Pratik Barve, Author at PhysicStuff

Nobel Prize in Physics 2018

By Pratik Barve | October 6, 2018 | Comments 0 Comment

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.

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.

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

How do they determine Stellar distances?

By Pratik Barve | August 16, 2018 | Comments 2 comments

Just looking at stars in the night sky seems as if every star is equally far away. Feels like we are bound by some spherical ball with stars on the sphere. Any star seems to be as far as any other and this led the ancient Greeks to believe that all the stars were the same distance away. Distance is one of the most important and difficult parameters which has to be measured in Astronomy. Astronomers use some smart methods to measure stellar distances. These distances are really too huge to be determined using traditional methods as they are beyond the scale of any physical instrument we use.

People started to think about ways to measure stellar distances but there were no instruments before Galileo built a refracting telescope in 1609 which would aid in measuring distances with sufficient accuracy.

Here’s a list of methods used to measure Stellar distances:

Parallax Method.
Using Variable Stars.
Using Colour of Stars.
Using expansion of Universe.

Parallax Method

Parallax is definitely observed by everyone. When we look out the window of a car or train we see objects closer to us pass by rapidly as compared to objects far away from us. Objects very far away from train/car like mountains in-fact appear stationary. A driver looking at the speedometer observes speed as 80 kmph but at the same time a person looking at the speedometer observes the speed over 90 kmph. So we define parallax as difference between the apparent position of object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.

For stars, the distance between the two viewing points needs to be large, as they are so far away. The best we can do is to use the opposite sides of the Earth’s orbit (Annual Parallax). A photo of an area of sky is compared with one taken six months previously. Robert Hooke outlined in 1674 the problems of looking for annual motion of stars and Isaac Newton tried to calculate the distance of Sirius by comparing its brightness to that of the Sun. However, until the 19th century, telescopes were not sensitive enough to detect the very tiny parallax motions. The first person to succeed was F. W. Bessel who in 1838 measured the parallax angle of 61 Cygni .

There are two sub-types:

Annual Parallax : It is caused by the Earth’s yearly orbit around the sun. The measurements are taken 6 months apart i.e from two opposite points on the orbit.

Annual Parallax.
Image Credits: space.com

Geocentric or diurnal parallax : Our observations are made from the surface of the Earth, not its centre. This is irrelevant when observing distant objects such as stars. But for closer objects (e.g. within the Solar System), a correction must be made. This is geocentric parallax, or diurnal (daily) parallax
(since it varies daily as the Earth spins around its axis).

Diurnal Parallax.
Two observations can be made of same object from diametrically opposite points.

Here is an example of calculation of Parallax. If anyone’s interested in the detailed geometric treatment of parallax, this is a good reference.

Even with recent telescope technology, the smallest parallax angle measurable from Earth has been about 0.01″. Only approximately 3000 stars have been observed with a reasonable degree of accuracy. In order to improve this figure, ESA has launched a satellite designed specifically to measure the tiny angles involved to excellent accuracy. Parallaxes as small as 0.002″ are possible.

Hipparcos by ESA which was launched in 1989.

Named HIPPARCOS (HIgh Precision PARallax COllecting Satellite) in honour of the Greek astronomer Hipparchus , the satellite was put into orbit by the European Space Agency in 1989.

Astronomy Through The Ages.
Image Credits: ESA

Variable Stars

As the name suggests, variable stars are those whose brightness varies (fluctuates) as seen from earth. There are different reasons of why the brightness of these stars varies.

They are classified as:

Intrinsic Variables: Whose luminosity changes periodically. For eg. the star might shrink or swell up periodically.
Extrinsic Variables: Whose apparent change in brightness is due to the change in amount of light that reaches Earth, or the star might get eclipsed due to other companion or planetary system orbiting around it.

It does sound weird, that variable stars aid us in measuring huge distances.One particular type of variable star has proved invaluable for helping to determine the stellar distances. This is a type known as a Cepheid Variable.

So what are these Cepheid Variables? They are named Cepheid Variables because the first star of it kind (The Delta Cephei) was discovered in the constellation Cepheus. All stars, late in their lifetime, change from being average stars for their mass ( main sequence stars ) to becoming swollen red giants . Most stars change from the swollen red giant phase to pulsating variable stars before they finally die, all reactions ceasing. These are Cepheid Variables, which expand and contract, glowing brightly and fading every so often.

Cepheid Variables are very large, luminous, yellow stars. They change in brightness very regularly with periods of 1 to 70 days between peaks.

Extrinsic variables have variations in their brightness, as seen by terrestrial observers, due to some external source. One of the most common reasons for this is the presence of a binary companion star, so that the two together form a binary star. When seen from certain angles, one star may eclipse the other, causing a reduction in brightness.

Variation in Brightness due to Eclipsing.

So how is it that we determine distances using these type of stars? Well the answer is very smart. The important feature of a Cepheid Variable that allows it to be used for distance measurements is that its period is related directly to its luminosity . This relation allows us to work out how much brighter than the Sun the star is. From there we can calculate how much further away the star must be than the Sun to make it the brightness we see from Earth!!

Delta Cephei light curves, Magnitude vs Time.
Credits: ast.cam.ac.uk

Plot of magnitude difference against distance.
Credits: ast.cam.ac.uk

Colour of Stars

I don’t know how people sometimes claim they see a “reddish” star just by looking at it through naked eyes, but personally through naked eyes I see stars as white dots. But when you click a long exposure photo or see a star through Prism you can see that star’s do have different colours. By obtaining the spectrum using prism or diffraction grating and analysing it we can even determine the surface temperature of the star!!

