Tag: technology

What is Doppler Effect?

What is Doppler Effect?

Everyone has observed the phenomenon of Doppler Effect at some point. An ambulance coming towards you sounds a bit high pitched and then when it goes away from you it sounds a bit low pitched. So basically Doppler Effect is the change in frequency or wavelength due to the relative motion between source and observer. This phenomenon was named after the Austrian physicist, Christian Doppler, who proposed it in 1842 during his time at Prague Polytechnic University.

Doppler effect can be observed in sound waves as well as electromagnetic waves i.e light. The apparent change in wavelength/frequency due to the motion of source object is called as Doppler Effect. Consider a scenario where an observer is observing a moving object. If the object is moving towards the observer the wavelength is shorter due to the motion of source, and hence the frequency increases (higher pitched sound). Whereas if the object is moving away from observer the wavelength is longer as source is moving away from the observer and hence frequency decreases (lower pitched sound).

In case of light, if the object is moving towards you its called as blue shift because the wavelength reduces i.e it shifts towards blue side of the Electromagnetic Spectrum and if the object is moving away its called as red shift because the wavelength increases i.e it shifts towards red side of the Electromagnetic spectrum.

Note that blue shift and red shift doesn’t actually mean the object appears blue or red, it just means that frequency increases or decreases. A stellar object’s spectrum may be in ultraviolet region which is already beyond blue, in that case blue shift means the spectrum shifts towards the higher frequency range.

Red Shift and Blue Shift of Electromagnetic Spectrum of a Star.


Some applications of the Doppler Effect

  • Police radars make use of Doppler effect. The device is pointed at the target (vehicle), radio waves are emitted which hit the target and are reflected back. Depending on whether the vehicle is moving towards or away the change in wavelength is measured and instantly speed of the vehicle is calculated by the electronic circuits in the device. Such device is a good for non-intrusive way of traffic rule enforcement.
    Handheld Police Radar.
    Image Credits : stealthveil.com

  • Doppler Radars are used by Meteorologists to study the weather. Similar to Police radar it uses radio waves, they have large enough wavelength to interact with clouds and precipitation. This can be used to determine the speed of cloud and using other parameters like wind speed, temperature, air currents,etc the prediction of weather becomes more accurate.
    Doppler Radar at the National Weather Service in Dodge City, Kansas.

  • Doppler Echo-cardiogram is a device used to take images of heart. It uses sound waves which makes it relatively safe medical imaging technique. The sound waves bounce off the walls of heart and the red blood cells hence we obtain an image which helps determine the rate of blood flow and direction.
    Doppler Mitral Valve.
    Image : Wikipedia.

  • In Astronomy and Cosmology Doppler effect is used to determine if a stellar object is moving towards or away from us. It is also used to determine the distances of stellar objects. Click here to read more about determining stellar distances.
  • When a planet orbits a star, the star wobbles around the center of mass of the star planet system also called as barycenter (common center of mass for star and their planets). So the wobble means that the star moves away from us and towards us. That’s it! We can use Doppler Shift to detect exoplanets!!

Infact our sun also wobble mostly due to Jupiter.


Star Wobbles due to exoplanet.

To read more in detail about Doppler effect and also it’s mathematical formulation refer to this pdf.


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

How do 3-D Glasses work?

How do 3-D Glasses work?

3D movies are really fun to watch as we get that amazing experience of something coming out of the screen! One kinda annoying thing about 3D movies – uncomfortable pair of glasses. If someone already has spectacles they have to wear 3D glasses above their spectacles which is even more uncomfortable – but worth it! So how do these glasses actually make things “look like” they are protruding out of the screen?

Lets go through few concepts first:

  • Let’s start by understanding what polarization of light is. The electromagnetic (EM) waves that compose electromagnetic radiation can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields.

    Electromagnetic Waves.

    In short polarisation means the EM waves are oscillating in only one direction, typically we talk about only the electric field.So polarized light means the electric field oscillates along one axis called polarization axis. Any other light gets partially or completely blocked. Sunlight is a source of unpolarized  light which means its oscillating along different directions. When this unpolarized light is passed through a polarizer it allows only those waves which are oscillating along the polarisation axis.

Sunlight, bulbs are sources of unpolarized light.

  • Which brings us to the next question, what are polarizers? Polarizer is an optical filter that allows light waves of certain polarization to pass through it. The material used in these filters has a molecular structure that can oscillate in only one direction (i.e the polarization axis). So it allows only that light which oscillates along the axis.
  • Now these polarizers are called linear polarizers because the light can pass only through one axis, so if you tipped your head when you watch through a polarizer the light gets blocked which is not a good thing for us.

Polarized sunglasses polarize light linearly.

  • If light can be linearly polarized it can also be circularly polarized. If its possible for light to have linear momentum then its also possible for light to have angular momentum. In circularly polarized light the electric field goes around in circles.

Circularly Polarized light.

