Category: DailyPhysics

Nobel Prize in Physics 2018

Nobel Prize in Physics 2018

Nobel Prize in Physics 2018. Image credits:

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:

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 a mirage?

What is a mirage?

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.

Inferior Mirage.
Image credits: Wikimedia Commons illustration by Ludovica Lorenzell. CC BY-SA

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.

Superior Mirage.
Image Credits:Wikimedia Commons illustration by Ludovica Lorenzell. CC BY-SA

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.

Schematic of Fata Morgana.
Image Credits: Wikimedia commons,by Brocken Inaglory  CC BY-SA

A person on the north pier in New Buffalo, Michigan with the mirage of Chicago, Illinois in the distance.
Image Credits: Joshua Nowicki – Photography

This is a very good image of Fata Morgana. What is seen here is the city of Chicago from the town of New Buffalo, which are roughly 45 miles apart.


How does lightning occur? | The physics of Lightning and Thunders

How does lightning occur? | The physics of Lightning and Thunders

The lightning we see during rains or storms are so fascinating in nature! A really huge spark of electrostatic discharge from clouds to the ground or sometimes within clouds itself. And after that bright flash follows a really loud bang which is called thunder. So what is it that really happens up above in the clouds that creates this huge electric discharge and a loud bang?

Branching of lightning bolt in slow motion.
Image credits: NOAA SciJinks

First we will understand how lightning works and then how thunder is generated.

Formation of Lightning

Lightning is the sudden discharge of electricity. Initially the charges get separated in the cloud formation itself. The primary source of lightning is the cloud type termed cumulonimbus, commonly referred to as the thundercloud. Due to the air currents inside the clouds and other factors the positive charges gets accumulated in the top region of clouds and negative charge is accumulated at the bottom region. Unfortunately we don’t know exactly why or how this charge separations occurs in the first place. One widely held theory is that the thunderstorm clouds have supercooled ice crystals and graupel (soft hail) and due to strong updraft they are pushed in the upper region of clouds and become positively charged.The heavier ice crystals which are negatively charged fall to the lower regions.

Now the lower region is accumulated with negative charge. As like charges repel, the negative charge on the ground is repelled and positive charge accumulates on the ground. Now unlike charges attract! When enough negative charge in clouds and positive charge on ground is accumulated, sudden discharge takes place which we see as a lightning strike. The negative charge starts flowing towards ground and the positive charge on ground starts travelling up towards the clouds. The point when these two flows meet is the point when we see a huge lightning bolt from cloud to the ground!

Negative charges from cloud (blue) and positive charges from ground (red) start moving towards each other eventually discharging suddenly creating a lightning. Image Credits: NOAA SciJinks

Its the same static discharge/zap we feel sometimes when we touch car doors or even other people, its just much larger in magnitude (and much lethal obviously).

Now the types of lightning:

  • Intra cloud: This type of lightning happens completely within clouds. It happens due to discharge between different charge regions within the clouds.

    Intra-cloud lightning.
    Image credits: NSSL, NOAA
  • Cloud to Cloud: This type of lightning strikes happen between two or more surrounding clouds.

    Cloud to cloud lightning.
    Image Credits: Wikimedia
  • Cloud to Ground: This is what we should be worried about! This is the type of lightning that strikes the ground.

    Cloud to Ground lightning strike.

    The branching can be clearly seen as the charges try to find the shortest possible path. This is the reason why tall buildings and trees are striked by lightning more often.

  • Cloud to air: This type of lightning occurs when there’s a discharge between clouds and oppositely charged air surrounding the clouds. The distribution of charges varies and depends on many factors so this type of discharge happens.

    Cloud to Air Lightning.
    Image Credits: NSSL NOAA

What causes Thunder?

Thunder is that very familiar loud rumbling sound we hear a few seconds after a lightning strike. We can actually determine how far away the lightning strike took place by counting the interval between flash and rumbling sound.

The average lightning bolt striking from cloud to ground contains roughly 1 Billion Joules of energy!! Now that’s just lot of energy and lightning strikes last for such a short duration. This immense energy superheats the air surrounding the lightning channel to plasma temperatures in a very short duration. Temperatures can go upto 30,000 Kelvins (53,540 degrees Fahrenheit). This creates a shockwave in the air rapidly and rumbling sound is generated. High frequency sound gets quickly absorbed by landscape so thunders due to distant lightnings sounds like low rumble.

How to survive a lightning strike?

If unfortunately you are in flat lands during a thunderstorm remember that you are one of the shortest possible routes for charges to discharge. Laying flat on ground won’t work as lightning can hit the ground and first and then your body. Avoid standing under trees, poles, any sort of tall conducting pointy things. It’s just safe to be indoors during a thunderstorm.

Staying inside a car or bus also might help as the frame or surface of vehicles act as a Faraday cage and lightning will flow on the surface and not through your body.

These are some tips just incase you are in the open during a thunderstorm.

  1. Crouch down low like a baseball catcher. Get as low as you can. The nearer you are to the ground, the less likely you are to be struck by lightning. But never lie down!
  2. If your hair begins to stand on end or your skin starts to tingle, a lightning strike is imminent. (You literally have a few seconds before you get hit). Immediately get into the crouching position. Lightning may strike without this warning, however.
  3. Place hands over ears to minimize hearing loss from the loud clap of thunder that will boom very close to you.
  4. The only thing touching the ground should be the front portion of your feet. Lightning can hit the ground first, and then enter your body. The more you minimize your contact with the ground, the less chance of electricity entering your body.
  5. Touch the heels of your feet together. If electricity from a ground strike enters through your feet, this increases the chances of the electricity going in one foot and out the other, rather than into the rest of your body.
  6. Don’t touch any possible conductors.

    How to survive a Lightning Strike?
    Image credits:


I hope you never get into such situation but knowing stuff is always good, ain’t it?