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Question:

If λcu is the wavelength of Kα X-ray line of copper (atomic number 29) and λMo is the wavelength of the Kα X-ray line of molybdenum (atomic number 42), then the ratio λcu/λMo is close to: 2.14, 1.99, 0.50, 0.48

1.99

0.48

2.14

0.50

Solution:

We know, The relation between λ and Z is given by,
λ = (Z-1)²
∴ λcu/λmo = (Zmo - 1)²/(Zcu - 1)²
∴ λcu/λmo = (42 - 1)²/(29 - 1)² = (41)²/(28)² = 1681/784 = 2.144