In this experiment you will observe and measure the imaging properties of a simple lens.
You will need a good lens (magnifying glass), a flashlight, a viewing screen (tri-folded white copy paper), a meter stick and perhaps some modeling clay to hold things in place. You will want to be able to set these items on a flat table about 1 meter wide in an area where the lighting can be dimmed.
The most basic optical imaging device is a simple lens like those found in a magnifying glass. These lenses have spherical surfaces and depend on the law of refraction for their image-forming properties. For the level of treatment in this experiment, the imaging properties of a lens is completely specified by giving the lens' focal length. The lens in a good magnifying glass will have a focal length typically around 5-15 cm.
The imaging capabilities of your lens can be demonstrated very nicely in a dark room with the picture from a TV set. Position the lens and screen about 2-3 meters from the TV. Adjust the distance between the lens and the screen until you get as sharp a picture as possible on the screen.
In the jargon of optics, the TV in this part of the experiment is called the object. The distance from the lens to the TV is called the object distance and given the symbol o. The picture on the screen is called the image. The distance from the lens to the image is called the image distance and given the symbol i. The focal length is given the symbol f.
Measure the image distance and record this on the data sheet.
Move the lens and screen an additional meter from the TV and repeat the measurements of the object and image distances recording your results on the data sheet.
When the object distance is much much greater than the image distance, the image distance will be approximately the focal length of the lens. Thus, the image distances measured above are approximately the focal length of the lens.
Cut out a circular piece of opaque paper (black construction paper can be used) roughly the size of the face of the flashlight. Then fold the circle in half and cut out a wedge shape along the fold so that when the circle is unfolded the cutout will be in the shape of a long pointed triangle that is 1 cm from the pointed tip to the base. Put some tissue paper backing over the triangular hole and tape this (tissue side down) to the face of the flashlight. When the flashlight is turned on the brightly lit white triangle should make a good object suitable for image formation. For the rest of this experiment the triangle will be used as the object.
Begin by setting the object distance accurately to 20 cm. Adjust the position of the screen (image distance) to get a sharp image of the edges of the triangle. Record the object and image distances on the data sheet. Also measure and record the size of the image of the triangle (tip to base).
Repeat this procedure as you adjust the distance between the lens and flashlight (object distance) from 20 cm to about 70 cm in steps of 10 cm. Each time you will have to move the screen to reform the image of the triangle.
Set the screen 50 cm from the lens (i=50 cm). This time, adjust the flashlight (object distance) to get a sharp image. Record again the object and image distances and the size of the image.
Try to draw a smooth curve through the data points. Remember, no measurements are ever perfect, so it is OK to let the curve miss the data points by a little.
Hint: The theory predicts that 1/o + 1/i = 1/f. Thus, if you construct (from your measured o's and i's) new variables x and y defined by x = 1/o and y = 1/i, the equation 1/o+ 1/i = 1/f can be rewritten y = -1 x + 1/f. This is the equation of a straight line of slope -1 and y-intercept 1/f. Plotting y on the vertical axis vs. x on the horizontal axis, the data points should lie on a straight line with the predicted slope and y-intercept.
If you like, take additional data and add these points to fill in your graph more completely.
Draw the best straight line through your data points using a straight edge. Determine the slope and y-intercept of this line and record these on the data sheet.
Last modified: April 7, 1995Robert DeSerio / firstname.lastname@example.org