Tag: physics

Nobel Prize in Physics 2018

Nobel Prize in Physics 2018

Nobel Prize in Physics 2018. Image credits: nobelprize.org

The Nobel Prize in Physics was awarded to Arthur Ashkin, Gérard Mourou and Donna Strickland this year, with one half to Ashkin and other half jointly to Mourou and Strickland. The award honours the inventions in the field of laser physics. 

Arthur Ashkin was awarded the Nobel Prize for his invention of optical tweezers that grab particles, atoms and molecules with laser beam. Viruses, bacteria and other living cells can be held too without being damaged.

Gérard Mourou and Donna Strickland developed a technique to create high intensity ultra-short optical pulses. This technique has broad industrial and medical applications. 

Let’s first understand what optical tweezers mean,  its brief history and applications followed by ultrashort high intensity beams and its applications.

Optical Tweezers

Tweezers literally means some sort of instrument which is used to grab very small objects. Optical tweezers means light based method to grab something very small. Arthur Ashkin used laser beam to trap/grab/manipulate particles which are as small as atoms. Optical tweezers take advantage of ability of light to exert force, or radiation pressure.

Radiation pressure is the pressure exerted by light on matter. There’s an interesting video by Vsauce about radiation pressure click here to watch!

The idea that light could exert pressure was put forward by Johannes Kepler in 1619, who postulated that pressure of light explains why comet tails always point away from Sun. In 1873, James Clerk Maxwell showed theoretically that light can exert pressure, based on his theory of electromagnetism. In 1900s, the existence of radiation pressure was experimentally confirmed by Pyotr Lebedev, Ernest F. Nicholas and Gordan F. Hull. Radiation pressure is extraordinarily weak under everyday circumstances.

Lasers were invented in the year 1960 and soon after Ashkin began to experiment with it. In lasers light moves coherently, unlike ordinary white light in which the beams are mixed in all the colours of the rainbow and scattered in every direction. Ashkin did an experiment designed to look for particle motion from force due to radiation pressure of laser light on small particle. A sample of transparent latex spheres suspended in water was used to avoid any heating or radiometric forces. With just milliwatts of power, particle motion was observed in the direction of mildly focused Gaussian beam. However, an additional unanticipated force component was soon discovered that strongly pulled particles located in the fringes of the beam into the high intensity region on the beam axis. The understanding of the magnitude and properties of these two force components made it possible to devise the first stable three-dimensional optical trap for single neutral particles.

Image Credits: Optical trapping and manipulation of neutral particles using lasers.
Proc. Natl. Acad. Sci. USA Vol. 94, pp. 4853–4860, May 1997

The trap consists of two opposing moderately diverging Gaussian beams focused at points A and B as shown in the figure above (Fig 1 (B))

The next advance in optical trapping and manipulation was the demonstration of the optical levitation trap in air, under conditions in which gravity plays a significant role. In the levitation trap, as shown in Fig.2, a single vertical beam confines a macroscopic particle at a point E where gravity and the upward scattering force balance. 

Image Credits: Optical trapping and manipulation of neutral particles using lasers. Proc. Natl. Acad. Sci. USA Vol. 94, pp. 4853–4860, May 1997

Optical tweezers are now a widely used tool in biological physics and related areas, and continue to find new applications. The method is used for non-invasively trapping and manipulating objects such as single cells and organelles and for performing single-molecule force and motion measurements. The study of single molecules is made possible by linking them to “handles” that can be easily trapped with the tweezers, such as micron-sized polystyrene or silica beads. The beads also act as probes to monitor motion and force. It is also used to trap living bacteria. One important breakthrough was the ability to investigate the mechanical properties of molecular motors, large molecules that perform vital work inside cells. The first one to be mapped in detail using optical tweezers was a motor protein, kinesin, and its stepwise movement along microtubules, which are part of the cell’s skeleton. 

High Intensity Ultra-short Beams

Laser light is created through a chain reaction in which the particles of light, photons, generate even more photons. These can be emitted in pulses. Ever since lasers were invented, almost 60 years ago, researchers have endeavoured to create more intense pulses. However, by the mid-1980s, the end of the road had been reached. For short pulses it was no longer practically possible to increase the intensity of the light without destroying the amplifying material. 

