The angular width of central maxima is given by 2θ=a2λ where, λ = wavelength of light used a= width of the slit For λ1=6000A˚,2θ=θ0 (given) For another light of wavelength λ2 (says), the angular wid th decreases by 30% so, 2θ=(100100−30)θ0=10070θ0=0.7θ0 As slit width is constant, so using Eq. (i) for these values, we get 0.7θ0θ0=λ2λ1⇒λ2=λ1×0.7=6000×0.7=4200A˚