Friday, February 25, 2011

Buckets of water

I was asked why a bucket full of water looks shallower than an empty one, so I pulled out an old physics book to find the answer. It's been many years since I've taken optics, although recently I've developed a new interest for it.

Refraction occurs because the speed of light changes based on the density – something I discussed here. The refractive index is the ratio of the speed of light in a vacuum to the speed in the medium. If we think about water with its refractive index of 1.33, we find that light travels 1.33 times faster in a vacuum than the water. The denser the medium, the greater the difference in speed of light and the bigger the refractive index.

Not only does light slow down, it also bends. When a ray of light hits a surface at an angle (angle of incidence) it gets bent to a new angle (angle of refraction) inside the surface. With a little trigonometry applied to these angles, we find that their ratio is also the refractive index, a trick discovered by Willebrod Snellius (of Snell's law fame) in 1621 – although an Arab scientist figured this out almost 500 years earlier.

So, what fun can we have with the refractive index? Ever looked into a still pool of water? Due to light rays bending in the water, the pool will look ¾ the depth it actually is. If a post sticks up through the water, it will look oddly disjointed at the surface – appearing to extend at one angle above the water and another below the surface even through the pole is straight.

From another point of view, what does a fish see when it looks up? A fish sees a lot more than expected. By looking up in a cone of 98 degrees, a fish gets a 180 degree view above the water due to refraction. The view above the water would be strange – someone fishing on the shore would look excessively squat, standing at an odd angle and probably distorted due to ripples on the surface. But, the fish would see the fisherman, making it much more difficult to be successful at fishing (spear fishing is even more complex due to refraction). By the way, if you put on your goggles and hopped into the local swimming pool, you would see what the fish sees.

Sunday, February 20, 2011

The physics of salad dressing

Ever made your own salad dressing? For a vinaigrette, ever wondered why the result is opaque even though most of the ingredients are clear? Vinegar is typically clear, as is oil – a trick of optics makes the results opaque, which is easily demonstrated at home (In case you want to check this one out for yourself).

Put oil and water (as a stand in for the vinegar) together in a jar (see picture). Oil floats on water with an easily seen interface. The background of my blue cutting board shows through for both layers. Vigorous shaking of the jar creates a emulsion of the two liquids. An emulsion is not the same as mixing, since the oil and water don't actually mix. Instead, both liquids form tiny bubbles that co-exist beside each other – over time they would separate back out into two layers. Once the emulsion forms it become opaque (see picture number 2).

An opaque liquid like mud (tiny dirt particles suspended in water) operates differently – mud's opaque because it absorbs much of the light incident upon it. Our oil and water looks opaque because of back reflection. Each tiny drop of oil and water remains clear. Now gizillions of surfaces form, separating the oil and water and each interface reflects light. Since the drops are round, the light isn't reflected perfectly back where it came from like a mirror, instead it scatters in all directions. This scattering creates a matte look to the emulsion – like the look of white paper.

As a tangent, under high magnification, white paper consists of random criss-crossing fibers that also scatters the light incident upon it. So a vinaigrette and white paper have something in common.

If you sit your vinaigrette on the counter for a few moments, the two layers will reform quite quickly. To keep an emulsion emulsified for a longer period of time, an emulsifier can be added. Mustard or honey are often added to vinaigrettes for this reason and their tastiness. Egg yokes can also be used and they typically act as an emulsifier for mayonnaise.

Thursday, February 17, 2011

fabrics with a fishy twist

Since I love the effect of structural colours, when I came across a story that could have me wearing them a few years from now, I had to share. Check out this.

Tuesday, February 15, 2011

A flash of green

The spectrum of sunlight peaks in the green wavelengths (520-570 nanometers), explaining why most plants are green: an attempt to optimize the available energy for photosynthesis. Our vision's colour sensitivity also peaks in the greens – the better to hunt green vegetables. With so many greens surrounding us, it would be easy to overlook a rare optical phenomenon in the sky: the green flash at dusk.

Just after the sun's final moment in the sky, a flash of brilliant green may be seen for a brief moment, but - only if the conditions are exactly right. A corresponding flash of green may occur just before the sun rises over the horizon. I've never been lucky enough to see either of these phenomenon. I assume folks saw this flash as soon as they started looking at the sky, however, the first conclusive scientific sighting occurred in 1865 by W. Swan. He described the sight as a 'dazzling emerald green' flash at sunrise. In 1926, a PhD thesis was written on the topic by P.F. Keuper. As I haven't been able to find a copy, I don't know if he found it dazzling or not.

