White light is incident normally on a glass plate (in air) of thickness 500 nm and refractive index of 1.5. The wavelength (in nm ) in the visible region (400 nm-700 nm) that is strongly reflected by the plate is

Ray-1 is reflected from a denser medium $(∆\varphi =\pi )$ while ray-2 comes after reflecting from a rarer medium $(∆\varphi =0°)$

                                        $$\begin{gathered} ∆x=2\mu t \\ =\left(2n-1\right)\frac{\lambda }{2} \end{gathered}$$for maximum intensity

or                                       $$\begin{gathered} \lambda =\frac{4\mu t}{\left(2n-1\right)} \\ =\frac{4\times 1.5\times 500}{2n-1} \\ =\left(\frac{3000}{2n-1}\right) nm \end{gathered}$$

Substituting $n=1,2,3,\dots \dots etc.$, we get $\lambda =3000 nm,1000 nm, 600nm etc.$

Therefore answer is 600 nm.