Lab - Concave and Convex Lenses

Purpose: investigate positions and characteristics of images produced by curved lenses.

Materials: double convex lens, concave lens, 2 meter sticks and supports, cardboard screen, holders for lens, screen, and light source, light source, metric ruler, He-Ne laser

Procedure: the lab room must be darkened so that images are easily observed.

A.  Focal length of convex lens.

1.  Arrange your lens, meter stick, and screen, as shown. Point the lens at a distant object (more than 10 m away) and move the screen along the meter stick until you obtain a clear, sharp image of object on the screen. Compute the distance 2F. Record this.

B.  Convex lens.

1.  Set up the apparatus. Place the light source somewhere beyond 2F on one side of the lens and place the screen on the opposite side of the lens. Move the screen back and forth until a clear, sharp image is formed on the screen. Record your measurements for ho , di, do, hi and your observations of the image.

2.  Move the light source to 2F. Move the screen back and forth along the meter stick until you obtain a clear, sharp image. Record di, do, hi, and your observations of the image.

3.  Move the light source to a position that is between F and 2F. Move the screen back and forth to obtain a clear, sharp image. Record di, do, hi, and your observations of the image.

4.  Move the light source to a distance F from the lens. Move the screen back and forth to obtain a clear, sharp image. Record di, do, hi, and your observations of the image.

5.  Move the light source to a position between F and the lens. Try to locate an image on the screen. Look through the lens at the light source and observe the image. Record your observations.

C.  Concave lens.

Place the concave lens in the holder. Place a screen on one \side of the lens. One of the following procedures can be used to determine the focal length.

SUNLIGHT: allow the parallel rays of the sun to strike the lens along the principal axis so that an image is formed on the screen. Caution: do not look directly at the sun since this can cause damage to your eyes. The image should appear as a dark circle inside a larger, brighter circle. Place the screen close to the lens. Quickly measure the distance from the lens to the screen and the diameter of the bright circle. Record the data. Move the screen and repeat the measurements for five additional sets of data.

He-Ne LASER: shine a laser beam through the concave lens so that an image is formed on the screen. Caution: do not look directly at the laser since this can cause damage to your eyes. Measure the distance from the lens to the screen and the diameter of the circle of light projected onto the screen.. Record the data. Move the screen and repeat the measurements for five additional sets of data.

Observations and Data

Table 1.

Focal length of mirror, f
2F
Height of light source, ho

Table 2.

Position of object / Beyond 2F / At 2F / Between 2F and F / At F / Between F and lens
do
di
hi
Type
Orientation

Table 3.

Distance from lens / Diameter of screen image

Analysis

1.  Use your observations from Table 2 to summarize the characteristics of images formed by convex lenses in each of the following situations.

A.  The object is located beyond 2F.

B.  The object is located at 2F.

C.  The object is located between 2F and F.

D.  The object is located at the focal point.

E.  The object is located between the focal point and the lens.

2.  For each of the real images you observed, calculate the focal length of the lens using the lens equation. Do your values agree with each other?

3.  Average the values for f found in question 2 and calculate the relative error between the average and the value for f from Table 1.

4.  Plot a graph of the image diameter on the vertical axis versus the distance from the lens on the horizontal axis. Allow room along the horizontal axis for negative distances. The rays expanding from the lens appear to originate from the focal point. Draw a smooth line that best connects the data points and extend the line until it intersects the horizontal axis. The negative distance along the horizontal axis at the intersection represents the value of the focal length. What is the focal length you derived from your graph? If your lens package includes an accepted focal length, calculate the relative error for the focal length you determined.