Certain science myths get recycled again and again. Here's my attempt to put the world right on some of them. To gain redemption, click on the links below, or scroll down. Obviously I'm sticking my head above the parapet here - If you believe that I am promoting untruths here, please contact me.
- FALSE: You can mix all colours using red, green, and blue lights
- FALSE: Visual illusions show how easily our brain is fooled
- FALSE: Bernoulli's principle says that fast-moving air always has a lower pressure than slow-moving air
- FALSE: Heat rises
- FALSE: Raindrops are in the shape of a traditional teardrop
Mixing coloured lights
We're all familiar with mixing paints (subtractive colour mixing) but rather less familiar with mixing lights (additive colour mixing). Nevertheless many people are aware that white light can be made by mixing lots of different colours of light, and that their computer monitors make colours by mixing red, green and blue lights. Unfortunately many books and websites recycle untruths and misunderstandings about additive colour mixing. This is an attempt to set the record straight.
Out of shortage of time, I haven't attempted to explain or derive the statements I make below. If you want to encourage me to do so, contact me.
FALSE: Red, green and blue are the primary colours for mixing coloured lights
TRUE: There is no set of lights that have special status when it comes to mixing coloured lights. There are no special "primary colours" in this respect.
FALSE: Using these colours we can mix all possible colours.
TRUE: There is no set of three lights that we can use to mix all possible colours. For any set of lights, there will be colours that we can't mix.
TRUE: Red, green and blue do give us a larger range of possible colours than other triplets of coloured lights will do.
FALSE: Red, green and blue are special because the spectra of the lights match the sensitivities of the 3 kinds of light-sensitive cone cell in our eyes.
TRUE: The good colour-mixing properties of red, green and blue are not because they each somehow uniquely stimulate one of the 3 kinds of light-sensitive cone cell in our eyes. An ideal set of "primaries" would do this, but it's an impossible aim because the cones (particularly the so-called "red" and "green" cones) respond to heavily overlapping regions of the spectrum - you can't stimulate the "red" cones" without also stimulating the "green" cones. A good red light for colour mixing would be a very deep red that was far from optimal for stimulating the "red" cones, but which is even worse at stimulating the "green" cones.
INCOMPLETE: White light is always made of a mixture of all of the colours of the spectrum
MORE COMPLETE: You can make white light using the whole spectrum - sunlight, or the light from a tungsten light bulb, are good examples. However, you can mix white using only (suitably chosen) parts of the spectrum, and the minimum requirement is a mere two wavelengths. For example, if you mix wavelengths of 482nm and 580nm (with the intensities suitably adjusted) you can get white. 440nm and 570nm will do as well. In fact there is any number of pairs of wavelengths that you can mix to get white.
FALSE: Additive colour mixing is about physics
TRUE: The rules of additive colour mixing arise from the properties of the three kinds of light-sensitive cone cell in our eyes. Additive colour mixing is about physiology.
Your brilliant brain
FALSE: All visual illusions show you how easily your brain is fooled
TRUE: Some illusions show us exactly the opposite.
This image of a real scene is my version of Edward Adelson's checkershadow illusion (with a little inspiration from Magritte).
The central square of the grid, in the shadow of the pipe, is actually darker than the square indicated by the arrow.
If you don't believe it, cut two holes in a piece of card to mask off the rest of the display. Or take a screenshot and use an image-processing program to measure the brightnesses.
Far from showing us how stupid your brain is, this demonstration shows you what a marvellous piece of equipment it is.
Think about the real checkerboard, not its image. The arrowed square is painted with dark grey paint, and the central square is painted with light grey paint. That's exactly what you perceive, despite the uneven lighting. Isn't that a good thing for your brain to do?
If you still don't believe me, try this thought experiment. Imagine that you live in a forest, and that there is a kind of fruit that, when unripe, is light grey and poisonous, and which ripens to become dark grey and nutritious. Suppose that you see two of these fruits next to each other, but in the dappled forest light the (light grey) unripe fruit is in shadow, and the (dark grey) ripe fruit is is bright light. Suppose that the depth of the shadow is such that the unripe fruit actually reflects slightly less light than the ripe fruit, just as the central (light grey) square reflects reflects less light than the arrowed (dark grey) square in the picture above? Would you really want your vision to tell you that the unripe fruit was the one to pick? Or would you want it to discount the irrelevant effect of the shadow and tell you which fruit was good to eat and which one would kill you? I know what I'd want.
