Category: Physics

Brazil Nut Effect – Saving Lives in Snow

By Anupum Pant

The effect

Have you ever noticed that the larger nuts (Brazil Nuts) in your Muesli boxes or bowls end up on the top? It turns out, scientists have a name for this effect. They call it the Brazil Nut effect or Muesli Effect. Well, actually scientists prefer to call it Granular convection.

If you haven’t heard of Muesli, think of a bowl full of random nuts. Or Indians could think of the Haldiram Navratan Mix – A popular snack mixture that contains several types of fried ingredients in various sizes. Irrespective of the kind of snack or even a mixture of sand you think of, it shouldn’t come as a surprise to you that the smallest of particles always end up at the bottom of a mixture when shaken enough.

If you think there is nothing cool about this effect, you are probably picturing it wrong. Watch this video where, on shaking, big marbles magically come on top of a rice heap.

Research studies

Normally, isn’t it common sense to assume that the largest (presumably the heaviest also) particles should end up at the bottom? Such is the counter-intuitive motion of particles in granular mixtures that it gives scientists a great insight about the real physics that goes inside. So they love to study more about this phenomenon.

This effect is so popular among scientists that some have studied it in martian and lunar gravity simulations. To do this they had to go on a plane that moves in parabolic arcs to simulate a specific reduced gravity conditions – A reduced gravity aircraft.

It saves lives

In the snow, sport persons or people taking part in extreme sports carry an avalanche safety bag with them.  On a free ride down a snowy cliff, sometimes these people get caught in avalanches. Being on top when the avalanche has settled helps the rescue teams find them. The avalanche safety bags are huge when they are inflated. And thanks to the Brazil Nut effect, these big bags attached to the sports person, often end up on the top.

The Reverse Brazil Nut Effect

And sometimes there is a reverse Brazil nut effect. Meaning, if certain conditions are met, the largest nuts can end up at the bottom of a container.

The shape of the nuts, shape of the container and the relative sizes of nuts plays a role in determining if the effect reverses or not. As a simple example, using cone shaped container reverses the effect.

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The Underwater Optical Man-hole

By Anupum Pant

Agreed, sometimes, when you find yourself being interrogated in a room covered with one-way mirrors, you can’t see the people who are observing you; Instead, you see yourself in the mirrors. Otherwise, If you can see something, it seems normal to assume that the thing can see you too.

A trout’s window to the outside world is something similar to what a person in the interrogation room experiences. However, unlike the person, a fish can actually see things that are out of the water, but the view is very limited.

The Snell’s Window

When a fish looks up from water, it sees only a circular window of light, from under the water surface. Everything that lies outside of this circle is darker. This darker area of vision is replaced by the reflection of the sea/lake bed (where there is no source of light to illuminate it). This effect isn’t due to any limitations of a fish’s eye. In fact, even human divers see only a circle of light when they are under water. This circle is called the Snell’s Window or the optical man-hole.

Irrespective of the fish’s visual acuity, some physical properties of water and air get together and have a great effects on what a fish can see. It sees a circle with diameter calculated by the Snell’s equation.
In short, the window is about 2.3 times as wide as the fish’s depth. So, a fish can see more if it goes deeper. At a depth of 1 meter, it can clearly see things on a circle that is 2.3 m wide on the surface of water.

So, even if you can see a fish in water, it will be foolish to assume that the fish can see you too. Some times it can’t. It looks something like this from under water:

In Wikipedia’s words:

Snell’s window is a phenomenon by which an underwater viewer sees everything above the surface through a cone of light of width of about 96 degrees.

Why does it happen?

It happens due to a simple optical phenomenon called the total internal reflection.
The physics behind this phenomenon can be read here. [Read here]

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There Is No Pink

By Anupum Pant

As we’ve seen before in a talk by David Eagleman, that there is nothing like colors really. They are simply electromagnetic waves with varying wavelengths. Colors are perceptions created by our brains that give us an evolutionary advantage to differentiate things easily. Without colors it would have been really difficult for us to spot fruits on trees. Of course that is just one of the millions of examples of how colors help us.

