achintya shantanu

When we project a mobile screen in front of a wall , we only see the white light but R unable to see the written matter on it why ? If this question seems to be undigesting , please try this for once n think.

Actually to explain the energy propagated by a wave , ther is a proposed obliqiuty factor ,1+cos(theta)=q . In the case when the backward wavefront is taken the angle becomes 180degrees and as a result q= 0 , it means no energy propagation in the backward...