Problem of the Week: Basic Sciences Problems: Light and Wave Propagation

Electronic distance measurements (EDMs) rely on an accurate estimation of the speed of light through the atmosphere. The ratio between the speed of light in a vacuum and the speed of light in the atmosphere is known as the refractive index of air. Derivation of the correct refractive index of air depends on knowing the wavelength under consideration, the temperature, the humidity, and which of the following?

A. Absorption
B. Atmospheric Pressure
C. Wind Speed
D. Emissivity

Solution: The speed of light in a vacuum is approximately 299,792,458 m/s. However, in the earth's atmosphere at sea level, the speed of light is approximately 299,702,532 m/s -- a difference of about 300 ppm. This speed is affected by the wavelength of the light, and the temperature, humidity, and pressure of the atmosphere. An electronic distance measurement (EDM) is usually standardized at a specific refractive index, usually about 12 degrees C and 1013.25 mbar. It is said that a change of approximately 1 degree , or 3.5 mbar, from the standard will result in approximately 1 ppm error.

The answer is B.

This is problem 2 (17-1) from the NEW third edition of “Surveying Solved Problems for the FS and PS Exams” by Jan Van Sickle, PLS (formerly "1001 Solved Surveying Fundamentals Problems"). Reprinted with permission from “Surveying Solved Problems for the FS and PS Exams” by Jan Van Sickle, PLS (2008 Professional Publications Inc.). For details on this and other FLS exam-prep books, call 800/426-1178 or visit