#physica

Exactly a year back on this very day, we explored how polarized sunglasses work and why Pilots prefer NOT to use them.

What about those with power prescriptions you ask ?

These days you can find a lot of people even with power prescriptions sporting glasses/lenses that turn dark when exposed to sunlight. These are known as Photochromic lenses. 

Let’s do a quick demo:

This is a photochromic lens getting exposed to 395-400 nm light turning dark as a result.

(Better quality video here)

How does it do that ?

To learn how this is achieved it must be understood that there are these class of reactions called ‘Reversible reactions’

A and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B. 

                    Example of reversible reaction of Bismuth Chloride

In the case of Bismuth Chloride, sunlight acts as a mediator to change it from a transparent solution to a dark colored solution and shaking in air brings it back.

In the case of Photochromic lenses, an organic dye is used that darkens under UV and lightens under its absence

                                                  Source                              

Alternate reality: Photo-degradation

If you ever have had the unfortunate experience of leaving your plastic in the sun or wondered why the colors of your clothes fade away, then the technical term to express your misery is Photo-degradation.

This is the other extreme of a reversible reaction where UV light from the sun break break down the chemical bonds in the plastic and clothes (called chromophores) and eventually leads to fading of the colors in the object

The catch

As any user of one of these Photochromic lenses will tell you, there are some aspects of these lenses that still need to be improved

-  The chemical reaction rate depends on the temperature of the surrounding. This means that on a sunny day if you are in a really warm climate, the lenses will adapt faster than if you live in a cold climate

- Most car windshields block UV light to some extent and therefore these lenses are not effective when you are inside the car.

- They do degrade after a while. Most of lenses do degrade having been used after a couple of years and need to be changed.

But having said that Photochromic lens are indeed a really cool piece of technology.

Have a great day!

Although there are no particles that are known to us to have negative mass, physicistsin the 1950s( primarily Hermann Bondi, William B. Bonnor  and  Robert L. Forward)explored the physics of what would happen if we had negative mass.

The most interesting case is when we have a positive and negative mass:

There is a push that repels the positive mass from the negative mass, and a pull that attracts the negative mass towards the positive one at the same time through eternity.

This is known as ‘runaway motion effect’ .

One bizarre consequence (among many others) of the runaway effect is that gas containing a mixture of positive and negative matter particles will have the positive matter portion increase in temperature without bound.

However, the negative matter portion gains negative temperature at the same rate without bounds but somehow mysteriously net temperature of the gas would still have not changed all this while. *

Just a random factoid that we thought we would share on this Valentine’s day. Have a great day!

*Check out more bizarre consequences 

** Source paper : Negative mass in General Relativity, Bondi

** This note was inspired from a comment on one of our previous post. Do check it out before you read this one.

The emission spectrum of atomic hydrogen is given by this amazing spectral series diagram:

Let’s take a closer look at only the visible portion of the spectrum i.e the Balmer series.

If a hydrogen lamp and a diffraction grating just happen to be with you, you can take a look at the hydrogen lamp through the diffraction grating, these lines are what you would see:

                                                     Source 

These are known emission lines and they occur when the hydrogen atoms in the lamp return to a state of lower energy from an excited energy state.

           Representation of emission and absorption using the Bohr’s model

Here’s another scenario that could also happen:

You have a bright source of light with a continuous spectrum and in between the source and the screen, you introduce a gas (here, sodium)

                              Source: Harvard Natural sciences

The gas absorbs light at particular frequencies and therefore we get dark lines in the spectrum.

This is known as absorption line. The following diagram summarizes what was told thus-far in a single image:

The absorption and emission spectrum for hydrogen look like so :

Stars and Hydrogen

One of the comments from the previous post was to show raw spectrum data of what was being presented to get a better visual aid.

Therefore,the following spectrum is a spectrum of a star taken from the Sloan Digital Sky Survey (SDSS)

                                 Plot of wavelength vs median-flux

Here’s the spectrum with all the absorption lines labelled:

                                              Source: SDSS

You can clearly see the Balmer series of hydrogen beautifully encoded in this spectrum that was taken from a star that is light-years away.

And astronomers learn to grow and love these lines and identify them immediately in any spectrum, for they give you crucial information about the nature of the star, its age, its composition and so much more.

                                             Source: xkcd

Have a great day!

*If you squint your eyes a bit more you can find other absorption lines from other atoms embedded in the spectrum as well.

Fluids

1. Buoyancy

   1. Origins of the Buoyant force
   2. How do hot air balloons work?
   3. Note on average density and how do ships not sink?

