Problem on Radiation Pressure

[jbriggs444]

Sorry but I didn't get you, what do you mean by , "the larger surface is partly shaded or displaced by the smaller one." ?

The portion of the light beam that strikes the smaller upper surface (the frustrum) is absorbed. It cannot then strike the larger lower surface. The lower surface is partly shaded by the upper surface.

Presumably the walls of the cone are constructed of a transparent material with negligible thickness or a refractive index of one.

It is important to assume that the base of the cone has specular reflection (like a mirror) rather than diffuse reflection (like a white painted wall). Not only does this assure us that the full factor of two is achieved, it also assures us that the light rebounding from the base does not strike the bottom side of the frustrum.

 

[Mohammad Ishmas]

Ok. However, if the diagram in post #25 is accurate, the larger surface is partly shaded or displaced by the smaller one. But since the ratio is given as large, presumably we can ignore that.

The portion of the light beam that strikes the smaller upper surface (the frustrum) is absorbed. It cannot then strike the larger lower surface. The lower surface is partly shaded by the upper surface.

Presumably the walls of the cone are constructed of a transparent material with negligible thickness or a refractive index of one.

It is important to assume that the base of the cone has specular reflection (like a mirror) rather than diffuse reflection (like a white painted wall). Not only does this assure us that the full factor of two is achieved, it also assures us that the light rebounding from the base does not strike the bottom side of the frustrum.

Thank you , I understood your point. It would have been more clearer if there were 2 light sources mentioned in the question like one that strikes the top of frustrum and the other strikes the base .

 

[Mohammad Ishmas]

If the total area is infinitesimally small, then there is no integration to perform; the total force is infinitesimally small, i.e zero. A non-zero force can only arise for a finite area.

Are trying to solve a different problem requiring integration? Maybe this:

View attachment 359228

If this was the question , where the light strikes the slanted surface of the cone then how we would find the area of where the light exerts force ?

 

[Steve4Physics]

If this was the question , where the light strikes the slanted surface of the cone then how we would find the area of where the light exerts force ?

@Mohammad Ishmas, there are now several different versions of your question on this thread. It is still not clear (to me anyway) which one (if any) is correct.

New replies to this thread are going to add to the confusion because it will not be clear which version of the question is being addressed.

If you have a specific question, then you could start a completely new thread with:
- an explanation that it is a ‘spin-off’ from this thread;
- a complete, accurate, unambiguous problem-statement, including a diagram;
- your own attempt at an answer - as required by the forum rules.