1) A point source of light is placed at the centre of curvature of a hemispherical surface.the radius if curvature is r and the inner surface is completely reflecting.Find the force on the hemisphere due to the light falling on it if the source emits a power W?
2)A perfectly reflecting solid sphere of radius r is kept in the path of a parallel beam of light of large aperture.If the beam carries an intensity I,find the force exerted by the beam on the sphere???
29Jesh in the first one completely reflecting means the change in momentum = 2c
in the second I.Ï€r2 = F.v ..
so F comes out to be I πr2 / c
1anyway i got the first two probs can u also try this!!
3) In a photo cell the plates P and Q have a separation of 10cm, which are connected through a galvanometer without any cell.Bichromatic light of wavelengths 4000 Ao and 6000 A0 are incident on plate Q whose work function is 2.39eV. If a uniform magnetic field B exists parallel to the plates, find the minimum value of B for which the galvanometer shows zero deflection.
13Engy withrespective lights ::
4000A0 ,E=3.09 eV
6000A0 ,E=2.06eV
So, only the 4000A0 wavelength waali light wud be responsible fr ejecting the electrons.
MAx, KE of ejected e- = 3.09-2.39 = 0.7eV.
29The current will stop..use the relation r = mv/qB..
so wen the radius is less than 10cm the partcles will get deflected and will not reach the opposite plate..
btw the answer i am getting is B ≥ 2.8 * 10-5 T
13Yeh, i was taking the mag. field perpendicular to the plates instead of parallel... :P, thnx.
1
A totally reflecting , small plane mirror placed horizontally faces a parallel beam of light. The mass of the mirror is 20g. Assume that there is no absorption in the lens and that 30% of the light emitted by the source goes through the lens. Find the power of the source needed to support The weight of the mirror. Take g=10m/s2
11Since the mirror is totally reflecting...
Prad = 2I/c (where I =intensity = power /unit area )
so 2 Ic x 30100 = mg
put all the values....
you get I = 108
