11) w/c
2) 4/(3c)
r dese correct?
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???
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12 Answers
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