2. Flow Instabilities

   1. Plateau- Rayleigh Instability

3. Pilot Wave Hydrodynamics (completed with Nicole Sharp, FYFD)

   1. Introduction
   2. Chladni patterns
   3. Faraday instability
   4. Walking droplets
   5. Droplet lattices
   6. Quantum double-slit experiments
   7. Hydro single- and double-slit experiments
   8. Quantum tunneling
   9. Hydrodynamic tunneling
   10. De Broglie’s pilot-wave theory
   11. Resources and references on pilot wave hydrodynamics

4. Others

   1. How to photograph shock waves?
   2. Ultrasonically levitated water droplets
   3. Catching prey using vortices
   4. Whale Flukeprint
   5. Estimating the energy of an atomic bomb using dimensional analysis

Physics

1. Astronomy

   1. Measurements

      1. [How far away is the moon and lunar laser ranging experiment](https://fyphysica.tumblr.com/post/117782860366/how-far-is-the-moon)

      2. Parallax method, 61-Cygni and the Hipporcas mission
   2. How to use the hubble legacy archive (HLA) - 101
   3. The Chandler wobble
   4. On the orbital plane
   5. Supernova Sorcerer: Robert Evans
   6. A note on the Hydrogen spectrum
   7. On the strong 5577Å spectrum line
   8. Celestial wonders – Binary stars
   9. Dropping a feather on the moon
   10. Saturn’s hexagonal cloud pattern
   11. Beauty of eclipses on earth
   12. On disappearing shadows of Birds and airplanes

2. Classical Mechanics

   1. So. this is called blobbing
   2. The simple harmonic oscillator
   3. Using complex numbers in classical mechanics

   4. Remembering the Laplacian in different coordinate systems

      1. [Jackson's form of the laplacian in spherical coordinates](https://ecstasyshots.wordpress.com/2019/02/18/jacksons-laplacian-in-spherical-coordinates-proof/)

      2. Solving the Laplacian in spherical coordinates
   5. Euler-Lagrange equation for a classical field
   6. Cooking up a Lorentz invariant Lagrangian
   7. Length contraction , Time dilation and Lorentz Transformation: Pokemon edition

3. E&M

   1. Chocolate bar in Microwave(#1)
   2. Chocolate bar in Microwave(#2)
   3. On Diffraction and the Helical structure of DNA
   4. On Optical Tunneling and Evanescent waves
   5. Diamagnetic Levitation -Ig Nobel prize (2000)
   6. A note on levitating frogs and superconductivity
   7. Can you crush a soda can with lightning ?
   8. Pokemons and bioelectricity
   9. The holy grail of Electromagnetic Radiation
   10. Combinatorics and the two state paramagnet

4. Chaos

   1. Chemical Turbulence : Belousov Zhabotinsky reaction
   2. Dynamics of Love affairs: Valentine’s Day Gift Box

5. Quantum Mechanics

   1. Commutation relation in QM (using Index notation)
   2. A note on what makes solutions discretized?

Math

1. Feynman’s trick

   1. Intro to parametric integration
   2. Using Feynman’s trick on Laplace transform problems
   3. Feynman’s trick applied to Contour Integration

2. Differential equation

   1. Legendre Differential Equation (#1)
   2. Legendre Differential Equation (#2)
   3. Legendre Differential Equation (#3)
   4. Legendre Differential Equation (#4)

3. Beautiful proofs

   1. Divergence of the harmonic series
   2. Euler’s Sum
   3. Area under a sine curve
   4. When Gauss was a young child…

4. Number and Graph theory

   1. Homeomorphic Irreducible Trees with 10 vertices
   2. Divisibility tests
   3. Holy matrimony of Pascal and Sierpinski’s triangle
   4. Geometric series formula
   5. The tale of taylor series coefficients
   6. Division by 7

5. Pi-day

   1. How did the Greeks know that pi=3.14?

6. Others

   1. Why is (-1) x (-1) = +1 ?
   2. Why is 1+1 = 2 ?
   3. Only Time will tell - A Complex Number Tribute.
   4. Complex numbers are not complex
   5. Basis vectors are instructions
   6. The generalized product rule (Leibniz formula)
   [Finding n roots of unity](https://fyphysica.tumblr.com/post/176415634641/multiplication-and-n-roots-of-unity)

   8. [Never forget De Moivre’s formula](https://fyphysica.tumblr.com/post/176137661561/never-forget-de-moviers-formula-if-you-ever-find)

   9. Caustics
   10. Curves of constant width
   11. On the direction of the cross product
   12. The Knight’s tour
   13. The geometry of a pringle

Engineering

1. Themed adventures

   1. Up, up and away

      1. [The magnus effect](https://fyphysica.tumblr.com/post/134198455876)

      2. Introducing the wind-tunnel
      3. Flight dynamics : Pitch , roll and yaw
      4. Physics of “stall”
      5. On Taj Mahal and lift in airplanes
      6. Vortex Generators _( How inducing a vortex helps an aircraft )_
      7. Wingtip vortices ( The why and how about wing tip vortex )
      8. Have you heard about the popular y250 vortex on a F1 car? It’s amazing!
      9. Von Kármán vortex street
      10. Aerodynamic Flutter
      11. Landing on aircraft carriers

