Does Light Exert Pressure to Move an Object?

 

Does light exert any pressure?

 

Light has energy. If light has energy, it will have momentum even though light has no mass. In which case light should be capable of exerting mechanical forces on objects?  When an electromagnetic wave is absorbed by an object, the wave exerts a pressure (P) on the object that equals the wave’s irradiance (I) divided by the speed of light (c): P = I/c newtons per square metre.

However, light exerts only trivially small forces on objects; this delicate effect was first demonstrated in 1903 by the American physicists Ernest Fox Nichols and Gordon Hull. Nonetheless, radiation pressure is consequential in a number of astronomical settings. We shall discuss this later.  

If a beam of light contains energy, then when it strikes an opaque object, the light energy is absorbed and converted to heat causing the particles of the opaque object to vibrate a tiny bit more than the motions caused by the surrounding heat energy.

Our question is, can the beam of light exert a direct force on an opaque object? In other words, can light cause an object to move by absorbing it? We know the effect of a massive body in motion on any object  that gets in its way.  The motion of a car at high-speed hitting a light stationary object such as a small road cone will cause the cone flying.  But light is a small packet of particles called photons with no mass. Can it nevertheless transfer its energy and exert a force on matter?

In 1873, the Scottish physicist J. Clerk Maxwell theoretically deliberated on this dilemma.  He demonstrated that light, even though light waves with no mass, could still exert a force on matter. The magnitude of the force depends on the energy contained in the beam of light per unit length.

Let us give an example. Suppose we have a flash of light that was on for just one second. The light it emitted in that one second contains a tiny amount of energy. But in that single second, light would have moved a distance of 299 792 458 m. If all the energy of light emitted out could stretch into a beam nearly 300,000 km long, then the amount of energy in one meter of it, or even one kilometer is very infinitesimal indeed. This is why we are not aware of any force exerted by light under normal circumstances for that matter.

Suppose we were to take a very light horizontally rod with vanes attached at each end and we suspend the rod at its centre using a thin thread. Suppose we touch the vanes with the slightest force, this would cause the rod to move or rotate around the thread as its axis.  If we now shine a beam of light onto the vanes, the rod will rotate only if we think the beam of light exerted a force.

Usually, the tiny force would not be noticed if there were the slightest wind pushing against the vane. In that case, we need to enclose the rod and the vanes inside in a chamber. Even then air molecules bouncing off the vanes due to heat in the chamber would  have forces much greater than that of light.  Let us now pump out all the air in the chamber almost into a vacuum. Once that, and other conditions such as shaking the chamber or causing vibrations on the table or stand are eliminated, it might be possible to measure the small displacement of the vane when a strong beam of light shines on it.

In 1901, two American physicists, Ernest F. Nichols and Gordon F. Hull, carried such an experiment at Dartmouth College and they demonstrated that light did indeed exert a force by just about the amount as predicted by Maxwell twenty-eight years earlier.

 At nearly the same time, a Russian physicist, Peter N. Lebedev, using a slightly more complicated arrangement, demonstrated the same effect.

When the existence of this ‘radiation pressure’ was demonstrated, astronomers were sure that they were able to explain something fascinating about the tails of comets as they approached the sun. We know the tail of a comet always points away from the sun, trailing behind the comet as it approaches the sun. The tail then changes direction as the comet moves around the sun at its closest approach. Then, when the comet is moving away from the sun, the tail paves the way first.

For half a century astronomers were confident that was the reason, but unfortunately, they were all wrong. The radiation pressure of sunlight isn’t powerful enough to push the comet’s tail.  It is the solar wind that drives comet tails away from the sun.

Perhaps most important to consider is either the solar wind, radiation pressure or the thermal pressure of the sun. The outward force of the light escaping the core of the sun, working with thermal and radiation pressure, acts to balance the inward gravitational forces of the sun may better explain the formation of cometary tails, in which dust particles released by cometary nuclei are pushed by solar radiation into characteristic trailing patterns.

Having explained all that, then what makes a radiometer, or a light mill spin here.

https://www.youtube.com/watch?v=r7NEI_C9Yh0

No physicist as far as I know has been able to give us, or at least to me a satisfactory answer how it works. Can you? I can’t.

Finally, having explained and said all that, there is only one light that is much stronger than any other light or any other radiation or solar wind. And that is, the red light at a traffic junction that can stop even long lines of the most massive vehicles in front of it.  I am of course just joking to conclude what I have just written.

Jb lim