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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Paper Bags Are Not Better Than Plastic Bags

By Anupum Pant

Plastic bags are terrible things. They choke animals, aren’t easy to recycle, do not break down, pollute our oceans, their production adds to our oil demands…and the list goes on. Some time back, we realized their ill effects and started taking steps that would encourage people to use bags made of alternative materials. Furthermore, several cities all over the world have banned the use of plastic bags.

Side Note: Interestingly, plastic bags aren’t actually banned for any of those reasons. They are banned because they tend to fly with the wind and move out of your trash fairly easily. They create a mess at places where they aren’t supposed to. That is the major reason as to why they are being banned.

In 2007, San Francisco banned plastic bags for supermarkets and pharmacies. Last year, it got expanded to all retail stores. Now, they have been banned for restaurant takeaways too. Also, the use of plastic bags at retail stores has been banned in several Indian cities. But the point isn’t to list out all the cities where it has been banned. There are many cities. I hope you get the idea…

When it comes to finding an alternative for plastic bags, paper bags seem to be the first choice. But it turns out, paper bags are not better than plastic bags.  Most of us underestimate their ill effects. Here are a few reasons that will make you realize why paper bags are not so good:

The point isn’t to make paper bags look bad or to make plastic look good or vice versa. It is to dispel the image of “the green paper bag” from our minds.


Production: Production of paper bags all over the world involves cutting down 14 million trees every year. It is estimated that the production of paper bags creates 70 % more air pollution than plastic bag production.
Production of paper bags also results in much more water getting polluted when compared to the production of plastic bags. This is because their manufacturing process requires a lot of water.
Almost the same amount of petroleum used for plastic bags (for the material) gets consumed in making of paper bags to fuel the machines plus transportation.

Weight: Paper weighs a lot more than plastic. It is estimated that to carry the same number of paper bags it takes 7 times the transportation it takes to haul plastic bags. More trucks, more pollution, greater greenhouse impact.

Space: Paper bags occupy a lot more space than plastic bags do. This creates a problem at landfills that are getting filled to the brim already.

Recycling: Paper bag activists would say, plastic bags live for ever in the landfills. Yes they do, but there, paper bags do not decompose within a meaningful time period either. In fact, most of the stuff lives on for a long time in landfills. Landfills aren’t meant to make things degrade. With a paucity of oxygen and water in landfills, it is hard for things to decompose there. Even food items thrown away at landfills last for years.
That said, even plastic bags are almost never recycled.

Also they tear easily. As a result, more number of paper bags have to be used.


Both of them – plastic and paper bags – are equally bad. Recently developed biodegradable plastic bags are not any good either (they have a bigger carbon footprint). Carrying canvas, cloth or jute bags and saving them for future use is probably the best alternative.

If you liked this, you’ll probably also like – Understanding the Impending Helium Crisis