Why is the contribution of the wavelets lying on back of secondary wavefront zero?

Why is the rate of assimilation energy at the herbivore level called secondary productivity?

A plane wave front AB propagating from denser medium (1)into a rarer medium (2) is incident on the surface P_1P_2 separating the two media as shown in the figure. Using Huygen's principle, draw the secondary wavelets and obtain the refracted wave front in the diagram.

Why is zero vector needed?

The magnetic field at the point near the centre but outside a current carrying solenoid is zero. Why?

Why tirtary carbocations are more stable than, secondary and primary carbocations? Explain.

Why primary cảrbanions are more stable than the secondary and tertiary carbanions ?