Question

Radiation of wavelength 200A° falls on a platinum surface.If the work function of the metal is 5eV.which of the following results are correct about experiment?
a)the velocity of photoelectrons increases with increase in intensity of radiation.
b)photo-emission of electrons takes place.
c)The threshold frequency of the metal is 1.21×10¹⁵sec-1.
d)The velocity of the photo-electrons is 4.48×10⁶m/sec.​

Answer 1

Solution

Wavelength of radiation (λ)=200nm=200×10−6m)

Etotal=hcλ=6.63×10−34Js×3×108ms−1250×10−9m

=7.956×10−19J

Work function (W)=5eV=5×1.6×10−19J=8.0×10−19J

Energy of photons

E=hv=hcλ

=(6.63XX10−34jS)×3XX108ms−1200×10−9m

=9.945×10−19J

Using Einstien's photoelectric equation

hv−KE+W

or KE=hv−W

=9.945×10−19−8.0×10−19=1.945×10−19J

KE=12mv2=1.945×10−19J

(Mass of electron=9.1×10−3g)

or v=2×1.945×10−19J(9.1×10−31)1/2

=6.54×106ms−1