Studio Session 9

Optical instruments

Do you wear glasses?  Are you farsighted or nearsighted?  What do you see when you hold your glasses or someone else's glasses far away from your face?  Look through your or another student's glasses as you move them towards or away from a printed page.  Describe what you see.

In the session you will explore image formation.  You will determine the focal length of a thin lens and observe aberrations.  You will then build a simple Keplerian telescope and a simple compound microscope.


Open a Microsoft Word document to keep a log of your procedures, results and discussions.  This log will become your lab report.  Address the points highlighted in blue.  Answer all questions.

Experiment 1

imageDetermine the focal length of a converging lens and observe coma.

imageRotate the lens approximately 20 degrees about the vertical axis in either direction.  Describe what happens to the reflected spot.  You should observe a lens aberration called “coma”.  It is observed when the light rays make large angles with the symmetry axis of spherical mirrors or lenses.

Experiment 2

imageObserve the formation of a real image using a converging lens.

Table 1

measured xo

measured xi

calculated focal length:
f = xoxi/(xo + xi)
measured ho

measured hi

measured magnification:
M = -hi/ho
calculated magnification:
M = -xi/xo
50 cm (1)              
50 cm (2)              
45 cm (1)              
45 cm (2)              

For the measurements of experiment 1 and 2 answer the following questions.

Experiment 3

Your lens is a bi-convex lens. Both sides are curved.  While most of the light from the lamp is transmitted through the lens, some light will be reflected from the front surface, and some from the back surface.

Experiment 4

imageBuild a Keplerian telescope with a magnifying power of ~3.1. 
Use the 12.5 cm focal length lens for the objective and the 4 cm focal length lens for the eyepiece. 
Let the center to center distance between the lenses be ~16.5 cm.

imageBuild your telescope on the optical rail.  (Remove the lamp.)  Make sure the center of both lenses is at the same height.  You can then easily slide the components along the rail without destroying the alignment.  Fix the objective to the rail.  Place your eye approximately 5 cm away from the eyepiece and move the eyepiece back and forth until you can see a sharp image of a distant target.  Look at the most distant target you can find considering the size limitations of the laboratory.

Record in your log:

image Evaluate the performance of your telescope. 

Experiment 5


Convert your log into a session report, certify with you signature that you have actively participated, and hand it to your instructor.