Perception kept aside for a while, we actually do know that there is a spectrum of visible light as we see it – ranges from violet to red. We see this spectrum on rainbows and thin films. Each of these colors on the spectrum is a wave (and particle) that has a particular frequency.
Mysteriously, the universal symbol of love, the color pink, is absent in this spectrum. There is no specific frequency for the color pink. There is no pink. Still we see it. So, what is pink, really? If it isn’t in the spectrum, why do we see it?

Why do we see pink?

Single type cone alone: We detect colors through these things called cones that are present at the back of our eye. There are 3 types of cones – let us call them red, blue and green. So, if an object absorbs all the white (sun) light and sends just the red color [waves] towards your eyes, red cones get activated and your brain tells you, you are seeing the color red. Similarly, green or blue cones get activated when the respective green or blue waves come towards your eye and then you are able to see the colors green or blue.

2 of them together: For other colors, things can get a bit complicated. To see pure yellow, both red and green cones have to get activated. Similarly, when green plus blue cones get activated, you see cyan, and blue plus red cones let you see the color magenta.

But cone aren’t switches that go either one or zero. They are like sliders. For instance, to see the violet color, your blue cones get fully active, while the red cones are activated only to a certain extent. As a result, your brain says, violet! That is 2 types of cones working together.

3 of them together: Now let us see how three of them work together. The color white activates all the 3 type of cones fully. Black activates none. And so on…

Pink does something similar as it uses three types of cones. To see pink, all three types of cones have to work together.  When red cones get fully active and the other two are only partially activated, we see the color pink.

So, even if objects don’t reflect magenta, yellow or pink (or several other RGB combinations like that), our cones can send mixed signals to our brains and the brain in turn creates these colors for us. In reality, they don’t exist.

[Read more]

What is pink really?

Henry Reich of minute physics, in his video explains this by referring to pink as white minus green. So, according to them, the color pink is actually minus green.  In short, absence of green color is nothing but pink. I’ve attached the video below:

Dancing Drops of Water and Dipping Hands in Molten Metal

By Anupum Pant

When you sprinkle water on a hot pan, you’ll find that the droplets start dancing on the surface, as if there was no friction at all. From far, this effect looks a lot like water droplets on a lotus leaf (a super-hydrophobic surface). But, the physics behind this phenomenon is completely different. Read on to find out what is the mystery behind these dancing drops of water.

The Leidenfrost Effect

Why does this happen?
Unlike the drops on a lotus leaf, this happens at a particular temperature for a specific liquid. Different kinds of liquids display this effect at different temperatures.
For water, at a temperature when a small amount of water in contact with the pan gets heated enough to form a thin-film of vapor below the drop, water is no longer stuck on the pan (water sticks to some surfaces due to low surface tension). The drop has a thin vapor film below it which enables the drop to move around on the film. The formation of this vapor film is a continuous process, till the whole drop turns into water, one film at a time. This is called the Leidenfrost Effect.

Some liquids like liquid Nitrogen are extremely cold. At normal room temperature, they start boiling. A normal room’s floor is like a hot pan for liquid Nitrogen. So, it forms these dancing drops on a floor which is just at room temperature. You can try this yourself – If you can find some liquid Nitrogen, you can simply drop it on the floor and watch droplets moving effortlessly. They won’t stop moving!

Dipping hands in Liquid Nitrogen

The temperature of liquid Nitrogen is around -195 degree centigrade. It is one of the coldest substances and is used with extreme caution in industries and laboratories. If it touches you, your skin can easily get burnt. Yes, burnt – at extremely low temperature. It could probably also make the dipped limb useless for life. So, you shouldn’t try stuff with liquid Nitrogen at home.

But, it turns out, you can safely dip your hand in it for a small amount of time and return unharmed. Thanks to the Leidenfrost effect. Our hot-pan like hand – for cold liquid Nitrogen – makes a thin film of vaporized Nitrogen around the whole hand. This film, protects our skin from the ill effects of extremely cold temperatures. Still, there is no reason for you to try this. It has been done already.

