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I recently came across a question in which we had to find the force acting on a cube due to light beam directed along its body diagonal with a certain intensity. I am having trouble visualising the situation and have no idea what to do. In which direction will such a force be directed if the light is completely reflected? Can we actually break the light into components and calculate force on individual faces? Would someone please help me out?

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Since this is a homework-like question, I won't solve it for you. However, I will give you some pointers.

  1. Draw a picture and calculate the angle between the surface normals and the body diagonal.
  2. The total reflection indicates that you have to use "angle of incidence = angle of reflection" (Snell's law).
  3. By symmetry you can deduce the direction of the total (=summed) momentum. Use this information: Consider a single ray and calculate the change of momentum in the direction of the final direction. This involves a projection onto this direction.
  4. We can definitely break up the calculation into individual surface. However, by using step 3 it becomes clear that we only have to consider the "cross section": If the length of one side of the cube is $a$, which surface area "sees" the incoming light?
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  • $\begingroup$ Thank you very much $\endgroup$ Jun 22 '20 at 9:33

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