Difference between effective area and actual area

[blueyellow]

it is said that the Total Collisional Cross-Section is the effective
area, normal to the direction of incidence, provided by a
target to an incoming projectile.

what is the difference between effective area and actual area?

 

[fzero]

The actual area is what you'd compute by considering the actual region of space that the target occupies. So for a bare nucleus, this might be on order of ten femtometers. The effective area includes the effects of the fields that the target acts as a source for. An incoming projectile will usually encounter the effects of such fields well before it gets very close to the target, so the effective area for collisions is larger than the actual area.