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When a ray passes through a lens, light is refracted and changes its momentum. In the case of an ideal lens (no absorption, flat) rays parallel to the optical axis pass through the focus and rays passing through the middle of the lens do not bend. The energy and magnitude of the momentum of the ray don't change, since we are dealing with an ideal lens, but the direction does. For the parallel ray, I imagine this should look something like that:

enter image description here

If we imagine the light ray as consisting of individual particles being reflected by a mirror, the mirror would be pushed in the direction of $p_f - p_i$. Also, the direction of this $\Delta p$ always has a component outwards away from the optical axis for a convex lens, since the problem can be symmetric around the axis. It seems this should lead to mechanical stress in a lens if the intensity of the light (=number $N$ of photons) is large enough. Of course, this momentum change is more or less also what is used to propel a solar sail.

Is this of practical consideration when designing (pulsed laser) optics and at what intensity (order of magnitude) would this rip a typical lens apart, if at all?

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  • $\begingroup$ I'd agree that a strong enough laser could heat the lens to failure. I don't know how to work out the problem, but one failure mode would be due to the transmission of the glass. The glass will adsorb some fraction of the photons. $\endgroup$
    – MaxW
    Commented Dec 16, 2019 at 9:27
  • $\begingroup$ @MaxW True, that is most likely the first point of failure. I was just interested if - ignoring thermal effects - also a mechanical component might play a role, even if we neglect absorption completely. $\endgroup$
    – DK2AX
    Commented Dec 16, 2019 at 9:57
  • $\begingroup$ Strong lasers may also break windows of vacuum equipment. It is important to have clean surfaces without scratches. Maybe some companies have specifications? $\endgroup$
    – user137289
    Commented Dec 16, 2019 at 13:10

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Yes, deflection of the laser light by the lens will cause a small mechanical stress in the way you have indicated. However, the amount of stress is very small, because the amount of momentum carried by a laser beam is very small. But sometimes it's not negligible. You might find it interesting to read about Breakthrough Starshot.

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