Atmospheric refraction causes the green flash (although other not yet understood phenomenon may be involved). As we know, sunlight is composed of many different wavelengths. In the vacuum of space, all wavelengths of light travel at the same speed (the speed of light, a speed we cannot exceed). Once they hit the atmosphere, some wavelengths are absorbed and some pass through. For now, let's consider the visible spectrum as most of it passes through the atmosphere without being absorbed.

Since the atmosphere is contains more stuff than the vacuum of space, when light enters it slows down. If the incoming light hits the atmosphere straight on, all the wavelengths pass through, albeit at a slightly slower speed. At an angle, the light is forced to bend as it enters the atmosphere – an effect called refraction. This allows us to see things slightly over the horizon, like a ship, because light is refracted the same direction as the earth curves.

The amount light slows differs for the different wavelengths resulting in slightly different bent angles. Higher frequency light (blue/green) bends more than the lower frequencies (red/orange). In the extreme angled case, like a setting sun, the sun's image separates by colour (difficult to see because the sun is so bright and dangerous to look at). If you could slow the sunset down, first red would set, followed by yellow, then green with a remaining glint of blue/violet. Typically, just a flash of green is seen, or if the conditions are exceptional a flash of blue.

If you are setting out to see a green flash, the ideal conditions include a sharp horizon (ie, far away from buildings), perfectly clear air (get even further from those buildings) and a homogeneous atmosphere (a remote desert might work).

For more info, check out wikipedia or Sunsets, Twilights, and Evening Skies by Aden and Majorie Meinel (1983), or The Field Guide to Natural Phenomena by Keith Heidorn and Ian Whitelaw.

Friday, February 11, 2011

Bubble colours

Who hasn't blown bubbles outside on a sunny day? If you haven't, devote some time to blowing bubbles the next time the sun is out - it's fun. As a bubble floats gracefully through the air, sunlight creates a virtual rainbow (actual rainbows are formed by a different process) of colours across the bubble's surface. The colour-making phenomenon at work is the same as what creates the colours on a slick of oil, a rooster's tail, a cardinal tetra or hummingbird's gorget – it's iridescence.

Bubble walls are constructed of several thin layers, two soap layers sandwich a layer of water between them. This wall encases a volume of air. As sunlight shines on a bubble, some of it reflects off the surface and some enters the soap film. Inside the bubble wall, light travels slower because both water and soap are denser than the air. At the interface between the soap film and the water, again some light reflects and some passes through. The reflected light may bounce back and forth between the two surfaces a few times or it may just pass back out of the bubble. Reflection or transmission of light occurs at every interface. Most of the light emerging from the interior of the bubble wall will be out of phase with the light that reflects off the interface. Out of phase means that the troughs from one wave line up with the crests from another so that the waves cancel each other out. Some of the emerging light will be in phase, that is, the crests and troughs line up with each other. These two light waves amplify each other, resulting in brilliant iridescent colours.

Layer thickness determines what wavelengths (thus colour) will be amplified. If you move and look at the bubble from a different angle, the colours will change. This is because your viewing angle has changed in relation to the layers. From different angles the distance the light has to traverse to reach you changes, thus the wavelengths that amplify each other also change.

Unfortunately, soap bubbles last only a short time. It doesn't take long before gravity pulls the liquid to the bottom and evaporation whisks fluid away. Bubble colours change as the bubble changes. When the bubble walls are thick, only red gets canceled out leaving blues and greens. As the walls thin, yellow is also canceled out leaving blue. Next green is removed and the bubble looks magenta. Blue goes last making the bubble look golden yellow. As the bubble wall's continue to thin, all the waves in the visible region cancel out and the bubble looks just clear. When the walls reach about 25 nanometres thick the bubble is in serious risk of popping.

Since the thickness of a bubble's walls aren't constant - the walls thicken towards the bottom (remember gravity acts to pull the water down), bands of colours seem to fall downwards on the surface. That pesky gravity also prevents us from dyeing bubbles. The dye will only mix with the water and drain to the bottom of the bubble. However, when gravity is absent dyeing bubbles becomes possible – so if you head out on a long voyage to Mars bring lots of bubble making supplies as you'll have years to perfect your bubble dyeing technique.

