Converging and Diverging Lenses
Obj: To examine the features of convex lens and use the lens equation.
Materials: optical bench, convex lens, diverging lens, 15W bulb
Convex (Converging, positive) lens
Part I. Approximating the focal length (+f)
1. Set your lens on the print of this lab paper, then pull back until the print blurs. Measure this distance.
2. Aim the optical bench towards the window such that light from across the courtyard focuses on the screen. Measure and record this distance in as the approximate focal length, f, and compare above.
3. Check your lens for chromatic aberration. Define chromatic aberration.
Part II. Three images and the lens equation
1. The room should be dark for this part. Draw a caricature face on the 15 W light bulb. Use the face on the light bulb as the object. Arrange the lens, screen, and object on the meter stick as discussed such that you obtain the following three conditions.
For each condition listed below (L)
(a) measure and record the object (do) and image distances (di) from the lens.
(b) Use do and di values to calculate the focal length of the lens using the lens equation.
(c) Determine the magnification factor from your measurements.
Condition 1: a real, enlarged image.
Condition 2: a real image, exactly equal in size to the object.
Condition 3: a real image smaller than the object.
Concave Lens (diverging, negative) lens
Part III. Determining the focal length (-f)
1. Examine a concave lens. Describe the image of any object at close and long range.
2. Set up the convex lens to produce an image on the screen approximately 20 cm from the lens and record the position on the meter stick of this image. This image will now be used as a virtual object for the concave lens.
3. Place the concave lens between the convex lens and the screen and then move the screen back 10 to 12 cm.
4. By adjusting the position of the concave lens you should get an image on the screen. [DO NOT ADJUST THE POSITION OF THE OBJECT OR THE CONVEX LENS.]
5. Record the position of the concave lens and the position of the screen as +di. The virtual object distance do is negative. Calculate the focal length of the concave lens using the lens equation. The object and image distances are measured from the concave lens. Be careful of ± signs with images and objects.
Part IV Conjugate Points
1. Place the illuminated object at the zero position of the optical bench.
2. Arrange the screen at a distance of about 5f. Record the screen position.
3. Keeping the distance between the object and the screen fixed (D), find the two positions of the lens that give sharply defined images. Record these two positions. The difference between these two positions will be the distance, d. The focal length can found using the equation
f = (D2 - d2) / 4D
4. Compare with previously calculated focal lengths.
1. Describe the conditions when a convex lens acts as a magnifying glass? Read this with your lenses: Physics is my favorite course.
2. Name two ways to determine the focal length of a convex lens without using the lens equation.
3. Describe the conditions for each type of lens for forming a virtual image.
4. Define chromatic aberration and spherical aberration. What is astigmatism?
5. Which lens type will correct for nearsightedness? Farsightedness?
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