Notes on Geometric Optics Lab

General

Each of the named items in the lab has a letter or number code which is indicated on the front sheet of the lab manual.  All of the numbered items are small items stored in the case.There is one item that does not have a code: the 'special' component holder, which is shorter than all of the other component holders.  It is  designed to sit on the rotating platform so that it is at the same level as the other component holders which sit on the optical bench.

Be able to assess the accuracy and most probable sources of errors in each section, but you do not need to perform detailed error analysis for this lab.

I. Angles of Incidence and Reflection:

Verify that the angle of incidence equals the angle of reflection. At first your results may be very off---you have to be really careful to line up the mirror surface itself with the marks on the rotating platform, not the mirror holder.  This is a little tricky to do, but you can tell when you've got it right because you know what the reflected angle should be, and you should be able to do this with an accuracy of a degree or so.

Do not worry about the vertical alignment (7).

II. Reflections from Glass and Acrylic:

Similar to (I).  You will probably find it easier this time to line up the front of the glass and acrylic precisely with the lines on the rotating platform.

Again, do not worry about the vertical alignment (5).

III. Index of Refraction

Note that it's the back of the glass plate that should be lined up precisely on the rotating platform.  It's not stated in the manual, but for this to work at all, you have to use a vertical line on the graph paper which is essentially at the center of the rotating platform, so that its position doesn't move at all when you rotate the platform back and forth.  If light were not refracted by the glass, then the shadow edge which you arrange to fall on the line would still fall on it no matter what angle the platform was rotated to.

The diagram shown in figure 2 is also incorrect, because the d shown is not the displacement of the shadow edge.  d should measure how far the shadow has moved from where it should have been if refraction did not take place; therefore, it should look like the d in figure 3, except measured along the back surface of the glass. I've tried to illustrate both the correct definition of d and the positioning of the glass plate in the below figure:

IV. Critical Angles

This derivation is not simple.  Try assuming that the critical point occurs when the light goes symmetrically through the prism, that is, the interior angles of the light with respect to the normal at each surface are equal.  This determines those angles uniquely.

Do not worry about parts (6)-(8).

Lenses

NOTE: The formula for magnification should be m=s'/s !

First Focal Length

Try calculating the focal length for each of the lenses--you'll find this method doesn't work very well for the longer focal lengths, and you should see why.  Use the technique in (7) to calculate the negative focal length of the concave lens.

Second Focal Length

You can do this rather quickly and qualitatively. Try using the 48 mm f.l. convex lens 'near' the light source. You can quickly slide the screen along the length of the optical bench to test for stability of the image. A few adjustments of the lens position should suffice to find the value that keeps the image size quite constant, regardless of screen position.

"Improved Method" and "Magnification"

Do both of these sections.