What is the distance between the convex lens of focal length 20 cm and the object ....

Dear Student

Placing a mirror as mentioned in the problem is eqivilent of  two lences of same focal length sepatrated by twice the distance between mirror and convex lence.

If the distance between mirror and convex lence = d

so the distance ebtween the lences = 2d.

Say tha focal length of given lence = f

Conbined focal length =F

1/ F =  1/f1   +  1/f2    -  D/f1f2   =  1/f   +1/f   -   2d /  f²   =   2(f-d) / f² 

====>    F = f² /  2(f-d)

 The distance between the 2nd lence and focal point is called back Focal Length.(BFL)

BFL=  f2(D-f1)  /  (D- (f1+f2)     =  f(2d-f)/ (2d-2f)

 Image is real,  size of the image same as object

 

 

You can plug in the numbers. 

 

Regards

Team

Topperlearning