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What is Doppler Effect?

By Pratik Barve | March 11, 2019 | Comments 0 Comment

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.

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

What is Tripple point?

By Pratik Barve | March 22, 2018 | Comments 0 Comment

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.
PV=nRT
This is the famous Ideal Gas equation.
P : Pressure
V : Volume
n : No. of mole i.e amount of substance
R : Gas constant
T : 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.

How do solar cells work?

By Pratik Barve | October 31, 2017 | Comments 0 Comment

The solar cell is an important candidate for an alternative terrestrial energy source because it can convert sunlight directly to electricity with good conversion efficiency, can provide nearly permanent power at low operating cost, and is virtually non-polluting. Solar cell also called as photovoltaic cell and are building blocks of solar panels. You must have seen these panels (large collection of solar cells) in green energy campaigns and also in developed cities in large arrays. It is used as a primary source of energy in space applications. ISS has large solar panels! Let’s understand the basics of solar cells.

The most commonly known solar cell is configured as a large-area p-n junction made from silicon. p-n junctions of silicon solar cells are made by diffusing an n-type dopant into one side of a p-type wafer (or vice versa). To get an idea about p-n junction click here. Now when light or photon hits the p-n junction it dislodges an electron and creates a hole in its place. Now the dislodged electron and hole are free to move in silicon crystal. Due to the electric field present, the electron moves to the n-type region and hole moves to the p-type region. The mobile electrons created in n-type are collected by thin metal fingers on the top of n-type region. Photons having energy equal to the band gap of silicon crystals are the only ones contributing in cells electrical output. Energy greater than band gap is lost as heat. We might also lose energy if electrons and holes recombine as soon as they are formed. So basically a solar cell works by knocking off electron of same energy as band gap of crystal material used and essentially converting light energy into electrical energy.

Schematic representation of silicon p-n junction Solar cell.

The radiative energy output from the sun derives from a nuclear fusion reaction. In every second, about 6 x 10^11 kg hydrogen is converted to helium, with a net mass loss of about 4 x 10^3 kg. We get a lot of energy from sun and if there is a way to harness this energy what’s stopping us from being completely reliant on solar power? There are various factors at play.

Terrestrially the sunlight is attenuated by clouds and by atmospheric scattering and absorption. Also we don’t receive sunlight during night time and during bad weather conditions. This is something we can’t do anything about. As mentioned above, only the photon having energy equal to band gap contributes in electrical output. Photons having energies other than band gap of semiconductor crystals either reflects back or goes through or the energy is just lost as heat energy. Antireflective coating is done to avoid reflection of photons. To deal with photons of energies other than band gap of that semiconductor crystal there is a different approach.

Spectrum Splitting :

Spectrum splitting is a very good way of increasing efficiency of solar cells by splitting sunlight into narrow wavelength bands and directing each band to a cell that has a band gap optimally chosen to convert just this wavelength band of light. There is one more way, by simply stacking cell on top of one another with the highest band gap cell at the top which automatically achieves an identical spectral-splitting effect, making this “tandem” cell approach a reasonably practical way of increasing cell efficiency.

This is an area of active research where scientists all over the world are trying to increase the efficiency of solar cells. The most efficient solar cell yet still converts only 46% of solar energy into electricity. And most commercial systems convert only 15-20% of solar energy. Solar energy is a source of green and sustainable energy and probably a candidate for future source of energy.