Question

Two lenses of power 2.5 D and -3.5 D are placed in contact. If an object be placed at a distance of 1.2

m from the lens combination where is the image formed?​

Answer 1

Answer:

If we assume them to be thin lenses, we can use the normal Gullstrand formula for the combined power

p12=p1+p2−p1p2Dn

As there is no refractive medium between the two lenses (other than between the cornea and the eye lens for which Gullstrand derived the formula back around 1910), we can set n=1. Adding the other values we get (using 0.2meters instead of 20cm since diopters are in units of meters, 1D=1/m)

p12=5+2−10⋅0.2=5D

Thus the combined focal length is 1/5m or 20cm.