Sunday, February 6, 2011

What makes water wet?

Water is pretty fabulous stuff. It makes life possible and over enough time acts as a universal solvent. On earth, water is everywhere from the glass on my desk to covering over 70% of the surface. Oceans contain approximately 1,360,000 cubic kilometers worth of water – a lot of water even before land based, sub-surface, frozen or any other place water hides is considered.

Our current understanding of water came relatively recently. We held onto the ancient Greek concept that water was one of the four basic elements (along with fire, earth and air) up until the Renaissance. Water is very stable, making it difficult to break into its parts, but it was done at a time when most people considered it a fundamental element. The French chemist Lavoisier managed to break water apart into its components (two hydrogen molecules and one oxygen on) on 28 February 1785. In time it was determined that covalent bonds hold water together, that is, by sharing electrons between atoms.

A water molecule resembles Mickey Mouse's head. Two ears on top, which are hydrogen atoms, and the head would be the oxygen atom. The top, where the hydrogen is, is somewhat positive and the bottom, where the oxygen is, is somewhat negative resulting in a polar molecule. Since positive and negative are attracted to each other a hydrogen atom from one water molecule is attracted to the oxygen atom of a neighboring water molecule.

How many water molecules must we have before it can be called a liquid? In the field of fluid mechanics, they have what is called a 'continuum hypothesis'. This hypothesis assumes that to be a fluid there must be over a million water molecules present within a reasonable volume (by reasonable, I mean some where between packing them in so tightly they become like a black hole or spreading them out so much it looks like a vacuum).

So, assuming we have enough water molecules to make a liquid, let's think about putting them all into a glass of water. In the middle of the glass, each water molecule attracts its neighboring molecules (remember the polar trait from above). This attraction occurs from all direction at once, resulting in a balance and no net force. Things don't work out so nicely at the surface. Here, the attraction doesn't balance because there is only attraction from the molecules to the sides and below as there are no water molecules above the surface. This causes surface tension. Molecules at the surface are pulled towards the center of the liquid, minimizing the surface area. A very small drop of water pulls itself into a sphere because a sphere has the smallest ratio of surface area to volume. Water in a glass will form a flat surface (ignoring the meniscus) as it is the minimum space it can take up.

Since wet is defined as 'consisting of, containing, covered with, or soaked with liquid (as water)'. Water acts to makes something else wet (as opposed to being wet itself) – so if I go walking outside on a rainy day, I end up wet from the rain.

Friday, February 4, 2011

My Forest

I grew up in a house surrounded by forest. We didn't live on a large lot, three acres or so, but the house couldn't be seen from the road and we couldn't see the neighbours from the house. The forest was typical of the area – temperate rain forest. Full of sappy fir trees, gray barked alders and majestic big leaf maples, each growing tall enough that their canopy perpetually shaded the forest floor. Under the trees a unique habitat thrived which I spent hours exploring. I considered it my forest and during daylight hours it felt friendly to me. I'm sure if I went back there it would seem small, but at that time/when I was little it felt huge and packed with things to see. The rich earthy smells have stuck with me and anytime I walk into a similar forest the smell immediately takes me back.

In the shade, huckleberries, salmon berries, salal, and vanilla plants (I have no idea if that is the true name for them, they grew as a single stalk with three large triangular leaves and to me they smelled like vanilla) grew. If I was really lucky, I would find a blooming trillium, which I would never pick because my mother told me if I picked them they wouldn't come back the next year. Tiny yellow violets could also be found hidden between ferns and stumps. On an particularly unlucky day, I found a not-so-fresh deer carcass – the smell from that find is still etched in my memory. The forest also provided a home for large wasp nests and many types of birds who were mostly heard, not seen.

Since we lived on a hill the forest had dry areas and boggy areas. The boggy low areas contained skunk cabbage and huge ferns beneath massive cedars. Here, a wrongly placed foot would end up swamped in mud. Little tea-coloured bogs/ponds teemed with tiny lifeforms. With rubber boots and a collection of jars, I would wade into the ponds and try to catch the little critters.

The stumps of the old growth forest remained as a reminder of a past majesty, like the ruins of an ancient civilization. These stumps could be found in varying states of decay everywhere. Most were topped with thickets of salal, except one which was hollow. This exception became a favorite hiding place and reminded me of a book I read about the same time where a boy did the same thing.

