Question

radiation pressure is the pressure exerted by light on any surface, right? so what are the possible effects of radiation pressure on the surface?

Answer 1

All electromagnetic radiation is made up a photons. When these photons hit and and are absorbed by an object, they impart to that matter their momentum.

Hence the 'radiation pressure' is the impulse the radiation gives to the matter. This is like a force. It is as if the radiation is pushing on the object. That is, it is as if the radiation is applying a force to the object.

Now what happens to an object that has a (net) force applied to it? It accelerates.

Answer 2

This is the theory behind some hypothetical space craft that would use solar radiation as a way to accelerate:

Answer 3

Actually, not entirely theoretical!
http://science.nasa.gov/science-news/science-at-nasa/2011/24jan_solarsail/

Answer 4

when light is shone on any material object, then can the force by the photons transfer momentum to the molecules of the object?

Answer 5

Yes, the momentum of the photons is imparted to the atoms in the object.

Answer 6

when the momentum is transfered ,can this cause an increase in the kinetic energy of the molecules of the object under irradiation?

Answer 7

the same thing explains photoelectric effect?
but that is absorbing energy from photons right? here is transfer of momentum due to collisions?

Answer 8

The photoelectric effect is a bit different, as it has to do with energy transfer. But they are not unrelated.

I should also point out that if the photons are reflected, even more momentum is imparted, as you might expect from intuition in classical physics:

Answer 9

what about the kinetic energy of the molecules of the object, does it increase when light is shown on it? and is the momentum transferred very small?

Answer 10

The photons all have energy and that energy has to go somewhere. Some of it will turn into kinetic energy yes. But don't neglect the fact that some of that energy will turn into other things, such as heat and ionizing matter.

Notice in the NASA solar sail, the surface is highly reflective. That is no accident. That is designed on purpose to maximize the reflection of the photons. This in turn maximizes the energy transfer to kinetic energy, vs. other forms of energy.

Answer 11

what about visible light?what happens when blue light is shone in an object? Also, when visible light is absorbed by matter it results in the electronic transition,right... but does it does not ionize?

Answer 12

i have a doubt...reflection increses momentum imparted means because of collision with moving objects(the surface is moving)?

Answer 13

All light is EM radiation. Regarding the effects of blue light vs. red light vs. gamma radiation vs. IR radiation, do a search on the photoelectric effect.

@salini: more momentum is transferred in reflection because the change in momentum of the photon itself is 2p, where p was its original momentum, vs. a change in momentum of just p if it is absorbed.

Answer 14

yeah imparted means final -initial momentum that is why
now i got it
thank u

Answer 15

photoelectric effect is for metals. I am talking about interaction of light with non-metals. Visible light is the non-ionizing EMR. It only causes excitation of electrons from lower energy levels to higer ones . Visible light has less energy as compared to the the EMR with even lower wavelengths... so it cannot cause ionization, right?
What I am trying to ask here is that the photons of visible light also transfer momentum, and increase the kinetic energy of molecules they are shone on? and is the transfer of momentum sufficient?

Answer 16

the photoelectric effect is *not* just for metals. It applies to all matter.

All photons impart momentum when they collide with an object, and are either absorbed or reflected. The momentum \( p \) of an individual photon of wavelength \( \lambda \) is
\[ p = \frac{h}{\lambda} \] where \( h \) is the Planck constant.

[Hence the lower the wavelength, the higher the momentum of an individual photon.

But unlike the photoelectric effect, the momentum transfer does very critically depend on the intensity of the radiation: more photons, more momentum transfer. With the photoelectric effect, more photons doesn't necessarily lead to more ionization.]

Answer 17

ahh, yes photoelectric effect is for non metals as well.
what is difference between the electrons flying out in the photoelectric effect and ionization? I am confusing the both..

Answer 18

If an atom is ionized, it losses (or gains) an electron. The photoelectric effect is a means of ejecting an electron from an atom. But not all of the photoelectric effect is about ejecting electrons; the photoelectric effect is also about moving electronics between energy levels within the atom (i.e., below the ionization level).

If you're interested in understanding this some more, watch this lecture:
http://ocw.mit.edu/courses/chemistry/5-111-principles-of-chemical-science-fall-2008/video-lectures/lecture-4/
and the one that follows.

Answer 19

ionization results in the chemical bond being formed.. and the ejected electrons are not involved in the chemical reactions?

Answer 20

That is another form of ionization, yes.

Answer 21

To add to JamesJ's answer, ionisation can be done through any means and is used in bond formation. But the photoelectric effect can only be done using photons of electromagnetic radiation. This effect is mainly utilized to produce electricity through free electrons(mainly with metals).

Answer 22

when visible light is shone on an object, the photon energy is fully absorbed by the object for electronic transition or the photoelectric effect or results in increasing momentum because of radiation pressure?

Answer 23

can all these happen simultaneously?

Answer 24

or am I just confusing everything up?

Answer 25

Just as in classical mechanics, we should distinguish between momentum transfer and energy transfer.

If the photon is absorbed or reflected, momentum transfer occurs.

If the photon is absorbed, energy transfer occurs but as mentioned above that energy after the interaction could take several forms.

If the photon is reflected, energy transfer might have occurred if there were a range of possible interactions; by classical analogy (and this isn't perfect), whether or not the collision was 'perfectly elastic' or not.

Answer 26

is the momentum transfer significant for visible light?

Answer 27

The cat sitting on the table sitting next to me is not being pushed by the sun shining through the window, no.

The momentum transfer is tiny. It took us to develop quantum theory in the early 20th century to even predict the effect, and then start experimenting for it.

Answer 28

Or I should say, the cat *is* being pushed, but neither he nor I notice it because the effect is tiny. Not zero, but very, very small.

Answer 29

lol,good example. I mean the momentum transfer on the atomic level or the sub-atomic level. The momentum transfer to increase the kinetic energy of the atoms?

Answer 30

Just like a collision of two billiard balls, a momentum transfer can lead to an energy transfer as well, yes.

Answer 31

so we can say that even if the visible light does not have enough energy to cause ionization, but it does increase the kinetic energy of atoms?

Answer 32

visible light does cause an heating effect so that means it increases the average kinetic energy of the atoms but it cannot ionize them...right?

Answer 33

here's the ionization energy of elements for electrons in the ground state:
http://chemwiki.ucdavis.edu/ @api/deki/files/6688/=Ionization_Energy_Graph_IK.png

Now, you tell me, what is the range of energy of individual visible light electrons?

Answer 34

2 -3eV

Answer 35

That being the case, it looks like the first twenty elements can't be ionized by visible light.

Answer 36

yeah, but photon energy is still being absorbed so that can be tranferred to kinetic energy?

Answer 37

yes, and that KE could manifest either as heat or motion.

Answer 38

and what about electronic transition?

Answer 39

yes, electrons could also go up a level due to the absorption of a photon. But if that happens, that energy will be released again a nanosecond later when the electron goes back down and a new photon is released.

Answer 40

thanks for clearing up all my confusions :)

Answer 41

Sure. You're asking good questions.

I think this topic is confusing because typically all of these effects are taught and discussed in text books separately. I'm going to look around a little and see if I can find a good coherent coverage that talks about all of them together.

Answer 42

okay, cool. :)

Answer 43

@jamesj that wud be awesome!
we were taught all this in chemistry!

Answer 44

Nothing is jumping out at me. If anyone has a good (I mean very good) link, please post it.