Question on a diffractive property of light

[caseytrimble]

I was walking through my house the other day and noticed a circle of light on the wall. I traced it back to a nearby window, but noticed that the opening was square. When I put my hand close to the opening, the light made a square, but as I walked backwards and kept my hand in the path of the light, the image slowly changed from a square to a circle. I'm sure there is a simple explanation for this phenomenon.

Can anyone explain?

 

[Andy Resnick]

The most simple explanation is that the further away from the hole you are, the less the shape of the hole can be discerned: all holes look the same when viewed sufficiently far away.

 

[sophiecentaur]

The hole in your window opening is too big to be noticing diffraction. The reason for the change of apparent shape is that the Sun is not a point source of light. At a distance from the hole, there will be a softening of the shadow (do you remember the word 'Penumbra' from School?) because, between all rays getting to your hand and no rays getting there (the shadow), there is a range of angles where some rays reach but others don't. This spoils the nice 'hard edge' that you get when 'proper'' optics are used to create shadows.
As the distance increases, the spread from each part of the hole overlaps the spread from all other parts and it will average out to a roundish blob.

If you use a smallish hole (say <5mm) you can get a really good 'pinhole camera' image of the Sun on your wall at the appropriate time and if you avoid too much other light in the room. You can see sunspots very clearly and the disc will be an impressive size if the 'throw' is several metres. I got a really magic image on the inside of the front door from light from the back of the house shining through a keyhole in the back bedroom door.

 

[caseytrimble]

Thanks, sophiecentaur, that makes sense. So basically if I use a light source that emits light with reasonably uniform vector angles, I wouldn't see this shape-changing effect?

 

[caseytrimble]

And to clarify, when I say I "traced [the source] back to a nearby window," I failed to include that the source was a small hole in the blinds where the drawstring is threaded, and the image was at least 6 or 7 meters away. I should have included the relative sizes in my post.

 

[sophiecentaur]

Thanks, sophiecentaur, that makes sense. So basically if I use a light source that emits light with reasonably uniform vector angles, I wouldn't see this shape-changing effect?

Do you mean parallel rays? Which "vectors" do you mean?

 

[caseytrimble]

Parallel's the word. I was thinking of the angle that the rays make with a cross section plane along the path of the light.

 

[sophiecentaur]

Actually, I should not have used the word "parallel" because the rays are, in fact, spreading out from their source. Perhaps the right terms would have been 'radial', because the spreading is what produces a larger and larger image as you get further away from the hole. I strongly recommend trying to get a good pinhole image. It can be really impressive if the throw is long enough. Keep your eye on that door every day and then, when you see the effect again, put a card with a small hole in it over the square hole. Remember to have a camera with you.

 

[caseytrimble]

I'll be sure to try it, thank you for the information!