Laser Light Pressure Supporting Foil · CAP 2009 #17
Physics · optics
Problem
An impulse laser may be treated as a source of photons that are emitted during the time interval of the pulse which is followed by a time interval when no photons are produced. Pulses are periodically repeated. A laser beam of diameter d = 10 microns is directed upward and is perpendicular to the thin foil surface which has an index of reflection ρ = 0.50 (see the sketch of the experiment). The index of reflection of the surface is the ratio of the reflected energy to the impact energy. A pulse with duration of 0.13 ms has a total energy of 10 J. What is the mass of the piece of foil that can be supported in the air solely by the light pressure of the laser beam?
(a) < 39 g
(b) < 3.1 × 10⁻¹² g
(c) < 39 mg
(d) < 3.7 g
(e) < 0.38 g