Which brings us to the next question, how the heck do you make electric field go in circles? We use something called as birefringent crystals, refringent means refraction and birefringent means the crystal refracts light in two different ways which creates two different images. This basically happens due the molecular structure of the crystal. Suppose we send linearly polarised light through the crystal, at the beginning the components of the wave are in phase with each other, but by the time they exit the crystal they are out of phase due to the crystal being birefringent. This makes the electric field go in circles with respect to the axis. How this exactly happens is kind of very complicated. Now if you make the material of just right thickness that it makes the light wave that comes out 1/4th of the wave out of phase then that piece of crystal is called quarter wave plate. Combining the components we find that its a circularly polarized wave. So circular polarizer is nothing but a combination of quarter wave plate and a linear polarizer.

Circularly polarized light enters quater wave plate and is linearly polarized and this light can be again circularly polarized using quarter wave plate.

We have to just make sure that the incoming light is at 45 degrees to the crystal.

Now finally lets get back to the main question. How do 3D glasses work? In reality we get the perception of depth due to the spacing between our eyes (which is about 2 inches). We actually look at things from two different perspectives and our brain combines these two images to give us the sense of depth (which mean 3D perception). Fun fact: In humans, each eye has a viewing angle (field of view) of about 150° but the binocular vision (i.e the image that can be seen by both eyes) is 114° which actually covers our nose. It means we see our nose continuously but our brain just chooses to ignore it for “convenience”. Back to 3D, so for the sense of depth we just need to look at images from two different perspectives and our brains will do the rest. This is where polarization comes in as we can project two different images (perspectives) of different polarization  on a 2D screen but by wearing glasses we can allow our eyes to see image of only one perspective and we get that feel of 3D image.

There are different methods of projection and types of 3D glasses. Here’s a list of some 3D glasses and their method of projection:

  • Anaglyph Glasses: These are the cheapest glasses that you can get. You might have seen these glasses as free gifts on some products. Anaglyph glasses have different colour filters for each eye (typically red and blue). The projected image has different colour elements and the filters allow both eyes to see different images. The quality of 3D is very poor as it is not able to resolve colours properly and its said to be very uncomfortable.

    Card-paper Anaglyph Glasses.

    These glasses are useful when there’s a large short term audience like in events or meetings.

  • LCD Active Shutter Glasses: LCD Active shutter means as the name suggests the glasses that we wear use LCDs (i.e Liquid Crystals) to make the glasses opaque or transparent at very high speeds like a camera shutter. It is commonly used in home theaters and 3D televisions. The television displays images with different perspectives at a frame rate of 120Hz which means it displays 120 images every second. The LCD active shutter is synchronized with the television which makes the LCDs in glasses become opaque at the same frame rate alternatively. So each eye gets effective frame rate of 60Hz (which is pretty good as we can see smooth video even at 27 frames per second).Due to the high frame rate, the brain however has the impression that it perceives both images at the same time and not in sequential order.

    LCD Active Shutter Glasses.

    Only disadvantage is that these glasses can be relatively expensive but nevertheless they provide very good quality 3D effect.

  • Glasses with Polarizing Filters: In these there are two types
    • Linearly Polarized Glasses: These glasses are used when two images are projected superimposed onto the same screen through orthogonal polarizing filters (Usually at 45 and 135 degrees). The viewer wearing linearly polarized glasses can see only one image in each eye (the one which has same polarizing angle). These glasses require the viewer to stay in the same orientation i.e if the viewer tips his head the image becomes darker as mentioned above in theory.
    • Circularly Polarized Glasses: These glasses are used when two images are projected superimposed onto the same screen through circular polarizing filters of opposite handedness.

      Right- Circular Polarized and Left Circular Polarized.
      Image credits: The science asylum.

      The viewer wears eyeglasses which contain a pair of Analyzing filters (circular polarizers mounted in reverse) of opposite handedness. Light that is left-circularly polarized is blocked by the right-handed analyzer, while right-circularly polarized light is extinguished by the left-handed analyzer.

      Illustration of Circularly polarized waves through glasses.
      Image credits: Science Asylum

      Thus the two different perspectives are projected on the same screen with different direction of circular polarization. Now in this case even if the viewer tilts his head he won’t lose the image and it becomes more comfortable. The quality and resolution of these glasses is excellent and these the typical glasses you use in movie theaters (and sometimes try to sneak out of theater with the glasses).

  • Virtual Reality Glasses: Everyone is familiar with the virtual reality tech that’s been blowing away people’s mind these days. These kinda give you the best 3D experience you can get. Both the eyes get different perspectives and when you tilt the head the images are displayed as if you are present in the scenario! These are used for realistic gaming, simulation, education and is kinda litt!

    Oculus VR.

    With virtual reality there’s also one other thing that is emerging – Augmented reality! This augments the reality in your virtual experience which means you can interact with real things and look at virtual animated things augmented into that vision! Microsoft Hololens is an example of Augmented reality and that tech is on whole another level! As technology gets better and quality of these virtual realities improve I don’t think we are far away from the point where we start doubting the reality! Kind of exciting and scary but who knows what new technology might appear and blow our minds!