(a) (i) One dioptre is the power of a lens of focal length one metre.
(ii) Using P = 1/f
-2.0 = 1/f(m)
or f = - 0.5 m
(b) This is due to the phenomenon of refraction of light. A ray of light starting from the lemons kept in water reaches the water - air interface and bends away from normal. To the observer, it appears as the light ray is coming from the point above the actual point. This apparent position makes them appear bigger in size.
(c) The light ray will travel fastest in medium with least refractive index i.e., medium A.