Does light exerts force?

One of my project will work with a great success if the light can exert force.
Can it be possible?

Related Other Physics Topics News on Phys.org
I don't think that light produces any force directly from its motion. The energy from light can cause a force to be generated, however.

ZapperZ
Staff Emeritus
One of my project will work with a great success if the light can exert force.
Can it be possible?
Light (as in photons) carries momentum. Any change in that momentum (such as absorption or reflection), will impart a force. So yes, it can exert a force. There's a simple experimental demonstration which has a set of 4 paddles being painted black on one side of each paddle, and while on the other, that spins simply when you shine light on it.

However, if your "great success" depends on using this to "move" objects substantially, you might be disappointed.

Zz.

Light (as in photons) carries momentum. Any change in that momentum (such as absorption or reflection), will impart a force. So yes, it can exert a force. There's a simple experimental demonstration which has a set of 4 paddles being painted black on one side of each paddle, and while on the other, that spins simply when you shine light on it.

However, if your "great success" depends on using this to "move" objects substantially, you might be disappointed.

Zz.
I think what you are talking about is called a radiometer. What causes it to spin is not the change in momentum from light "particles" hitting the paddles. In fact, the black side of the paddles absorbs much of the incoming light, causing the black side to heat up, while the white side stays cool because it reflects the light. When air molecules inside the radiometer hit the black side, kinetic energy is imparted to them, and the change in momentum of the air molecules (away from the black side) means an equal and opposite change in momentum happens to the paddle. Because there is a partial vacuum in the radiometer, these air molecules mostly flow freely until they hit either a paddle or the side of the radiometer, and it is more likely for them to hit the glass surface and slow down before hitting the white side of the paddle (which would cause turning in the opposite direction).

Last edited:
ZapperZ
Staff Emeritus
I think what you are talking about is called a radiometer. What causes it to spin is not the change in momentum from light "particles" hitting the paddles. In fact, the black side of the paddles absorbs much of the incoming light, causing the black side to heat up, while the white side stays cool because it reflects the light. When air molecules inside the radiometer hit the black side, kinetic energy is imparted to them, and the change in momentum of the air molecules (away from the black side) means an equal and opposite change in momentum happens to the paddle. Because there is a partial vacuum in the radiometer, these air molecules mostly flow freely until they hit either a paddle or the side of the radiometer, and it is more likely for them to hit the glass surface and slow down before hitting the white side of the paddle (which would cause turning in the opposite direction).

Zz.

dst
Unless your project involves space and several square kilometres of material, don't count on getting any useful work done.

pam
If light is reflected from a metal surface, it exerts a pressure (in dynes/cm^2) given by
p=2I/c, where I is the intensity in ergs/(cm^2-sec) and c is the speed of light.
As dst said, I don't think you can measure this small pressure in the laboratory,
although it is a possible option for space travel ("solar sailing").

Andy Resnick
One of my project will work with a great success if the light can exert force.
Can it be possible?
Yes, a focused beam of light can pull objects into the focal volume. Google "Laser Tweezers". Polarized light can exert angular momentum on a birefringent object as well- polarization is an analog to spin. Light can also have angular momentum- Bessel beams are a good example.

as light conssit of photon which is massless, i think the pressure or force are the virtual concepts of light to explain some phenomena like compton effect etc

The short answer is yes, light does exert a force. But it is an extremely small force. As has been mentioned, you pretty much have to be in vacuum and you have to have a very large sail if you want to get anything out of it.

Or you need a VERY powerful flashlight, probably orders of magnitude more so than anything we have.

jtbell
Mentor
D H
Staff Emeritus
While photons are massless, they do have energy E=ℏω and momentum p=ℏω/c=E/c.

The force is quite small. Suppose the entire power consumption of the US, 3.4 terawatts, is converted to coherent light and aimed at some perfectly reflective object in space. The thrust on that object is 2E/c (the light is reflected), or about 23,000 newtons.

Unless your project involves space and several square kilometres of material, don't count on getting any useful work done.
Or high energy radiation like X-rays and Gamma rays.

sophiecentaur
Gold Member
The force, for a given flux of energy, is not dependent on the frequency. The momentum of each photon of high frequency radiation is higher but the energy carried is greater, so fewer photons are involved. The two cancel out so it just depends upon the power. So em pressure can be calculated 'classically' (ignoring quantisation) and you get the same / right answer. Strange / reassuring, I think.

If light is reflected from a metal surface, it exerts a pressure (in dynes/cm^2) given by
p=2I/c, where I is the intensity in ergs/(cm^2-sec) and c is the speed of light.
As dst said, I don't think you can measure this small pressure in the laboratory,
although it is a possible option for space travel ("solar sailing").
I didn't know people still used medieval units. Care to express that in thumbs per teaspoons?

:)!

pervect
Staff Emeritus

rcgldr
Homework Helper
On a semi-side note, is it the light (photons) that have momentum, or is the momentum effect due to some intermediate state and/or interchange as electrons capture and/or emit photons which causes the electrons to gain and/or lose energy?

Andy Resnick
The electromagnetic field itself possesses momentum, expressed as the Poynting vector.

My only hope is that the OP is with full understanding that, in all circumstances, the energy derived is LESS than the energy required to emit the photons.

Yes, but you're not going to push over a building by shining a flashlight at it.

At a certain point it's like gravity--you don't notice it because you've lived with it your whole life. But it's REALLY weak.

When we feel forces in the real world, we feel electrostatic repulsions, right? But photons don't have a charge (I think?), so how can they exert a force?

When we feel forces in the real world, we feel electrostatic repulsions, right? But photons don't have a charge (I think?), so how can they exert a force?
Go here and scroll down to "physical properties":
http://en.wikipedia.org/wiki/Photon

When something with some momentum hits something else and there is a change in momentum, there is a force involved in changing the momentum. So when you shine a flashlight at a wall you're exerting a force on it.

If you keep reading you'll see that the momentum of light is proportional to Planck's constant (order 10-34) and inversely proportional to wavelength. For light with wavelength 650 nm (red light) that puts you at Planck's constant / (650 nanometers) = 1.01939508 × 10-27 m kg / s (put the part before the = into Google if you want). As you can see this is an extremely small amount of momentum per photon.

Now I'm not sure how many photons are coming out of say your typical flashlight, but from experience you should know it's not enough to knock anything over.

Also you are right that day to day forces are electromagnetic in nature (mechanical friction for instance is really a macroscopic E&M effect). But there are also the strong and weak force (and gravity). I'm not sure what the interaction mechanism is when a photon hits something but suffice it to say there is more than one type of force.