The crazy duo from Myth Busters tried this with molten lead. It worked!  They, of course had to wet the finger with water – for the vapor film formation.

Water flowing uphill

Recently, an undergraduate research student group from the University of Bath found out a way to manipulate the movement of water on a specially designed surface, using this phenomenon. They found that machining ridges on the surface (and heating it) would make the thin vapor films under water droplets move in such a way, that they could use it to propel drops against gravity. They were able to demonstrate this by showing water moving uphill on a slope. It is enthralling to see it for yourself. I’ve attached their video below.

Prince Rupert’s Drop – Exploding Glass

By Anupum Pant

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What is it?

At first, a Prince Rupert’s Drop is an interesting yet harmless looking drop of glass with a long tail. It looks like a tadpole: [image]

It is no different from an annoyed person who refuses to let out his resentment – A slightest something might make him explode suddenly, but it isn’t easy to make him let it out. Confused? Read on…

Now, think of a glass drop that has immense amounts of potential energy stored inside it – It explodes (actually implodes) when the tail is disturbed, but it is impossible to hit it hard with a hammer and break it.

How?

A Prince Rupert’s Drop is formed when a drop of molten glass is suddenly dropped into a water bath. This quick cooling, solidifies the surface fast, while the inner part remains molten. Now, glass formed on the surface, being a poor conductor of heat doesn’t allow the inner part to cool quickly. When the inner part starts cooling, it tries to shrink and pulls the surface towards it. As a result, great amount of potential energy gets stored inside, in the form of stresses (stresses are seen using a polarized filter). This stored energy gets released when the tail is disturbed – It explodes into very tiny pieces of glass.

Toughened glass – a stronger variety of glass used in several places – also uses a similar technique to make strengthened glass.

On Wikipedia, a user asked about the possibility of utilizing the energy released from this explosion, being used to fire a bullet from a barrel. An interesting possibility, I must say.

The Name

Prince Rupert of Rhine did not discover the drops, but played a role in bringing them to Britain. He gave them to King Charles II, who in turn delivered them to the Royal Society for scientific study. Prince Rupert’s Drop was a widely known phenomenon among the educated during the 17th century – far more than now.

Watch it being explained better

Probably the best demonstration of this glass drop exploding is right here on the internet. Couple of months back, a YouTuber, Destin (Channel: SmarterEveryDay) posted a video demonstrating the physics behind it. He recorded  the progression of the explosive fracture using a hi-speed camera (at more than 100,000 frames per second) and calculated the speed of the fracture travelling through its tail (~ 1.5 miles per second). I’ve attached it below for you to watch.

Longest Continuously Running Experiment – 83 Years and Counting

By Anupum Pant

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An experiment so slow that a professor overseeing it, died without having seen the results for half a century! The Pitch Drop experiment, started by Professor Thomas Parnell of the University of Queensland in the year 1930, is the probably slowest science experiment and also holds the Guinness World Record for the world’s longest continuously running experiment ever.

What is the experiment?

It is an experiment designed to measure the flow of a solid looking piece (image) which is actually an extremely viscous liquid (actually a Viscoelastic Polymer) with a viscosity of approximately 230 billion times that of water. The name used for this class of extremely viscous liquids is, Pitch – Bitumen, Asphalt, Resin and Rosin are a few examples (not Glass). These things are so viscous that you can strike them with a hammer and see them shatter into sharp flakes (like glass), but it flows. The experiment is explained in detail, in the first few minutes of this radio show attached below. (the second half is pretty interesting too, but that is for some other day)

Other unbelievable materials previously covered in this series – Aerogels and Superhydrophobic surfaces.

Side note: The overseer of this experiment, Prof. John Mainstone actually lived through the drops of pitch falling three times, but unfortunately missed watching it happen every time (for 3 times in 50 years). In all, 8 drops have fallen since 1930.

