## List of atmospheric optical phenomenon

A list of wonderful atmospheric optical phenomenon can be found here. My personal favorite being “The moon illusion”.

• #### On the strong 5577Å spectrum line

• The night sky emits strongly at 5577.338Å and is observed in all astronomical spectra. This post explores the origins of this sky emission.

• #### Oxygen: The greenish color of life

• This article from astronomy.com dwells deeper into the topic of Oxygen  and Astronomy.

• #### Breath

• Talking about the miracle molecule which keeps us alive, I recommend listening to  the episode titled “Breath” from Radiolab. This is a great episode where ” try to climb into the very center of this thing we all do, are all doing right now, and now, and now.

## Using ray diagrams to better understand everyday reality – Demo

One of the easiest way to visualize ray diagrams for lenses is by using an array of arrows on the board as the object and the camera on your phone as the screen.

(i) Convex Lens : Arrows on the board are placed outside the focal length of the lens -> Inverted image

(ii) Convex Lens :Arrows on the board are placed inside the focal length of the lens -> Magnified Erect image

(iii) Concave Lens: Irrespective of whether you place the arrows inside or outside the focal length, you get an erect image.

(iv) Convex mirror: Virtually erect image formed from a Christmas ornament

The following video from UCLA extends the ray diagram analysis for concave and convex mirrors:

## Physics of invisibility

(This page will be updated as the quarter progresses with other techniques that you will eventually learn about)

There are many way to make things “disappear” and impart the illusion of invisibility. Based on what you have studied so far, here is one possible ways you can do so. (There are certainly other ways and I strongly recommend spending some time thinking about this)

### Using the refractive index

If you want to make something invisible,you have to ensure that the index of refraction of the object and the medium where you are hiding it remain the same and also that the object is transparent and colorless.

The refractive index of a polymer ball is identical to that of water and if you immerse a colorless polymer ball in water, you can make it ‘disappear’ like the animation above shows you.

When you immerse colored polymer balls in water on the other, they seem as if they are 2-d objects although they are spheres!

## Leslie speaker, Doppler effect and the Nobel Prize in Physics 2019

The Nobel Prize in Physics 2019 was awarded “for contributions to our understanding of the evolution of the universe and Earth’s place in the cosmos” with one half to James Peebles “for theoretical discoveries in physical cosmology”, the other half jointly to Michel Mayor and Didier Queloz “for the discovery of an exoplanet orbiting a solar-type star.”

In this sub-section we will try to understand how Michel Mayor and Didier Queloz discovered the first ever exoplanet – 51 Pegasi b . Let’s first take the example of a Leslie speaker.

## Leslie Speaker

This speaker has two horns from which the sound emerges out.

The two horns are placed on a rotating platform which can spun at high speeds.

Therefore, if you play a tone at frequency ‘f’ and begin to spin the horns,  you can make the listener hear a higher frequency(f1) and a  lower frequency tone(f2) instead of ‘f’.

If the horns stop spinning, the listener will only hear frequency ‘f’ .

This is due to the Doppler Effect and leads to some really cool sound effects. This video offers a great demo around the 7:30 mark:

## Planet or no planet?

In our solar system the Sun, Earth, and all of the planets in the solar system orbit around a point called the barycenter. This is where the center of the mass of the solar system lies at :

This means that the motion of the sun and jupiter looks like so:

Top and side view (exaggerated for more clarity)

This same ‘wobbling’ idea applies to planets revolving other stars as well (called ‘Exoplanets’).

The star moves around in a circle like the horns of a Leslie speaker.

The spectrum of the star when it is moving towards us would be doppler shifted to a higher frequency and when the star is moving away would be doppler shifted to a lower frequency!

Measuring this wobble is one way to find whether a planet is orbiting the star or not.

Michel Mayor and Didier Queloz were awarded the Nobel prize for their discovery of 51-Pegasi b, an ‘exoplanet’ orbiting a sun-like star 51-Pegasi using this technique.

When they published their results in 1995 it was the first exoplanet to be discovered.

Today more than 4,000 exoplanets are confirmed to be in orbit around other stars but their research definitely stands as the cornerstone in what has now become a field of its own.

Source of gifs: NASA , UOregon

* Check out other techniques to find exoplanets here.

## Van Gogh’s The Starry Night, Turbulence and Adaptive optics

Van Gogh’s The Starry Night is a stunning painting that artistically brings out the effect of turbulence in our atmosphere.

And this turbulence of air in addition to the effect of varying refractive index of the layers in our atmosphere causes the twinkling of stars:

If you are an astronomer trying to study the cosmos from the earth, this turbulence of air and twinkling of stars is a total nightmare.

The last thing that you want the light that painstakingly took millions of years to get to the earth is to be wiggled away from your telescope through refraction and turbulence!

But Astronomers found a way to deal with this, a technique called ‘Adaptive Optics’ which uses deformable mirrors to account for the disturbances in the atmosphere.

## With and Without Adaptive Optics

Using this technique, the following is the difference between capturing an image with and without adaptive optics.

## What can you find with this technique?

Here’s an interesting question: What exactly is at the center of our galaxy? Is there a black hole ? How do we go about studying it?

Prof.Andrea Ghez and her research group at the UCLA’s Galactic center group were inspired by the same question and decided to look at a region in the sky which they believed was the center of our milky way galaxy.

And this is what they found of the trajectories of stars surrounding the proposed center of the galaxy:

The first thing that you notice about these stars is that they are orbiting a point in space. This is very similar of how planets in our solar system are orbiting the sun.

One of the special stars in that animation is S0-2 which completes its elliptical orbit in only 15 years!

( it takes the sun approximately 225-250 million years to complete one journey around the galaxy’s center )

But having this knowledge of how small the orbit is, we can use Kepler’s law to find out the Mass at the center of the galaxy. And we get the mass of the center of our galaxy as a staggering 4 million times the mass of the Sun

## How massive is that?

Let’s take a look at the orbits once again:

The radius of this object at the center, in order to avoid collision with the rest of the objects has to be about the diameter of Uranus’s orbit.

So, an object that has 4 million times the mass of the Sun. and diameter of Uranus’s orbit .. Hmm.. The only astronomical object that would fit this characteristic is a Super Massive Black Hole (SMBH)

And that’s why we believe that at the center of our galaxy is a SMBH: Something we would not have been able to realize without adaptive optics.

So, the next time you go out to gaze at the cosmos, just remember that whatever you are seeing in the night sky right now is through the looking glass of our beloved atmosphere.

And astronomers put in immense effort to nullify the dynamic atmospheric effects that it loves to entertain us with.

All images/animations featured in this post were created by Prof. Andrea Ghez and her
research team at UCLA and are from data sets obtained with the W. M.
Keck Telescopes