I think that it is wrong to call this effect an illusion (and so does Adelson). There is nothing illusory about what you see. You brain tells you the truth about the scene in front of you.
Bernoulli's principle
OVERGENERALISATION: Fast-moving air has a lower pressure than slow-moving air
CORRECT GENERALISATION: If a body of air moves from one place to another in such a way as to increase its speed, AND if there is no external input of energy, its pressure will fall.
For example, if air flowing down a tube encounters a narrowing of the tube, at the same level as the rest of the tube, the air will speed up through the narrow section and the pressure in that section will be lower than in the wider parts of the tube.
On the other hand, if air flowing down a tube encounters a fan that increases its speed, its pressure will not necessarily drop, and may rise. The fan is an external input of energy. A falling stream of fluid can gain energy from gravity as well.
If separate air streams pass through two pipes at different speeds, we can say nothing about their relative pressures. Bernoulli's principle applies only between points on a streamline, that is, where a body of air could travel from one point to the other. It does not apply to completely independent bodies of air.
The pressure in any free (ie unpiped) jet of air is atmospheric except very close to the nozzle. Thus the common explanation of the "Bernoulli Blower" exhibit - that the pressure in the moving air is lower than in the surrounding air - is incorrect. Note also that this explanation also ignores the fact that the moving air stream has passed through a fan.
A load of hot air
OVERGENERALISATION: Heat rises
CORRECT GENERALISATION: Hot and cold regions of the same fluid will tend to arrange themselves so that the warmer fluid is above the colder fluid.
If the statement "heat rises" were universally true, then you wouldn't be able to make toast under a grill, and we wouldn't feel the warmth of the Sun as it travels down from the sky.
But in our common experience, heat does seem to rise. Warm air rises to the top of a room or a building, it's hotter at the top of an oven, and you put your hands over a domestic radiator, not under it, to warm them. So what's Ben making all this fuss about?
The point is that the last paragraph dealt with only one way of moving heat from one place to another. It's called natural convection. If you warm part of a fluid, it will expand, and its density will fall. This will make it buoyant compared to cooler regions of the same fluid, and so it will tend to rise. As we live in a body of fluid (air), subject to all sorts of uneven heating processes, the rising of heat by natural convection is very common. Not surprisingly, we tend to think that heat rises.
(Note that if we heated an enclosed body of air completely uniformly, nothing much would happen. Hot air can only rise if there's some cold air to fall.)
But there are other processes of heat transfer, and they are very important. For example, all of the Sun's heat (the power for nearly all life on Earth) arrives by the process of radiation. This isn't nuclear radiation, it's electromagnetic radiation. Visible light is a form of electromagnetic radiation, and the radiation we feel as heat on our skin is light's cousin - invisible infra-red radiation. Radiation is much more important than you think. All objects emit radiation (the hotter they are, the more they emit) and you'd soon notice the difference if the walls around you stopped radiating. And the important thing is: electromagnetic radiation doesn't care about gravity - it goes down just as easily as it goes up.
(Actually, Einstein tells us that e-m radiation does care about gravity, but the effect is tiny in ordinary circumstances, and anyway, is such that e-m radiation is pulled down by gravity like everything else.)
The other important process of heat transfer is conduction, which is the way heat travels through stationary substances. The outside of a kettle gets hot when you boil water in the kettle, because the heat has been conducted through the wall of the kettle. Conduction doesn't care about gravity at all - it will take heat down as easily as it takes heat up.
I should mention that there is another kind of convection called forced convection, in which the warm fluid is moved about by some force other than its buoyancy - a fan for example. As we can move fluids in any direction we want, forced convection shows no preference for moving heat upwards either.
Raindrops and teardrops
FALSE: Raindrops are the traditional teardrop shape, round at the bottom and pointed at the top
TRUE: Raindrops are about the shape of a bread roll, tending towards spheres as they get smaller. The picture comes from an article in Scientific American in 1954. It shows the true shape of a large raindrop.
That pretty much says it all. Click on the link to see some pictures of raindrops.
This non-fact is often embellished by saying that the reason that raindrops are teardrop shaped [which we know they aren't] is that this shape falls most easily through the air. Now it is true that the teardrop shape, with its smooth front and tapered tail, is not a bad shape as far as air resistance is concerned. But there is no principle in physics that says that things moving through fluids automatically shape or align themselves to offer the least resistance - if there were, parachutists would be in trouble. You can see this for yourself by dropping a playing card onto the floor. The card would fall most easily if it was vertical, but it won't maintain a vertical position even if you start it off that way.