In a photocell, bi-chromatic rays of light of wavelength 2475 Å and 6000 Å are incident on cathode whose work function is 4.8 eV. If a uniform magnetic field of 3 x 10-5tesla exists parallel to the plate, the radius of the path describe by the photoelectron will be (mass of electron =9 x 10-31 kg)
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a
1 cm
b
5 cm
c
10 cm
d
25 cm
answer is B.
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Detailed Solution
Energy of photons corresponding to light of wave length λ1=2475Å is E1=123752475=5eVand that corresponding to λ2=6000Å is E2=123756000=2.06eV As E2W0photoelectric emission is possible with λ1 only. Maximum kinetic energy of emitted photoelectrons, K=E−W0=5−4.8=0.2 eVPhoto electrons experience magnetic force and move along a circular path of radius,r=mvqB=2mKqB=2×9×10−31×0.2×1.6×10−191.6×10−19×3×10−5=0.05m=5cm.