Lab 8

Optical instruments

Do you wear glasses?  Are you farsighted or nearsighted?  Look through your or someone else's glasses as you move them towards or away from a printed page.  What do you see?

In the lab you will explore image formation.  You will determine the focal length of a thin lens and observe aberrations.  You also examine what a camera records when it looks through 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

image Determine the focal length of a converging lens.

Observe coma.


Experiment 2

Observe the formation of a real image by a converging lens.

Table 1

screen-lamp
distance
measured xo

measured xi

calculated focal length:
f = xoxi/(xo + xi)
inverted?

50 cm (1)        
50 cm (2)        

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


Observation 1

In the video clip below the lens initially projects a focuses real image of the cross onto the screen.  The lens is rotated about its vertical axis and the image becomes blurred.  Play the video clip in full-screen mode.

Describe what you observe as the lens is moved away from the screen.  Identify the aberration.


Observation 2

imageThe 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.  The lens surface act like spherical mirrors.  In the picture on the right you see two images of the cross, one is upright and one is inverted.  To be able to judge if these images appear behind or in front of the lens, watch the video clip below.

 

Describe your observations.
Which surface of the lens acts as the mirror producing the upright image?  Does this image appear behind or in front of the lens, as seen from the side of the lamp?
Which surface of the lens acts as the mirror producing the inverted image?  Does this image appear behind or in front of the lens, as seen from the side of the lamp?


Observation 3

imageA simple Keplerian telescope with a magnifying power of ~3.1 is mounted on the optical rail.
It uses 12.5 cm focal length lens for the objective and the 4 cm focal length lens for the eyepiece. 
The center to center distance between the lenses be ~16.5 cm.

image

The telescope is pointed at a book sitting standing on a cart near a door across the room.  The camera looks through the eyepiece and takes a picture.  The lens of the camera produces a real image at the location of the camera sensor.  The picture on the left is taken with the camera in the same position and the telescope moved to the side.

image      image

Observation 4

A (very poorly designed) simple compound microscope with a tube length g of 16 cm, an objective with focal length fo = 6 cm, and an eyepiece with focal length 12.5 cm is mounted on an optical rail.  16 cm is the most common tube length for laboratory microscopes.  The center to center distance between the lenses be 34.5 cm.  The target is placed ~8.5 cm in front of the objective.  The magnifying power of this microscope ,when the human eye is used as the detector, is MP = -(g/fo)(25 cm/fe) ~ 5.3.  Here the camera is used as a detectot.  The camera lens is positioned close to the eyepiece and moved and forms a reasonably sharp image of the object on the camera sensor.

Schematic and picture of the simple compound microscope.

image

image

Below you see the image of the target produced by the camera lens.  The eyepiece lens is large, there is no actual tube, and the camera lens has a wide acceptance angle, so the camera can "look around" the objective. The picture on the left is taken with the camera in the same position and the microscope moved to the side.

image      image
image      image 

Convert your log into a lab report.

Name:
E-mail address:

Laboratory 8 Report

Save your Word document (your name_lab8.docx), go to Canvas, Assignments, Lab 8, and submit your document.