Human machinery had pushed some of these old stumps aside. A large bulldozer would do the trick. Someone before we moved there must have done it because I don't remember big bulldozers in my forest. The stumps were pushed (or pulled) until their massive root balls came clear of the soil. These root balls had a diameter much greater than the stumps. On their side, the roots would extend up in a gnarled mass of hand holds - so I would climb them. Some had been there so long that ferns and huckleberry bushes grew at the top. All of them were dirty. I had to be careful, because sometimes a choice hand hold would only be a dried ball of mud and would crumble the instant I held onto it. I always felt satisfaction when I would get to the highest root and look around.

The light in my memory is always the same: dappled sunlight that would shift as a breeze moved the canopy of fir needles above. This indirect light allowed for dark recesses in the forest that added mystery. Was something watching me from these recesses? (This very question made this forest a scary place at night.) A cougar? A bear? They were in my forest too, but I never saw them. In reality, I was never that far away from our house. I think I was lucky to have had the opportunity to roam alone through this wild place – many kids don't.

Wednesday, February 2, 2011


Although only a fortunate few can ever visit the deep sea, the precise instruments of the oceanographer, recording light penetration, pressure, salinity and temperature, have given us the materials with which to reconstruct in imagination these eerie forbidding regions
- Rachael Carson, The Sea Around Us

I wouldn't call the deep ocean eerie. Alien is perhaps a better descriptor, as it's a place where survival is only possible with extreme measures similar to what we would need to survive in outer space – minus the rockets. I like the idea of being in a profession (ie oceanographer) that includes exploring an alien world right here at home. The more I think about the exploring part of my last post, the more I wonder if I consider myself an explorer? I've been reading about early attempts made by the British to explore Africa's interior. Most expeditions ended in a typical horror movie ending – everyone dies. What prompted them to set off on expeditions that would most likely kill them?

Mungo Park, an Englishman who barely made it home the first time, traveled to West Africa for the second time in 1805 – the trip didn't end well. 40 soldiers accompanied him (not by his choice) along with the few friends he really wanted to travel with. A twist of traveling fate started them walking into the jungle in the rainy season, resulting in malaria and dysentery. Wild dogs, crocodiles and lions attacked the group, and locals stole their gear. Party members dropped one by one. By the time they reached the Niger River, only 12 of the original group remained. In the end, when the party was whittled down even further, they constructed a 'boat' to ride the Niger back to the ocean. Angry locals, who weren't paid the tributes they expected, ambushed the boat. Park was never heard of again. Years later, Mungo Park's son, Thomas Park, vanished into Africa looking for his father – all that was ever seen of him again was a freshly laundered shirt labeled 'T Park' that was delivered with another explorer's laundry on the coast. (from The Age of Wonder: how the romantic generation discovered the beauty and terror of science by Richard Holms).

I've gone on expeditions to various places for various reasons, but I've always left confident I would come home okay. I'm not sure how well I would fare if I knew everyone who went before me had died, but I suspect I wouldn't be keen on going.

If I look in an issue of Outside Magazine, there is always a tale of travel to some remote place. In the issue on my desk, it's to Mongolia (it's an issue from last summer). These expeditions are more about adventure and less about exploration. We've already explored the surface of our globe – which doesn't negate the value of exploring a place because it is new to you. The only places in this planet that are new to everyone are in the water or under the ground.

My field – physical oceanography, includes exploring the waters beneath the ocean surface. The term 'oceanography' comes from the ancient Greek 'oceanus' and 'graphia' and literally means recording and describing the ocean. 'Oceanology' might be a better descriptor for the field as it means 'the science of the ocean,' but, it isn't in common usage. Oceans make up a significant part on the surface of earth and includes vast unexplored areas. Huge unanswered questions about the physics of the oceans still exist – for instance we know how much energy is put into the ocean through the tides but we can't account for where all that energy goes.

For my work, I still consider the pressure, salinity and temperature in the same way they did back in the 1950's when The Sea Around Us was written, except now these values are measured electronically. This means I can get significantly more information. In fact, there is probably significantly more oceanographic data out there than oceanographers to analyze it.

Back to my original question – can I consider myself an explorer? Since, there is stuff out there for everyone to explore, I'll conclude we all can be explorers if we choose to. For some ideas on how to start check out How to be an Explorer of the World by Keri Smith. Here are some of my ideas: What about the exploration of ones own mind? One's own world? Or even cyberspace? How about exploring the world through food?