Backward v/s Forward Ray Tracing

We use the technique of backward ray tracing to render our objects.

What is Backward Ray Tracing and why we use backward ray tracing technique?

For This it is important to understand what we mean by forward ray tracing;

In real life we see objects only when light is present otherwise we can't see anything. This is through forward ray tracing. In forward ray tracing ray start from light then hit the surface and enter our eye as shown in the picture above (see the arrow sign). According to physics the color that we see of the object is one that object reflect. Object absorb some colors and reflect another color.

In Backward Ray Tracing, what we implement in our ray tracer, is opposite to forward ray tracing. In backward ray tracing we shoot a ray from our eye (camera) toward object and then render the color of object by shooting the ray from the intersection point on the object toward light

The problem with the implementation of forward ray tracing is that the scene contain many objects and photon ray (light ray) shot from the source hit many surfaces (directly or indirectly) before entering our eye which can be very difficult and tedious to render.