physics Archives - Page 3 of 4

How do magnets work?

By Pratik Barve | January 26, 2018 | Comments 0 Comment

We all are very familiar with magnets! Everyone gets so curious when they play with magnets and feel the attractive/repulsive forces.Typically when we place other objects like piece of wood or stones, they don’t really interact with each other (in the sense they don’t attract or repel). But if we place magnets near each other they are attracted to each other (or repelled). So what really happens in magnets? What makes magnets, magnets!?

We are familiar with the illustrations we see, of how the magnetic lines of forces originate from north pole and end at south pole. Opposite poles attract and like poles repel each other due to magnetic lines of force.

But what creates this force in the first place? Lets go down to atomic level and see why. Every atom has certain number of electrons whizzing around the nucleus in specific orbits. Every electron has fundamental properties like mass, charge, spin etc. Now to explain the origin of magnetism one needs to use quantum mechanics (which we are not going to do right now). In simple terms we imagine electrons as charged particles which create magnetic field due to their motion in orbitals. Except these don’t actually contribute to the magnetic field. The outermost shells of some atoms which remain partially filled or half filled contribute to the majority of magnetic field of the atom. The electronic configuration of atoms follows Hund’s rule in which spin up electrons occupy energy levels first and then spin down. So in some atoms the outermost shells remain half filled and the intrinsic magnetism of individual electrons actually creates majority of magnetic field of the atom.

Electronic Configuration of Iron which shows partially filled outermost shell.

Okay so now that we have established that outermost electrons of an atom actually creates magnetic field we can say that each atom works like a tiny magnet (very tiny magnet). When very very large number of such magnetic atoms come together to form a solid we call it a magnet! So one natural question pops up in our mind that why are there few elements which can be magnets ?

Which brings us to the topic of domains. Now when magnetic atoms come together to form solids they have got options. They can align their magnetic fields in one particular direction which creates a magnet, or they can align in alternating fashion which essentially cancels out the magnetic field. They align themselves in a fashion requiring least energy to do so. A permanent magnet is a Ferromagnet i.e the magnetic field of every atom is aligned in one direction. Whereas Anti-Ferromagnets are the one’s in which every atom is aligned in an alternating way such that the magnetic field cancels out and makes the solid very non-magnetic.

(a) Ferromagnetic
(b) Anti- Ferromagnetic

But these are the two extreme cases, it may happen that some elements are somewhat magnetic or can be magnetised.Paramagnets are materials in which the direction of magnetic field of individual atoms is random. Domains are chunks or very small part of solid consisting of electrons aligned in the same direction as other in that chunk. It is possible in solids that one chunk of it has its atoms magnetic field pointed in one direction and other chunks in slightly different or opposite direction. However if we apply a very strong magnetic field we can force all the atoms in these domains to align with the external magnetic field hence creating a magnet!

That’s why only few elements like iron, nickel, cobalt, etc can be very good magnets. Other elements of the periodic table have completely or mostly filled outermost shells so those elements are not magnetic. Ferromagnetic materials are the only ones attracted to a magnet strongly enough to be considered as magnetic but all other elements do respond weakly to magnetic fields.

Atom which have full or partially full outermost shells are not magnetic but atoms which are at the middle of major blocks having partially filled shells are magnetic.
Image Credits: Minute Physics.

The elements in red are magnetic. But Chromium as an atom is very magnetic but as a solid its one of the most anti-ferromagnetic element as the atoms align to opposite directions in alternating fashion cancelling out the total magnetic field. Rare earth elements (Lanthanides) have partially filled outermost shells and as these are huge atoms they are very magnetic. The most common types of rare-earth magnets are samarium-cobalt and neodymium-iron-boron (NIB) magnets. But these rare earth elements which make best magnets and have endless applications are somewhat expensive. The United States Department of Energy has identified a need to find substitutes for rare-earth metals in permanent-magnet technology, and has begun funding such research.

So lets just summarise how magnets work:

Each atom of magnet has to have half-filled or partially filled outermost shell of electrons.
These atoms have to be aligned with each other in the same direction of magnetic field.
And all the domains have to be aligned.

So yeah that’s a very short description of how magnets work! If you go deep enough and ask why do electrons have magnetic field in the first place, the answer is no one actually knows! Its just the way the universe works and we know this because of observations made that every particle having charge has magnetic field. The case of electromagnets is totally different and it works due to special relativity which is a topic for another post.

How do 3-D Glasses work?

By Pratik Barve | January 12, 2018 | Comments 0 Comment

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.

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!

What is Graphene?

By Pratik Barve | January 3, 2018 | Comments 0 Comment

Let’s start with a very short story, two physicists were having fun experimenting and they used scotch tape to remove layers of carbon from a lump of Graphite (material used in pencils). Voila, they made Graphene. And this thing got them the Nobel prize in Physics in 2010. Those two physicists were Andre Geim and Konstantin Novoselov. Now it sounds just too simple but there is still a lot to be known about Graphene.

