#Physics, #Household math - Light reduction through screen doors and windows...

Question

When I have my window open, it's always so much brighter when the screen is open as well. Why?

Answer

Diffraction of light 

Analysis

First off - great question! I'd never really noticed just how much the light is dimmed in my house until I did a quick comparison of having the screen open and shut.

So ok - what's going on?

As far as I can tell, there are 3 things going on:

• The wires that make up the screen catch debris.

Now, I'm not claiming that your screens are dirty, but I'm realizing that mine are, so I'll be back in a minute after I wipe them down... and I'm back. And the light still dims noticeably. Another explanation is needed.

• The wires block light.

Depending on the type of screen you have with the size of the holes and the gauge of the wire, any calculation will vary in results. I did some searching online to see if there was a standard size for wire screens and I found that there are several different kinds of metal mesh screens. So what I'll do is use my screens as at least an approximation of what you may be experiencing.

I found that my screens have, per cm^2, 7 wires in one dimension and 9 in the other.

The width of each wire can be quite hard to find (I couldn't find a figure online). One way to find the measure of a small amount is to measure many of those things together and then divide by the number of elements. In this case, I am fortunate enough to have a cat who likes to climb the screens, bending them in places where I was able, to a rough degree to measure a number of wires compressed together. I found that roughly 5 wire widths are equivalent to 1 mm, or in other words each wire is roughly 0.2 mm per wire.

We can then do some multiplication to find the area of wires per cm^2:

7 X 0.2 X 9 X 0.2 = 1.4 X 1.8 = 2.52 mm^2

Keep in mind that 1 cm^2 is the same as 100 mm^2, so the area of wire to space is 2.52 / 100 = 2.52%

But let's face it - the amount of light that comes through the screen is less than a very small 2.52% reduction. We need another explanation.

• Diffraction

And here's where physics comes into play...

Light is a funny thing. You'd think that, knowing that only 2.52% of the area of a window is blocked by a screen mesh, that 97.48% of the light would just zoom on through. If light only acted like a particle, this would absolutely be the case and we'd have much brighter rooms when the wire screens are closed.

Here's the thing - light also acts like a wave and it's this form of light that gives rise to the light reduction. Here's how it works:

Ambient light consists of a whole bunch of different colours (or wavelengths) and they vibrate in all the ways possible (think of it this way - when you shine a flashlight at something, some light vibrates up/down, some left/right, etc. In fact, if you think of the circular shape of the flashlights' light, you can pick any given spot on the "clock" of that circle and the light will vibrate through that spot (say 2 o'clock) and also through the opposite side (8 o'clock).

All these vibrations come streaming towards your window and some of these vibrations "bump" into the wire mesh and scatter (and the tighter the mesh, the more apparent this effect will be). Gone are the lovely beams of light streaming into the room and instead the light heads off in a gazillion different directions. Let's face it - the screen mesh acts much like a cloud - it scatters light in a way so that we don't get lovely beams but rather a medium-glow. And just like a cloud, there doesn't need to be a high density of things blocking the light to scatter it.

If you are of a mind to learn more about diffraction (and this article in particular deals with laser light, which is different from ambient light in that the laser light is all the same colour and vibrates in same direction), here's an excellent paper on it:

http://www2.optics.rochester.edu/workgroups/berger/EDay/EDay2008_Diffraction.pdf


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Questions and comments always welcome!