Question

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

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

Apparatus: An optical track with two lenses (spherical and cylindrical); a ruler; and a lamp-house with a coarse wire mesh on it. 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 combination until an

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

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

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

combination until a second appropriately sharp real image is formed. Again,

measure object distance and image distance. From the object and image

distances, calculate the focal length corresponding to each image. 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 image and object

distances.

Answer 1

procedure:

step 1:

collect the focal lengths of the two lenses from the instructor.

step 2:

combine the two lenses together and put them on the track.

step 3:

move the combination till a sharp and distinguishable image is formed.

step 4:

calculate object distance and the image distance

object distance=distance of the object from the lens combination

image distance=distance of the screen from the lens combination

note these two down in your notebook.

step 5:

move the screen till a sharp image is formed.

and repeat step 4.

calculation:

let object distance be u and image distance be v. (magnitude only, no sign)

if focal length is f, then for the given case, lens equation will be written as

(1/v)+(1/u)=1/f

step 1:

for the recorded values, calculate f for the combined les using above formula.

step 2:

theoretically, for 2 lenses with focal length f1 and f2, combined focal length f is given as

1/f=(1/f1)+(1/f2)

step 3:

compare experimental value from step1. and theoretical value from step 2.