CPA – Chirped Pulse Amplification

Strickland and Mourou’s new technique is known as Chirped Pulse Amplification (CPA). They took a short laser pulse, stretched it in time, amplified it and squeezed it together again. When a pulse is stretched in time, its peak power is much lower so it can be hugely amplified without damaging the amplifier. The pulse is then compressed in time, which means that more light is packed together within a tiny area of space – and the intensity of the pulse then increases dramatically. The CPA-technique invented by Strickland and Mourou revolutionised laser physics. It became standard for all later high-intensity lasers and a gateway to entirely new areas and applications in physics, chemistry and medicine.

Cutting materials using ultra short laser pulse.
Image Credits: assemblymag.com

There are some interesting applications of this. Things happen so quickly at the molecular and atomic levels that it was difficult to describe the process. Only before and after picture was possible to be described. But with pulses as short as a femtosecond, one million of a billionth of a second, it is possible to see events that previously appeared to be instantaneous. Ultra-sharp laser beams also make it possible to cut or drill holes in various materials extremely precisely – even in living matter. Many applications for these new laser techniques are waiting just around the corner – faster electronics, more effective solar cells, better catalysts, more powerful accelerators, new sources of energy, or designer pharmaceuticals.

To read the published papers click on the links below:

Arthur Ashkin 

Gérard Mourou and Donna Strickland

What is Tripple point?

What is Tripple point?

Tripple point is a term coined in 1873 by James Thomson, brother of Lord Kelvin. Tripple point of a substance means the temperature and pressure at which a substance exists in all the 3 states (gas, liquid, solid) simultaneously in thermodynamic equilibrium. Now that sounds pretty weird. For instance let us consider the substance to be water. Water boils at 373 K (100 degrees celcius), it is liquid above 273K (above 0 degrees Celcius) and it’s solid i.e ice below 273K (below 0 degrees Celcius). So how is it even possible that all three states are able to coexist.

First let’s go through some basic thermodynamics.
This is the famous Ideal Gas equation.
: Pressure
: Volume
n : No. of mole i.e amount of substance
R : Gas constant
: Temperature

Now when we try to achieve tripple point of any substance, the system or the container is isolated from the surrounding. So the volume remains constant, amount of substance is constant, the gas constant will always be a constant. So to keep the equation valid Pressure has to be directly proportional to Temperature.

Here things become interesting. So if we reduce the pressure in the system we actually reduce the boiling point of the substance. That’s why in vacuum chamber water boils instantly. So now all we have to do it carefully maintain pressure and temperature of the system in such a way that it freezes the substance into solid but low pressure starts boiling it and in between there will be liquid state. That’s how you obtain a tripple point of a substance.

And believe me it just looks too weird that a substance, for instance water, is boiling but freezing at the same time in the same container!
The tripple point for water is achieved at 273.16K (0.01 degrees celcius) and at the Pressure of 0.0060 atm i.e 0.611… kPa.

Take a look at this interesting video : https://youtu.be/r3zP9Rj7lnc

Just like water has tripple point at specific parameters of pressure and temperature other substances have thier own tripple points where they coexits in all 3 states.

What is the significance of tripple point?

What exactly is this parameter used for? Well triple points make ideal reference points for the calibration of thermometers. They can be realised by using a sealed, evacuated, cylindrical glass cell filled with the pure substance, with an axial re-entrant well for the insertion of the thermometer. This device is called tripple point cell used for calibration of thermometers.
The triple point of water has a unique place in metrology since it is the basis of the definition of the units of temperature, the kelvin and the degree Celsius. Its temperature is 273.16 K and 0.01 °C by definition. Additionally, the triple points of mercury and several gases – argon, oxygen, neon and hydrogen – are used as low temperature reference points on the ITS-90. Triple point cells containing organic substances can also be made. Ethylene carbonate has a triple point temperature of 36.315 °C which, being close to body temperature, makes it a highly useful reference point for the calibration of clinical thermometers, while benzoic acid has a triple point temperature of 122.33 °C, close to the sterilising temperature of medical drip solutions.

Triple point cells are so effective at achieving highly precise, reproducible temperatures, an international calibration standard for thermometers called ITS–90 relies upon triple point cells of hydrogen, neon, oxygen, argon, mercury, and water for delineating six of its defined temperature points.

