Question

a real image 4/5 size of the object is formed 18cm from the lens calculate the focal length

Answer 1

Utilize the equation, 1/V-1/U=1/f, Where V=distance of the picture from the lens(negative esteem), U=distance of the protest from the focal point.

utilize the general recipe for focal points. 1/f=1/u+1/v where, f=focal length u=distance of protest from focal point, v=distance of picture from focal point, now utilizing general focal point equation, 1/f=1/u+1/v, 1/f=1/22.5+1/18, 1/f=0.1, f= 10 cm.