  1. 1979 – He missed it because he wasn’t in the laboratory for the weekend.
  2. 1988 – Missed it because he went out for a tea break.
  3. 2000 – A camera was installed as a precautionary measure, the equipment malfunctioned; missed again!

He recently died waiting to see it in action. Since then, three web cameras have been installed as a fool proof measure to record the extremely rare event. You can watch it happening online here, although you might have to wait for several years to see it happening. (To confirm the live stream, look at that clock in in it and confirm with time here). There is also a time-lapse from 28th April 2012 – 10th April 2013 compressed into a 10-second-long video of the drop forming, embedded below.

A parallel experiment running at Trinity College, Dublin also wasn’t able to capture the rare scientific event on camera in spite of several drops falling since the commissioning of the experiment (1944). Finally, after 70 years of patient wait, on July 11, 2013 it was recorded on camera.

Why is a Metal Plate “Colder” Than a Plastic Plate?

by Anupum Pant

No, it isn’t!

What is Cold?

According to the dictionary, a body at a relatively lower temperature, especially when it is compared to the temperature of a human body is described as a colder one. So, any object below the normal human body temperature – about 37 degrees Celsius – is a cold thing. But wait a minute!

When you touch an object, what does it tell you about the temperature of the object? Can you really judge if it is a cold one or a hot one? Unfortunately, our bodies aren’t thermometers, we are not so smart when it comes to judging the temperature. Consider the following case.

A book and a steel plate kept in the same environment for a long time attain the same temperature eventually (it is called thermal equilibrium). This can be checked by using a thermometer on both the objects. But, when people are asked to touch a metal plate and a book, they find the former to be much cooler. You can try this out yourself by touching different materials around you. You’ll see how some things ‘feel colder’ while the others feel warmer. A YouTube channel Vertasium conducted a social experiment to record this on camera. See the video below:

There is no cold – only heat

So, in the video, ice melts faster, if kept on steel plate than on a plastic plate, even when the steel plate ‘feels colder’. Common sense dictates that the colder thing is supposed to sustain the ice block for a longer time, just like your refrigerator does. So why does the opposite happen?

A better way to understand this ‘contradiction’ (not really a contradiction) can be this:

According to thermodynamics, simply put, everything has heat in it. So, even a cold ice block has some amount of heat stored in it (say, around 273.15 Kelvin or 0 degree Celsius). When one object comes in contact with other object, it loses or gains heat till their temperatures get equal or till they attain ‘thermal equilibrium’. Which object loses heat and which one gains it, is decided by their relative temperatures. In case of ice and steel, ice has a lower temperature than steel (assuming it isn’t already freezing out there). Therefore, here, ice gains heat from steel till they attain the same temperature and ice melts.

Side note: The ice is also in contact with a relatively ‘hotter’ atmosphere. Hence, it gains heat from there also. In this case, we are only concerned about the steel and ice interaction.

Why does it melt faster on steel?

There is a particular property which depends on the kind of material and is called thermal conductivity. This is the parameter which decides which objects lose heat quicker and which ones do it slower.

Here, for instance, steel has a higher thermal conductivity than plastic. Hence, the steel plate gives away heat to the ice block faster than a plastic block does. As a result, ice melts faster on a steel plate than on a plastic one.

Incidentally, this effect can also be used to explain why one plate feels colder than the other, in our hands. Think of it like this, the ice is replaced by our hand. So, a steel plate, due to its better thermal conductivity, draws heat faster from our hand than a plastic plate. This makes us feel that the steel plate is colder than the plastic one.

As checked by a thermometer, both the plates have the same temperature, our bodies are only fooled into believing that the thing we feel is temperature; it isn’t. None of the plates is actually colder than the other (according to the dictionary – see first paragraph). We don’t feel the temperature. What we feel is actually the rate of heat being drawn away from our hand. Faster an object draws heat, the colder it feels.