What is a Lunar Eclipse?

What is a Lunar Eclipse?

There’s flood of images of blood moon on social media since 31st Jan. The popular term being used for the phenomenon is “Super Blue Blood Moon”.  That phenomenon was nothing but a Total Lunar Eclipse. Now we will understand what happens and how it happens. First of all lets understand what is a lunar eclipse and then we will discuss its types.

What is a Lunar Eclipse?

The most common definition we hear is, when the sun earth and moon are aligned in the same line/ when the moon enters the Earth’s shadow. That all is fine, but the most natural question that pops up in our mind is; then why doesn’t eclipse happen every month? As it takes about 27.3 days for one complete revolution around Earth shouldn’t the sun, moon and earth line up twice in a month which would lead to one total solar eclipse and one total lunar eclipse every month? But that doesn’t happen right? Eclipses doesn’t happen that often. Well that’s because the Moon’s orbital plane is inclined by about 5.1° to the ecliptic plane (the plane in which Earth orbits around sun). So Eclipses occur only when the moon passes through that exact point where these two planes coincide. And this doesn’t happen that often.

By Peter Sobchak - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=35889221
The Moon’s orbital plane differs a little than the Earth’s orbital plane.

Types of Lunar Eclipses

Umbra – The region where no sunlight reaches.
Penumbra – Region where sunlight reaches partially.

Penumbral Lunar Eclipse

Penumbral Lunar Eclipse happens when the moon passes through the penumbra region of the Earth’s shadow. This type of eclipse is usually very hard to notice as there is only slight change in the brightness of moon.

Partial Lunar Eclipse

When part of the moon passes through Umbra we get a partial Lunar eclipse. It is easy to notice as there is significant drop in Moon’s brightness. It might happen once or twice in a year.

Total Lunar Eclipse

Now this is something which doesn’t happen quite often. Total Lunar Eclipse happens when the moon passes through the umbra i.e all the sunlight is blocked by Earth. This event happened recently on 31st January 2018.

The types of Eclipses. Image credits: Addison Wesley.

So what was so special about the Lunar eclipse of 31st January? And why was it called Super Blue Blood Moon? Lets understand it word by word.

Super moon

The moon’s orbit around earth is not exact circle, its an ellipse. So there must be two points; when the moon is farthest from Earth and when it is closest to Earth. The farthest point is called apogee (406,300 km) and closest point is called perigee (356,700 km). Super moon is when its a full moon at perigee and its called super moon because it looks a little bit bigger than usual.

Comparison of Moon at Apogee and Perigee.

Blue Moon

This has nothing to do with the colour blue. When two full moons appear in one month then the second full moon of the month is called a blue moon! There is a phrase “once in a blue moon” so its evident that this is something rare. Blue moon happens once in 2 to 3 years. And whats more rare is a double blue moon which is two months in a year which has two full moons each! And this happens 3-5 times in a century!! The last double blue moon happened in 1999 and next will happen in 2037!

Blood Moon

Blood moon sounds so surreal! When a total lunar eclipse occurs all the sunlight is blocked by earth and moon is in the umbra region. Now Earth has a thick layer of atmosphere. Rayleigh’s scattering of sunlight in atmosphere is what gives sky its blue colour. The atmosphere scatters blue light more efficiently than red light (In better terms, the scattering is inversely proportional to the fourth power of wavelength). So the Red light passes through the atmosphere without getting scattered and enters the umbra region onto the moon! That’s what gives the moon a reddish hue! Hence the name blood moon.

The atmosphere bends red light towards the umbra region.
Image credits: lovebigisland.com

Super Blue Blood Moon

Now what are the odds of super moon, blue moon and blood moon happening at the same time !! That’s why people were hyped for the Super Blue Blood Moon!!! I got to watch the eclipse and also clicked images at different stages of eclipse. The first image is when the totality occurred, the moon was visibly red even with naked eyes. And then gradually it turned white into a full moon! Isn’t it beautiful!?

Composite image of different stages of Total Lunar Eclipse.

The next Blue Blood Moon will happen on 31st December 2028 and the next Super Blue Blood Moon will happen on 31st January 2037! So if you missed this event you need to have a lot of patience.