While light is incident on a glass plate of thickness 5000 Å and refractive index 1.5 the wavelength in the visible region (4000 Å - 7000 Å) that are strongly reflected by the plate is

Both (A) & (B)

Light of wavelength λ is strongly reflected if $2μt = (2n + 1)λ/2$

If λ = 4000 Å . . . (i)

solving it for n we get n = 3.25 similarly for λ = 7000 Å

we get n = 1.66

Hence, within 4000 Å and 7000 Å the integer can take value 2 and 3 from equation (i) $λ = 4μt/ (2n+1)$

Substituting n = 3 and n = 4 we get

λ = 6000 Å and 4290 Å.

So these two wavelengths are reinforced.