Answer to Question #261037 in Optics for maheshi

Question

An object is placed 25 cm in front of the convex lens whose focal length is 10 cm. (a) Find the image distance and where it is located Now a concave lens is placed 6 cm behind the convex lens and a new image is formed 20 cm behind the convex lens. Draw a rough diagram and indicate the length measurements (b) Calculate the focal length of the concave lens (hint: the image formed by the convex lens will be the object to the concave lens)

Answer 1

 Object distance - $d=25cm$ , Focal length - $F=10cm$

a) from lens formula for convex $\large\frac{1}{d} + \large\frac{1}{f} =\frac{1}{F} \to f = \frac{dF}{d-F} = \frac{25*10}{25-10}=\frac{50}{3}=$ $16.66cm$

it is located right side of the convex lens

b) $d = 20cm -6cm =14cm$ $f = \large\frac{50}{3}-6 = \frac{32}{3}$

from lens formula for concave $\large\frac{1}{d} - \large\frac{1}{f}=-\frac{1}{F}\to F = \frac{df}{d-f}= \frac{3*14*32}{3(42-32)}=$ $44.8cm$