Question

reason for internal reflection in ater droplet to form rainbow?????????

Answer 1

the refracting index of  water = 1.33.  Perhaps with other particles and chemicals mixed in water, it may be a little higher or lower.

The critical angle for water to air interface is : Sin⁻¹ 1/1.33 = 48.6 deg.  When Sun rays from distant Sun in the morning or late in the afternoon, fall on water drops with angle of incidence around 40 deg. Then they get refracted at 28 deg, nearly. Then they fall on the other spherical surface of the drop at the other end, the angle of incidence gets more than 48.6 degrees.   Hence, they get totally internally reflected.  A little dispersion occurs already at the first refraction.

After the total internal reflection, the colors disperse a lot when refracting a second time.