How Loud Can it Get?

by Anupum Pant

Wives and moms can scream really well. But is it loud enough to inflict physical pain? Can sounds get louder than a nuclear bomb? How much damage can a loud sound cause? How about mass extinction? Read on to find out the answers.

What is sound?

Sound, as most of us know is a longitudinal, mechanical wave. That means, it is just a series of pressure changes [compressions and rarefactions] in a particular medium. So, the property of sound is as good as the medium it uses to travel. For instance, sound cannot travel in a vacuum due to the absence of any medium, but it can travel much faster in solids than in air. That is the reason you can’t hear someone talking in space (yes, movies that show loud explosions in space, lie). Also, the faster speed of sound in steel rails is exactly the reason why, you can tell a train is approaching, if you stick your ears to the rails (do not try this on electric rails).

Two of the fundamental parameters that describe a sound wave in numbers are pitch and amplitude. Pitch is measured in hertz – we’ll talk about it some other day. But, the amplitude of a sound wave determines how powerful it is; greater the amplitude, louder the sound. The loudness of sound is measured in Decibels (abbreviated dB).

More about decibel scale

Like most other linear scales, Decibel isn’t as easy to understand. A 10 point rise in the dB scale can be visualized as a 10 times increase in the loudness. Adding dB levels of different sound sources also doesn’t really work, the calculation is much more complicated; the resultant loudness depends on the coherency of the source [See this decibel addition applet]. Also, the perceived loudness is obviously lesser as you go away from the sound. Normally, a decibel scale ranging from 0 dB to 130 dB is enough for measuring the loudness of most things. But, things can get louder…much louder.

To get an idea of the decibel scale: 10 dB is 10 times more powerful than 0 dB, not 10 points greater. Similarly, 20 dB is 100 times more powerful and 140 dB is 100,000,000,000,000 times more powerful than a o dB sound.

0 dB is the loudness of near silence (a mosquito 10 feet away), while 120 dB is the loudness of a loud car horn heard from 1 meter away. Humans can hear sounds starting from 0 dB. But it can be quieter than 0 dB [the world’s quietest room]. It measures record setting -9 dB and can literally drive you crazy. In fact, the longest someone stayed in that room was for 45 minutes.
On the upper side of dB spectrum, a whisper is around 15 dB, conversations range from 40 – 60 dB and a jet engine measures 130 dB on the decibel scale. Like I said before, the perceived loudness depends on your ear’s distance from the source, so the loudness of a lawnmower can range anything from 90 dB to as much as 110 dB if you stand 3 feet away from it. [see the Decibel chart]

90 Decibels or a sound as loud as only a raised voice can cause gradual hearing loss (Refer to the hearing safety chart here). While 140 dB can cause physical pain. After 150 dB (firecracker) sounds can be felt in the form of shock waves. The pressure difference they cause in the medium can actually be felt by your body.

Beyond Decibels

Since the loudness depends on the medium, the maximum loudness a medium can propagate is dependent on its density. Our atmosphere can do nothing more than 190 dB, that, by the way, is enough to make you deaf or cause death. Sounds in water can get louder. A pistol shrimp is able to create a 200+ dB sound at 97 km/h to stun or kill its prey by snapping claws really fast. This is a very short lived pulse which doesn’t carry enough energy to do us any harm.

For events like the Saturn V launch, volcanic explosion, nuclear bomb explosion, earthquakes, star-quakes the concept of sound doesn’t really apply anymore. They are measured in terms of the shock wave they produce using the Richter scale. On this scale, 9 means total destruction (8.2 was measured during the explosion of the largest bomb ever, Tsar Bomba). An earthquake or earthly event measuring 10 has never been observed.

However, in the universe beyond earth, the starquake on the magnetar SGR 1806-20 registered 22.8 to 32 on the Richter scale. The magnetar released more energy in one-tenth of a second than our sun has released in 100,000 years. An event which thankfully took place 50,000 light-years away from earth. Had it been even 10 light-years away, the energy released would have wiped off life on earth. [read this BBC article for more information on this event]