In the given diagram, the portion BO of straight stick is immersed in water, but it appears to be bent at point B. The ray OC and OD coming from lower end of stick gets refracted away from normal in direction CX and CY. These rays when produced backwards appear to meet at ‘I’ so the virtual image BI is visible to us.

Thus, when a stick is half immersed in the water, then it appears bent at the surface due to refraction of light when light passes from water to the air.