Explain with the help of diagram why a pencil partly immersed in water appears to be bent at the water surface.

It is due to the refraction of light.

Consider a pencil AB. Such that CB portion of the pencil is immersed in water as shown in the figure. Rays of light from the tip (B) of the pencil bend away from the normal as they go from water to air. The refracted appear to come from point B’. Thus the immersed portion of a pencil in water appears as CB’. Hence, the pencil appears to bent at the water surface. Moreover, the immersed portion of the pencil in water also appears to be shorter than the actual length of the pencil.