Venus Tunay BSPT2

Experiment
Part I: The Focal Length of a Converging Lens
For this part, use the large bi-convex lens, an "object" (lamp), and an imaging screen. See Figure 12.1 for set up.
Assuming the thin lens equation (12.1) is satisfied, we may use the optical bench to measure u and v for several images in order to obtain a value for the focal length of the biconvex lens. The sign convention for equation (12.1) is that v is taken to be positive to the right of the lens and negative to the left.
Procedure
Place the object on the optical bench near one end, and place the screen at the opposite end, for example at 0 cm and 100 cm.
Slide the lens along the bench between object and screen and find all (two or less) positions where the lens produces a sharp image on the screen.
For each position of the lens, record u and v in a table and describe the image using the terms real or virtual, erect or inverted, and magnified or reduced.
Move the screen about 10 cm closer to the object and try to find all images again. Record u and v. and the image attributes.
Repeat, moving the screen closer to the object by 10 cm until there are no more sharp images on the screen. At this point the two images have come together until there is only one. There are no more images for closer separations.
Part II: The Focal Length of a Concave Mirror
For this part use the concave mirror, the lamp, and the imaging screen. The thin lens equation is still obeyed, only u and v are defined as in figure 12.2, where v is taken to be negative behind the mirror.
Figure 12.2
Procedure
Place the mirror at one end of the optical bench as in figure 12.3. It will remain there the whole time. At the opposite end put the lamp.
Figure 12.3
Now slide the screen along the rule and find all the positions where the mirror produces a sharp image on the screen. There may be some difficulty in locating the image because the screen will block the rays from the object. You will have to aim the object slightly to one side (or angle it up) and hold the screen next to the optical bench.
In the table on your lab sheet record the values of u and v and the attributes of the image.
Next move the object 10 cm closer to the mirror, finding again the place(s) where image(s) are formed. Record u and v and the image attributes.
Repeat these observations moving the object each time 10 cm closer to the mirror, until it is no longer possible to form an image on the screen.
Part III: The Compound Microscope
The compound microscope uses two lenses. The one closest to the object is called the objective lens; the one closest to the observer is called the eyepiece.
The objective lens has a small focal length, possibly only a few millimeters. The object is placed slightly beyond the focal point of the objective in such a location as to form an enlarged real image inside the focal point of the eyepiece.
The eyepiece is then usually adjusted to form an enlarged virtual image about 25 cm from the viewer, as 25 cm is the standard value for the "near point" of the eye, or the closest point upon which the eye can focus. By having the image at the near point, its apparent size is maximized. The total magnification M of the microscope is the product of the lateral magnifications of the objective lens m1 and of the eyepiece m2 (i.e., |M| < 1 ="=""> image reduction, if M is positive the image is erect, and if M is negative the image is inverted.),
M = m1 m2. (12.2)
If the eyepiece forms a virtual image 25 cm behind the eyepiece, the image distance v is equal to -25 cm. We then may use the thin lens equation to write the magnification as
, (12.3)
where fe = 20cm is the focal length of the eyepiece.
We will use the above equation for m2 along with a measured value for m1, to investigate the total magnification M of the microscope.

Procedure
Construct a compound microscope on the optical bench as in figure 12.3 by combining the short focal length bi-convex lens with the large bi-convex lens used in the first section.
Figure 12.3
Tape a resistor to the side of the imaging screen and illuminate it with a desk lamp. Adjust the positions of the two lenses so that you obtain the best magnified focused image.
Holding the ruler between your eye and the eyepiece, make a rough measurement of the apparent size of the object.
Keeping the distance between the ruler and the object(and your eye) the same, remove the object from the microscope and estimate its actual size (as in Figure 12.4). The ratio of these sizes gives an estimate of the magnification M for the lens system.