I have a practical for physics lab soon. Can you walk me throughhow to do the following and the equations I would need.

Measure the combined focal lengths of a two-lens combinationconsisting of a spherical and cylindrical lens in contact.Qualitatively compare measured and expected values.

Apparatus: An optical track with two lenses (spherical andcylindrical); a ruler; and a lamp-house with a coarse wire mesh onit. The focal length of each lens will be given to you.

Procedure:

Put the spherical lens and the cylindrical lens together on

the track as close as they can go. Put the screen on the track,at the op-

posite end from the lamp-house. Move the two-lens combinationuntil an

appropriately sharp real image is formed. It may also benecessary to make

adjustments by rotating the cylindrical lens in its holder.Measure object

distance and image distance. Next move the screen closer to thetwo-lens

combination until a second appropriately sharp real image isformed. Again,

measure object distance and image distance. From the object andimage

distances, calculate the focal length corresponding to eachimage. Then, us-

ing the focal lengths of each individual lens as given to you,calculate the

expected focal lengths of the two-lens combination.Qualitatively compare

these expected focal lengths to the ones calculated from imageand object

distances.