A lump of graphite, a graphene transistor, and a tape dispenser. Donated to the Nobel Museum in Stockholm by Andre Geim and Konstantin Novoselov in 2010.
Image credits: Wikipedia

Graphene in simple words is a very thin layer of carbon atoms, and by very thin I mean only one atom thick!! It’s a sheet of carbon atoms in hexagonal lattice. Fun fact: When you write with a pencil there’s a chance you might have accidentally created Graphene!

Let’s get straight to the properties of Graphene to help us understand why it is of such significance.

Properties of Graphene:

Structure: Graphene is a crystalline allotrope of carbon and is tightly packed in regular hexagonal pattern in one plane.Graphene’s stability is due to its tightly packed carbon atoms and a sp² orbital hybridisation – a combination of orbitals s, p_x and p_y that constitute the σ-bond. The final p_z electron makes up the π-bond. These π-bonds are responsible for most of graphene’s notable electronic properties, via the half-filled band that permits free-moving electrons. In simple words sp² orbital gives graphene its strength and p_z electron helps electrons to move easily. It’s actually a perfect 2D structure as its just one atom thick, well in atomic scale its 3D but its the best 2D structure we can get. Something so thin will be obviously very light weight. Apparently less than a gram of graphene sheet can cover an entire football field. And to top it off the structure of Graphene is so strong that it would take an elephant balancing on a sharp pencil to pierce a Graphene sheet with the thickness of Saran Wrap. (This fact about the strength of Graphene sheet is totally mind blowing and hard to digest). In technical terms the tensile strength of Graphene is 130 GPa , for comparison the tensile strength of stainless steel is 860 MPa !!

Hexagonal structure of Graphene — Hexagonal structure formed by 6 Carbon atoms.

Thermal Conductivity: Now we know Graphene is super strong very light weight one atom thick sheet of carbon atoms. As if it wasn’t enough Graphene is better at conducting heat than any other material. It is 10 times better at carrying heat than copper.
Electronic Properties: Graphene is also a very very good conductor of electricity. It has very less resistance due to the uniform flat structure of graphene, so the electrons flow very easily. At room temperature it can conduct electricity faster than any other known material.
Optical Properties: As Graphene is extremely thin it is almost transparent as one might expect. Infact Graphene transmits about 97% of light which is more than a glass pane.

Summary: So in short Graphene is ultra light, ultra thin, super strong, transparent and a very good conductor of heat and electricity!!

Graphene sheet.
Image credits: www.dailymail.co.uk

Now lets get to the possible applications of this futuristic material.

Applications:

Graphene is transparent and flexible conductor which is very promising for applications in LEDs, Solar cells, flexible Touchscreens for wearable gadgets, etc.
Graphene super-capacitors serve as energy storage alternatives to traditional electrolytic batteries. Some advantages are fast charging, long life span and environment friendly production.
According to Moore’s Law, Silicon transistors are becoming smaller and smaller and hence approaching its limits. Graphene can be an exciting replacement in electronic devices due to its amazing properties and many big companies are working on it.
Graphene can be used to coat materials to increase their structural strength.
Such super materials have a direct application is sports. Like Graphene is used in tennis rackets and is said to have better performance than normal rackets. One other application is in Formula 1 cars (Which is already on a whole another level in terms of engineering and technology) or in sports cars. BAC’s 2016 Mono model is said to be made out of graphene as a first of both a street-legal track car and a production car.

2017 BAC Mono Graphene
Image credits: www.topspeed.com

The multifunctional nature of graphene means that it is going to have limitless applications we haven’t even thought of yet. It can be used in aerospace applications, motor vehicles, flexible wearable electronic devices, and many applications in medical and biomedical devices.

Inspite of all these benefits of a material of amazing properties whats stopping us from actually using it in the above applications? Well the method used by Andre Geim and Konstantin Novoselov produces very less amount of Graphene and it may not be a perfect single layer of Graphene. The various methods of Graphene production are Mechanical Exfoliation (which means using scotch tape and other adhesives to peel of layers from Graphite), Chemical Vapour Deposition (in which we can produce comparatively big sheets of Graphene than exfoliation), Dispersing graphite in a liquid medium can produce graphene by sonication followed by centrifugation, etc. But all these methods produce less amounts of Graphene and new methods are being developed to reduce defects and production costs. As Graphene sheets are very difficult to make it is one of the most expensive materials on the planet as of now.

Production and development of Graphene is a very active field of research in the field of material science and it is said to be the material of the future. There might be other materials not yet discovered maybe similar or better than Graphene we don’t know!