The angular width of the central maximum in the Fraunhofer's diffraction pattern is measured. The slit is illuminated by the light of wavelength 6000 A ∘ . If the slit is illuminated by the light of another wavelength, angular width decreases by 30 % . The wavelength of light used is

Step-by-step solution

For the first diffraction minimum, d sin ⁡ θ = λ And if the angle is small, sin ⁡ θ = θ d θ = λ i.e., Half angular width, θ = λ d Full angular width w = 2 θ = 2 λ d Also, w ' = 2 λ ' d ∴ λ ' λ = w ' w or λ ' = λ w ' w = 6000 × 0.7 = 4200   Å