When the convex side of the concavo–convex lens is coated with silver, the combination becomes a mirror. This combination consist two lenses and one mirror placed close to each other. The powers of all the three will be added. When the water is filled on concave side, we get plano-convex water lens whose focal length is found by lens makers formula. This combination consists of four lenses (two lenses of glass and two lenses of water) and one mirror.
(a) The refraction takes place from the first surface, reflection from the lower surface and finally refraction from the first surface of focal lengths fg , fm and fg , respectively. The combined focal length F is given by
For the image to be informed at the same point as the object
u = 2F = 2 x 7.5 = 15 cm.
The object should be placed at a distance of 15 cm from the lens on the optical axis.
(b) If fw is the focal length of lens in water, the focal length F’ of this combination is given by