The image can explain my question
In the image light is clearly trapped.Even if the mirror absorbs energy the light is continously being added, will there be enough force to break the mirror?
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$\begingroup$ possible duplicate of Two mirrors facing each other $\endgroup$– ACuriousMind ♦Commented Mar 22, 2015 at 14:18
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$\begingroup$ It's not a duplicate, my question is a bit different $\endgroup$– SaadCommented Mar 22, 2015 at 14:27
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$\begingroup$ Your edited question is indeed not a duplicate, but now it is dependent on the exact reflectivity of the mirrors, the amount of power added, and other exact values, and essentially devolves into a homework-like calculation. $\endgroup$– ACuriousMind ♦Commented Mar 22, 2015 at 14:29
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1$\begingroup$ Looks very much like a laser, although it doesn't use light to start the bouncing between mirrors. $\endgroup$– LDC3Commented Mar 22, 2015 at 14:40
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$\begingroup$ if using general values $\endgroup$– SaadCommented Mar 22, 2015 at 14:41
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1 Answer
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It doesn't quite work the way you envision it (if the refraction angle is such that you can add light, it will escape the same way), but there are optical resonators that do essentially what you want: Light incident on a mirror gets added to a light field trapped between two or more mirrors.
In such setups, not quite enough light usually builds up to cause mechanical (or thermal damage)---but you might argue that is because one usually prefers to design them in a way that they continue working. Quite a lot of power can be present in an optical resonator: Quality factors (ratio of internal to incident power when on-resonance) can reach about one million for near infrared wavelengths, and interferometric gravitational wave detectors use relatively high-Q resonators with, for a quantum-optical precision experiment, rather powerful lasers, for example about 200 W in the advanced LIGO experiment.
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$\begingroup$ And in fact a lot of work goes into designing intracavity optics so that the peak power levels in a laser do not break anything. $\endgroup$ Commented Mar 22, 2015 at 14:43
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2$\begingroup$ But note that even in an optical resonator with a huge circulating power, the absorbed power of any optical component can never exceed the input power (at least in steady state). So, e.g. in the Advanced LIGO experiment the main cavity mirrors can have several hundred kilowatts impinging on them, but they can only ever absorb a fraction of the 200 W input power. This is not enough to produce any physical damage, but it can deform optics in an undesired way. Also the radiation pressure is a serious technical concern. $\endgroup$– EmilCommented Mar 22, 2015 at 15:03
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$\begingroup$ The problem is not force damaging the optics. It is heat. High power lasers go to a lot of trouble to use components with low absorption. Even so, components do not last forever. They get dirty. Dirt absorbs light and heats up. Also thermal expansion deforms optics, and defocuses the beam. Some infrared laser cavities use a copper mirror. A special high conductivity grade of copper must be used, and even then it may need to be water cooled. $\endgroup$ Commented Mar 22, 2015 at 15:36