(a) With the help of a labelled diagram, explain why a tank full of water appears less deep than it actually is.
(b) Name the phenomenon due to which a pencil partly immersed in water and held obliquely appears to be bent at the water surface.

(a) Refraction of light causes a tank full of water to appear less deep than it actually is.
Suppose, we have a tank of water as shown in the following figure.


Here, we take a point O at the bottom of the tank. We see this point because of the light rays coming from it. Now, a light ray OA from point O passes through water and enters air at point A. It gets refracted away from the normal in the direction AX. Similarly, another ray OB gets refracted at point B and bends away from the normal in the direction BY. These two refracted rays, i.e., AX and BY, meet at point I on producing them backwards. This point I (nearer to the water surface than O) is the image of point O.
Therefore, point O appears to be nearer at position I. Similarly, this can be applied to all points on the bottom of the tank. This causes the tank to appear less deep than it actually is.

(b) It is the phenomenon of refraction due to which a pencil partly immersed in water and held obliquely appears to be bent at the water surface.