Stopped Clock Illusion

By Anupum Pant

When you quickly move your eyes to focus on the seconds hand of an analogue clock, have you ever noticed that the first second you see actually seems to linger for a slightly longer time? Yes, it does. And there’s a reason why it happens.

When you rapidly move eyeballs to focus from one point to the another, it’s called a Saccade. If you ever try doing this rapid movement with a camera, a motion blur occurs in between the first point focus and the last point focus.

Unlike cameras our eyes (work closely with the brain) has a built-in mechanism to erase this motion blur. The brain erases all the motion blur during those few milliseconds and replaces the motion blur frames with the final image in the end.

This is why you see the longer first second when you quickly focus your eyes on the seconds hand – the stopped clock illusion or chronostasis. This also explains why you can never see your eyeballs moving when you try to spot their movement while staring at your own eyes on a mirror.

Michael Stevens from Vsauce explains…

Helium Balloon in a Car

By Anupum Pant


Whenever I choose to write about Helium, there’s something I almost never forget to mention – Helium is precious (Click to know why). So, if you’ve read that, you’ll know that you shouldn’t use it in party balloons, nor should you use it to make your voice sound funny. These are the most silliest things you could do with a borderline non-renewable resource. However, if someone uses it in party balloons to make science look cool to 5-year-old, it’s beautiful.

The Experiment

Destin, a super-cool dad, from the YouTube channel Smarter every day, did exactly that. This is what he did:

Pendulum in a car: First, he tied a pendulum to the roof of his car. Then he accelerated the car. As everyone must have guessed, the pendulum moved back as the car accelerated. It’s natural for our brains to assume that everything would move back in an accelerating vehicle.

Helium Balloon in a car: Next, he tied a balloon filled with Helium to the base of the car. Then, right when he was about to accelerate his car, he asked his 5 year olds sitting in the back seat – “Where do you think the balloon would go if I accelerate?”

Answer the question before proceeding, and reason it if you can. (Even if you don’t know it, it’s easier to guess it right now because of the build up I gave in the previous paragraphs)

The Answer

Unless you already have dealt with this “anomaly”, it’s pretty tough to guess that the balloon would actually move forward as the car accelerates. Yes, it moves forward! Something moving forward in an accelerating car sounds counter intuitive. I knew it because someone had asked me it in a physics puzzle sometime back. Just for the record, I had answered it wrong then. There’s no way I could have guessed, or reasoned it accurately the first time. Did you?

The balloon seems to be defying the laws of physics. But a helium balloon moving forward as the car accelerates can be completely explained by physics. It’s just our brain fooling us again.

Simple Analogy

Here’s how Destin explains it with a simple analogy – using a glass jar filled incompletely with water (so there’s an air bubble inside). Assume that the glass jar is like the car. The water in it, is like the air in the car. And the Helium balloon is like the air bubble in the jar – Since an air bubble is lighter than water, it is safe to assume that because even Helium is lighter than the air.

Now when the jar accelerates forward, the water in the jar moves back – so does the air in the car. As a result, the air in the jar moves forward – just like the Helium balloon does.

Here, watch the video if that sounds too confusing…

Most of you probably know this. But I’m sure that many don’t. Moreover I found the video really cute – A super cool dad explaining science to his little kids in a car. Plus they ask you to go to their audible link that would get you a free audio book. At the same time, it would help a cool dad fund his children’s education. My heart melted. If nothing, the video will at least make you smile.

Yes, Light Can Push Physical Objects

By Anupum Pant

Tim is a 71-year-old eccentric who has been collecting interesting toys for 50 years now. Today he has a collection of around 250,000 absurd toys in suitcases, labeled and stacked in a room from the floor to the ceiling. He shows them off in a Youtube channel regularly. I almost never miss any of his toys. Usually most the toys he displays amaze you, but do not blow off your mind. The last one did.

Before this, I had not known that light can push or move a physical object. So, I decided to investigate a bit.

The Extraordinary Toy

In his last video, he showed off a beautiful glass bulb mounted on a wooden stand, that he says has been made by some German company. The bulb has a very good vacuüm (not complete vacuüm, just enough to not create unnecessary drag for the vanes) and encloses a fan-like structure that starts rotating when a bright light is switched on near it. Some people call it the light mill – like the wind mill moves with the wind, this one moves when photons hit it. If give enough time, the mill can accelerate to really good speeds (at thousands of rounds per minute). Watch the video below. [Video]

Theories on how a radiometer really works

The device, a special kind of radiometer, was invented around 140 years back in the year 1873, by Victorian experimenter Sir William Crookes to measure the radiant energy of heat or light. It has four vanes mounted at the edge of four stiff wires to make a fan, each of which has a black side and a silvered side. All of this is enclosed in a bulb which is evacuated enough to not cause drag due to air. In complete vacuüm the vanes move in an opposite direction, but that experiment is really difficult to recreate!

  1. When light is turned on, the fan moves in a way that makes it look as if light is pushing away the black colored side. The theory of photons pushing the black side was accepted initially. But soon a problem was seen with the theory. If light is absorbed at the blackened side and reflected at the silvered side, the fan should be moving in the opposite direction.
  2. Then came in the other explanation which explained that the heated black side due to absorbed radiation rarefied the air near the black side and hence caused the gas to rush in and push that side. The greater the heat, the more this back side would get pushed and it would spin faster. Later even this theory was proved to be wrong. But, Britannica, till date decides to go with this explanation – to some extent this theory goes in the right direction, you will see why…
  3. One more theory claimed that the heat evaporated the impurities on the black side, whose force made it spin.

How the radiometer really works?

The correct explanation was given by a  prominent Anglo-Irish innovator, Osbourne Reynolds. He explained it by mentioning a porous plate, where the air inside the holes would flow from the colder sides to the warmer side (obviously) and make the vanes spin in the opposite direction. He called it “Thermal Transpiration” – makes sense and is easy to understand. But the vanes here are not porous. So…

He said that “Thermal Transpiration” in the vanes takes place at the edge of the vanes and not the faces. Think of edges as small pores, he said.

Due to a temperature gradient formed, the air starts moving along the surface from colder to the warmer side through the edges. The net pressure difference around the vane is created which pushes it in a direction that is away from warmer air and towards the colder air – makes sense for the apparatus.

This is the reason, if it is cooled, it moves in the opposite direction.

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