2. Train, Flights and cars

   1. Flight

      1. [What Is That Hole in the Tail of an Airplane?](https://fyphysica.tumblr.com/post/142619755241)

      2. Why do pilots use non polarized sunglasses ?
      3. Reverse Thrust
      4. Supersonic shock diamonds
      5. The radial engine
      6. Variable sweep aircrafts
      7. The Joukowski transform
      8. Vapor cones

   2. Rockets

      1. [ How to turn during flight ?](https://fyphysica.tumblr.com/post/132215818276/rockets-101-how-to-turn-during-flight-to-be)

      2. Why do shuttles orient belly up after a launch ?
      3. Spin stabilization
      4. How do you place a satellite in orbit?
   3. Pantograph and Trains
   4. This is a Dragster Blowover
   5. UV light, Thou shall not enter!

3. Technology

   1. The Touch Screen
   2. ‘Hook and Loop’: The fascinating tale of velcro
   3. That’s how a revolving fan works
   4. ABS - Anti-lock braking system
   5. Why are there rivets on your jeans?
   6. Making images from pixels

4. Others

   1. Walter lewin chalk trick | 1000 fps
   2. Why do shower curtains encroach your showering space ?
   3. Can dominos knock down the empire state building?
   4. Types of damping
   5. The Miura fold : Making flat surfaces smaller
   6. Powering a 13W CFL bulb using a 3V battery
   7. Square wheels on cars?
   8. De-constructing Backyard Brain’s TED talk
   9. Why can’t you cycle backwards?
   10. Baby powder and fire breathing
   11. Why do bullets spin when they leave the barrel ?
   12. Polish a surface well enough and you can turn it into a mirror !
   13. Small balloons exert higher pressure ?
   14. How does turning this knob control your fridge temperature?
   15. How does sand from Sahara end up in your windshield ?
   16. A note on Vibration and Washing Machines
   17. You cannot drown in Quick Sand!
   18. The great pyramid of Giza has 8 sides not 4!
   19. Why not just build a solar panel around the sun to solve all energy problems?
   20. Major types of engines - giffed

With a heavy heart we wish to say that FYPhysics! is coming to an end. This being its final post!

When we started this blog almost 5 years ago, we had only one thing in mind: Just explore physics for the love of it and blog about it. And from that starting point to now, it has been an absolutely breathtaking journey.

Now with over 130,000+ followers, it still seems magical that we were able to make it this big on this platform. But we have decided to end this grand project of ours.

Why end it now?

When you do something repeatedly for a really long time, you develop a bit of comfort zoneand when you get too comfortable with what you have, it’s very hard to make impactful work.

And for us, we seem to have gotten a little too comfortable with Tumblr as a platform that we noticed that we were not pushing the limits of what could be done in terms of content creation.This hit us really hard and  made us feel uncomfortable with our own content.

There are definitely other contributing reasons for why this turned out that way. But we believe that the best line of action that would keep us pushing harder would be to end this chapter in our lives and move forward to try other things.

But having said that, this single project has taught us innumerable valuable lessons that we are sure none of the other platforms would have been able to provide.

Plans for the future?

One immediate plan that we have is to wrap upISAFEHQ-  A Collection of 250 open source high quality scientific illustrations (current progress - 75 illustrations completed) which will be permanently hosted on CosmicNoon by June 2020. This will also collectively wrap up any unfinished series on FYPhysics!

But beyond that we are still unsure on how things would unfold. But you can send us an email at [email protected] or follow on twitter for updates.

The posts will continue to exist

There are currently no plans to do anything to the posts present on the blog. All posts will remain on the platform and will continue to be accessible to everyone.

Finally, Who are ‘we’?

My name is Keerthi Vasan G C and I am currently a graduate student in Physics at University of California, Davis. I am the primary editor of this blog. I have been constantly helped in this venture by a lot of my very close friends (who do not wish to be named) but contribute in many ways to the successful running of this blog .

Final Thoughts

The physicist Leo Szilard once announced to his friend Hans Bethe that he was thinking to keeping a diary: “ I don’t intend to publish. I am merely going to record the facts for the information of God.” “Don’t you think that God knows the fact ?” - Bethe asked. “Yes” said Szilard. “He knows the facts, but He does not know this version of the facts”

We are extremely happy to have been able to delight you with our version of the facts for all this while. It has been a great honor to have been part of this platform for so long and a big shoutout to @staff​ and @engineering​ for making this happen. Thank you to all our amazing followers for the loads of love and support. We wish you the very best in your future endeavors.

–

FYPhysics!

** If you have any other questions feel free to post them below/ send them privately / email them to [email protected], we will surely answer them.

*** [July 2020] Defaulting to a new url - fyphysica.tumblr.com. Posts will still remain on the blog.