Answer:
Explanation:
Due to  change in the refractive index of the medium , wavelength changes. If λ  be wave length in vacuum ,  wave-length in refractive index μ
will become  λ / μ
Initially no of wavelengths in length L were  L / λ
Finally no of wavelengths in length L were L / λ / μ = Lμ / λ
Given
Lμ / λ  -  L / λ  = N
L / λ ( μ -1 ) = N
μ = ( Nλ / L + 1 )