaries for the BBC and her first book, Storm in a Teacup, which looked at the physics of every day things, was a bestseller. Recently she was awarded the prestigious William Thomson, Lord Kelvin Medal and Prize from the Institute of Physics.

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