Q.

Heat treatment of muscular pain involves radiation of wavelength of about 900 nm. Which spectral line of H atom is suitable for this 

Given: Rydberg constant RH=105cm-1, h=6.6×10-34Js, c=3×108m/s                  [2025]

1 Lyman series, 1  
2 Balmer series, 2  
3 Paschen series, 53  
4 Paschen series, 3  

Ans.

(4)

λ=900 nm=900×10-9m

   =900×10-9m×100 cm=9×10-5cm

RH=105cm-1

By Rydberg’s equation:

1λ=RHZ2(1n12-1n22)

19×10-5cm=105cm-1×(1)2(1n12-1n22)

(1n12-1n22)=19

n1=3,  n2=