Purpose
Measure the focal point of a convex lens, and observe the positions and characteristics of images produced my convex lenses.
Concept
Light rays that that strike a convex lens, parallel to the principal axis, pass through
the focal point (f). The focal length of a lens depends on the both the shape and the
index of refraction of the material. The farther objects are away from the lens,
the closer the image appears to the focal point.
Materials
2 meter sticks, convex lens, lens holder, index card (image screen), card holder, light source.
Procedure Part A
1. Set up the apparatus.
2. Point the lens at an object that is at least 6 m away from the lens.
3. In a darkened room, adjust the screen by moving back and forth until you see a clear image on the screen.
4. Measure the distance from the lens to the image. Record this as the focal length.
Part B
1. Place the light source 2 times the focal length from the lens.
2. Adjust the screen until you see a clear image of the light source.
3. Measure the distance from the lens to the image and record.
4. Observe and describe the orientation and size of the image.
Part C
1. Repeat part B with the light source 1.5 times the focal length from the lens.
Analysis
1. Using the object and image distances from part B and C, calculate the focal length of the lens using the lens equation.
2. Does your calculation agree with your measurement in part A? Explain.
3. As the object distance increases, what happens to the size of the image? Explain.
