how line following machine works

AS YOU CAN SEE IN IMAGE HOW WHELLS REACT ACCORDING TO SITUATIONS 

how sensors work :

As we know that black colour absorbs light similarly as we don't able to see the in dark. This concept works in sensors 

There are two bulb shaped projections at edges of the sensor. One of them is transparent l.e.d and another is black to absorb rays from led. 

When line following machine starts moving transparent led release some rays. 

1) if surface is non black or white it reflects rays again to black bulb and wheels continue moving. 

2) if surface is black rays from led light got already absorbed by surface so they don't reach to black bulb and wheels stop moving. 

Line following machine moves on this concept