Worksheet Lensmaker Equation
Refer to the following information for the next two questions.

A converging lens [n = 1.560] has a focal length in air of 40 cm.
 a. What is its focal length when immersed in water [n = 1.33]?

 b. What is its focal length when immersed in methylene iodide [n = 1.74]?

Refer to the following information for the next question.

A converging lens [n = 1.560] has a focal length of 30 cm when immersed in benzene [n = 1.50].
 What is its focal length when it is immersed in water?

Lenses in close combinations

 Find the power and focal length of the following thin lenses when they are placed in close combination: f1 = 80 cm, f2 = −30 cm, f3 = −60 cm.

 Is this combination going to act like a converging or diverging lens?

 What must the focal length of a thin lens be if it is to be placed in close combination with a 2-Diopter lens to form a combination lens that has a focal length of −80 cm?

Challenge Problem

A certain piece of equipment contains a lens, made of glass [n = 1.60], that has a focal length of 40 cm. In order for the equipment to operate correctly this focal length must be strictly adhered to.

Unfortunately that portion of the apparatus containing the lens - which was originally designed to operate in air - must now be submerged in a rectangular vessel of water to cool the surrounding mechanisms. In order for the apparatus to operate properly while under water, a second lens, of the same type of glass, must be placed in the water in close combination with the original lens to bring the lens system back into focus.

 In order to buy this second lens, determine its focal length in air.