Correct Answer – C
Number of waves in a medium (N) `=(“Thickness”)/(“Wavelength in the medium”)`
`:. N_(“glass”)=N_(“water”)`
`:. (4)/(lambda_g)=(5)/(lambda_w) :. (lambda_g)/(lambda_w)=(4)/(5)`
but `lambda prop (1)/(R.I.) :. (n_w)/(n_g)=(lambda_g)/(lambda_w)`
`:. n_W=n_g xx (lambda_g)/(lambda_w)=(5)/(3) xx(4)/(5)=(4)/(3)=1.33`

Correct Answer – C
No. of waves in the glass slab of thickness `t_g` = No. of waves in the water column of height `h_w`. The optical path in a medium of thickness t, and R.I. `mu` is `mu xx t` .
Using this we get ,
`mu_g xx t_g = mu_w xx h_w`
`:. “”mu_w=(mu_g xx t_g)/(h_w)=(1.5 xx 6)/(7)=(9)/(7)=1.286`