Answer 1

β = 0.12 cm
θ = 40 seconds = 1.9392 x 10^-4 radians

`β = λ/(2θ)`

λ = β x 2θ
= 0.12 x 2 x 1.9392 x 10^-4
= 4654 A^o

Wavelength of light used = 4654 A^o