The famous constellation of Orion. This long exposure photographs show colours of stars. On the top left we have red supergiant Betelgeuse and on the bottom right we have blue supergiant Rigel.
Credit & Copyright: Matthew Spinelli

Astronomers divide stars into seven types according to their spectrum: O,B,A,F,G,K,M. The order of these letters can be easily remembered by the mnemonic – Oh be a fine girl kiss me! O stars are the hottest (50 000 degrees C) and are blue. M stars are the coolest (3 000 degrees C) and are red.

Now we are able to determine the surface temperature of a star. So how is distance measured using this parameter? Well we use Stefan’s Law which relates the star’s surface temperature to its luminosity. Once we know the luminosity, the absolute magnitude (a measure of the total amount of light being given out by the star in all directions) can be found and so the distance. The absolute magnitude is directly related to the star’s luminosity and the apparent magnitude (brightness of a star as observed from here on Earth) can be measured here on Earth. From the Inverse Square Law , we can deduce an equation connecting the magnitudes and the star’s distance. This allows us to calculate the distance in parsecs if we can find the star’s apparent and absolute magnitudes.

Plot of Surface temperature vs Spectral Class.

Expansion of Universe

Edwin Hubble was an American astronomer who discovered that universe is expanding. Furthermore it’s not only expanding but further the object (galaxy/star) is the faster it is receding away from us. Now how can this be used to determine the distances?

For that we need to first determine how fast the object is moving. We use the Doppler Effect for this. For galaxies coming towards you, the light appears slightly blue. For galaxies going away, it appears slightly red. By looking at the spectrum of a galaxy, astronomers can work out exactly how much the light has been changed and so determine the speed of the galaxy away from the Earth.

Now that we have determined the speed, we can approximate the distance using the Hubble Graph. The slope of this graph is the Hubble’s constant.

Plot of Velocity vs Distance (Hubble Graph).

These are some methods astronomers use to determine distances which are beyond the scales of any measuring instrument. It’s just fascinating that we can determine such huge distances with an impressive accuracy.

What is a mirage?

By Pratik Barve | May 7, 2018 | Comments 1 comment

Mirage is an optical phenomenon which creates an illusion of water. The most common occurrences are during hot sunny days and most of us are familiar with mirage we see on highways. We have heard popular stories of weary traveller who sees a lake at a distance, that’s just the reflection of the sky above which creates the illusion of blue lake.

Reflection can be seen on the road which looks like there’s water on the road. This is the usual highway mirage which is formed as the surface of the road heats up the air just above it.

So how is this illusion created in the first place? When light travels through a medium of equal temperature it follows a straight line path. But when there’s a temperature gradient i.e different layers of medium (air) have different temperature, light doesn’t follow a straight line path. This has to do with the refractive index of cold air and hot air. Refractive index is the ratio of velocity of light in vacuum to velocity of light in medium. So if a medium has higher refractive index the speed of light decreases in that medium. Hot air is less dense as compared to cold air so light travels faster in hot air than in cold air. During hot summer days the road or surface of earth gets heated a lot. This heats up the layers of air just above the surface and temperature gradient is created (regions of hot air above the surface). So the light coming from objects far away instead of following a straight line path towards us bend towards hot region as it travels faster through it creating a reflection like illusion.

Vertical Temperature gradient is how the temperature varies as we move from surface towards vertical direction. The surface heats up the air just above it. So the light rays from tree which should travel in a straight line towards us bends towards hot region as it travels faster in hot region creating an inverted image of the object.

Another explanation according to quantum electrodynamics is that the photons take the path of minimum time when travelling from one point to another. Even if the path is curve it will bend to reach other point in minimum time. So when a vertical temperature gradient is present during hot days mirages are formed.

Types of Mirage

Inferior Mirage

Inferior mirage is when the image is formed under the real object. Usually in desert or highway mirage the real object is sky and the mirage is formed below the object which looks like reflection of sky from water. The light rays from object bent in hot region by same amount. Therefore inverted image is formed. This is the most common type of mirage. It is not much stable as the hot air rises above cold air which creates distortions in the image. As you walk towards mirages they seem to be moving away from you.

Superior Mirage

A superior mirage occurs when the temperature of air below the line of sight is colder than the air above it. This is unusual since warm air above cold air is unusual gradient and hence it is called temperature inversion. So now the light rays are bent downwards from the hot region, this creates the image above the object. This looks kinda weird and is not usually observed. They tend to be more stable than inferior mirages as there is no turbulent flow between cold and warm air. Superior mirages are common in polar regions especially over large sheets of ice that have a uniform low temperature.

These mirages can be pretty weird, some light from objects beyond horizon can bent and form an image above but the object cannot be seen as it is beyond horizon. This may explain some stories about flying ships or coastal cities in the sky, as described by some polar explorers. These are examples of so-called Arctic mirages, or hillingar in Icelandic.

Fata Morgana

Now this is something cool. Fata Morgana is an unusual type of superior mirage. A Fata Morgana may be described as a very complex superior mirage with more than three distorted erect and inverted images. Because of the constantly changing conditions of the atmosphere, a Fata Morgana may change in various ways within just a few seconds of time, including changing to become a straightforward superior mirage. The rays will bend and create arcs. An observer needs to be within an atmospheric duct to be able to see a Fata Morgana. Fata Morgana mirages may be observed from any altitude within the Earth’s atmosphere, including from mountaintops or airplanes.