point for the lens. Measuring the optical bench distance from lens to focal point gives the focal length. Compare this value to the value you obtained in part C, when you simply imaged a “distant” object on an index card. Which method is more accurate? Which method is easier?
Replace the positive lens in Figure L-2 with the negative lens. The challenge now is greater since the two laser beams diverge on the right side of the lens—and do not form a real image anywhere. Can you design a method to locate the spots of the two parallel beams at the lens and the spots for the two diverging beams on the right of the lens, then “trace” your way back to locate the focal point on the left side of the lens? If you can locate the focal point on the left, you can then measure its distance from the lens to get the focal length of the negative lens.
Student Project (optional)
Design a 10X beam expander using first a combination of two positive lenses and next a combination of a positive and a negative lens. Carefully draw each design to scale. Refer to publications such as the Melles-Griot Catalog or the Edmund Scientific Industrial Optics Catalog to obtain lens diameters, focal lengths, and approximate costs for each beam-expander design (less housing). Test each design on an optical bench and measure the size of the incident and exit beams. Determine how closely each beam expander meets the 10X specification. Is there any reason for choosing one design over the other?
Other Resources (optional)
- The Education Council of the Optical Society of America (OSA) has prepared a discovery kit designed to introduce students to modern optical science and engineering. The kit includes two thin lenses, a Fresnel lens, a mirror, a hologram, an optical illusion slide, a diffraction grating, one meter of optical fiber, two polarizers, four color filters, and instructions for eleven detailed experiments. OSA offers teacher membership opportunities. Contact the Optical Society of America, 2010 Massachusetts Avenue, NW, Washington, D.C.
- K-12 Optics Outreach kit, available from SPIE, Bellingham, Washington.