# When a Photon is Emitted from an Atom, the Atom Recoils. the Kinetic Energy of Recoil and the Energy of the Photon Come from the Difference in Energies Between the States - Physics

Sum

When a photon is emitted from an atom, the atom recoils. The kinetic energy of recoil and the energy of the photon come from the difference in energies between the states involved in the transition. Suppose, a hydrogen atom changes its state from n = 3 to n = 2. Calculate the fractional change in the wavelength of light emitted, due to the recoil.

#### Solution

Difference in energy in the transition from n = 3 to n = 2 is 1.89 eV ( = E).

If all this energy is used up in emitting a photon (i.e. recoil energy is zero).

Then,

E = (hc)/lamda

rArr lamda = (hc)/E ..........(i)

If difference of energy is used up in emitting a photon and recoil of atom, then let ER be the recoil energy of atom.

E = (hc)/lamda + E_R

rArr lamda ' = (hc)/(E - E_R) ............(ii)

Fractional change in the wavelength is given as,

(Delta lamda)/lamda = (lamda'-lamda)/lamda

rArr (Deltalamda)/lamda = 1/lamda((hc)/(E-E_R ) - (hc)/E)

rArr (Delta lamda)/(lamda)=E/(hc)  (hcE_R)/(E(E-E_R))    (therefore lamda = (hc)/E)

rArr (Deltalamda)/lamda = ((E_R)/(E- E_R))

Concept: The Line Spectra of the Hydrogen Atom
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#### APPEARS IN

HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 21 Bohr’s Model and Physics of Atom
Q 37 | Page 385