Consider a two lenses of focal length f1 and f2 arranged coaxially but separated by a distance d can be considered. For a parallel ray that falls on the arrangement, the two lenses produce deviations δ1 and δ2 respectively and The net deviation δ is,
δ = δ1 + δ2 ……. (1)
From Angle of deviation in lens equation, δ = \(\frac{h}{f}\)
δ1= \(\frac{h_1}{f_1}\); δ 2 = \(\frac{h_2}{f_2}\); δ= \(\frac{h_1}{f}\) ..... (2)
The equation (1) becomes,
Substituting the above equation in Equation (3)
The above equation could be used to find the equivalent focal length.
