What happens after refraction, when: (i) a ray of light parallel to the principal axis passes through a concave lens? (ii) a ray of light falls on a convex lens while passing through its principal focus? (iii) a ray of light passes through the optical centre of a convex lens?

After refraction, the path of the light ray changes according to the lens type and the angle of incidence. (i) A concave lens spreads out the light rays, so a ray parallel to the principal axis will pass through the principal focus after refraction. (ii) A convex lens converges the light rays, so a ray passing through the principal focus will emerge parallel to the principal axis after refraction. (iii) The optical centre of a convex lens is the point where the lens is thinnest, so a ray passing through this point will pass undeviated after refraction.