# Chemistry Class 11 NCERT Solutions: Chapter 2 Structure of Atom Part 15

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Q: 51. The work function for caesium atom is Calculate

(a) The threshold wavelength and

(b) The threshold frequency of the radiation

If the caesium element is irradiated with a wavelength 500 nm, calculate the kinetic energy and the velocity of the ejected photoelectron.

Answer: This is given that the work function

(a) From the expression, , we get:

Where,

Substituting the values in the given expression of

Hence, the threshold wavelength is 653 nm.

(b) From the expression, we get:

Where,

=threshold frequency

H = Planck’s constant

Substituting the values in the given expression of:

Hence, the threshold frequency of radiation

(c) According to the question:

Wavelength used in irradiation

Kinetic energy

Kinetic energy of the ejected photoelectron

Since K.E

Hence, the velocity of the ejected photoelectron (v) is