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.”

Noble prize winners

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.

Demo of leslie speaker

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

working of leslie speaker diagram

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 :

barycenter of sun and jupiter.

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

Wobbling of orbits of a star and its planet.

                        Top and side view (exaggerated for more clarity)

Wobbling of orbits of a star and its planet.

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!

Nobel prize in physics 2019 radial velocity method

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

Von Gogh's starry night

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:

Refraction of light from a start in our atmosphere
     Source: Enhanced Learning

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!

f you have seen lasers coming out of telescopes. That's part of the Adaptive Optics system used to correct for atmospheric disturbances due to turbulence and refraction.
If you have seen lasers coming out of telescopes. That’s part of the Adaptive Optics system used to correct for atmospheric disturbances due to turbulence and refraction.

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.

Adaptive optics- On vs off

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:

  Trajectories of stars surrounding the proposed center of our galaxy.
    The star in the middle is the proposed center of our galaxy.These images were taken through the years 1996 – 2016 (see top right of gif).

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.

Solar system animation

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

S0-2 completes its entire elliptical orbit in just 15 years!
S0-2 completes its entire elliptical orbit in just 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:

Orbits of solar system vs SO-2 star

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

Rolling Shutter and online guitar videos

When you search for videos online of plucking a string on an instrument such as the guitar, a surprising number of searches lead you to videos such as the following:

Rolling shutter effect of a plucked guitar string which appears in a lot of online videos.

This is not how plucked strings look like! And they don’t have anything to do with Harmonics either! The reason why you are seeing those shapes on the guitar is due to the rolling shutter effect on your camera.

But if you do want to see how plucked strings look like, the following videos would be your best bet:


What is Rolling Shutter?

DIY: Tutorial running you through how you can recreate the effect on a guitar for yourself

Visualizing Doppler Effect using ripple tanks

Ripple tanks are really cool ways to explore the way a wave behaves under the influence of a perturbation.

They are fairly simple to make, and are usually available in college and school laboratories to render better understanding of the wave phenomenon.

How does it work ?



There is a usually an oscillating paddle( above– used to produce plane waves) or a point source/s ( below – used to produce circular waves ) which are actuated by eccentric motors, solenoids, etc + a shallow tank of water.

And that’s about it! One is ready to visualize wave phenomenon


Here are some of my favorite renditions of physical phenomenon on a ripple tank. Check sources for more. Enjoy!

1. Diffraction


2.Double slit experiment


3. Reflection


4. Refraction


5. Parabolic Reflectors


6. Doppler Effect

Doppler effect is the increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move toward (or away from) each other.

If we have a speaker that is moving to the right (see animation above), if you are standing infront of the speaker you will hear a shorter wavelength- higher frequency sound and if you are standing behind the moving speaker, you will hear a longer wavelength – lower frequency sound.

We experience the Doppler effect everyday whenever a car whizzes past us. Here’s a demo:

** Source videos : Educational Services Inc-1964  and Aerodynamic generation of sound

Even and Odd Harmonics of a vibrating string

In the previous section we took a look at the vibrating string fixed at both ends and found that in order for the boundary condition to be satisfied, the following are the only solutions possible:†

Even and Odd harmonic animation

The solutions on the left of the image are often termed as ‘Odd Harmonics’ because they have odd number of anti-nodes and the ones of the right have even number of anti-nodes hence ‘Even Harmonics’

If you pluck a string right at the center, you are essentially exciting only the odd harmonic components.

Odd Harmonics of a Triangular wave

You can test this out by taking a string instrument and plucking it at the center. Download a spectrum app on your phone and take a look at the spectrum

Harmonics spectrum
Source video

You will see peaks in the spectrum only at the frequencies which correspond to the odd harmonics:

Odd harmonic spectrum

String instruments like the violin or guitar are plucked off-centered, so you get both the odd and even harmonics. But what differentiates one instrument from the other is the amplitude at which these harmonics are expressed when you play them.

Do watch the following video if you would like to know more: