Until the 1600s, scientists believed light could travel any distant instantaneously, but Galileo disproved that theory when he conducted a simple experiment that revealed light travels at a specific speed. Students can replicate this famous lantern experiment with a couple of high-powered flashlights. Partners will need to go out at night and stand one mile from each other. The first student will turn on his flashlight briefly, then extinguish it. As soon as the partner sees the light, the second flashlight is illuminated briefly. Repeat the process nine times. Although light travels too quickly to measure, the experiment should reveal that time ever so briefly does elapse before it reaches a given point.
Advanced physics students will observe light speed when they design their own light signal detector. Using an He-Ne laser, detector module, beam splitter and mirrors, students split light into multiple travel paths and calculate the speed at which light travels by comparing the distance of each path and its completion time to the other paths. Students will gain familiarity with critical formulas as they compare signals received by the oscilloscope and derive the velocity of each light path from that information.
Although light travels at a constant rate through the vacuum of space, traveling through substances on Earth often slows its path as it exerts work to overcome the resistance of the object through which it passes. This phenomena is visibly observed and measurable in the form of refraction. Students should prepare square plastic food storage containers with tap water, ice, rubbing alcohol and clear gelatin. The angle of incidence should be calculated prior to shining a laser through each substance perpendicular to its surface. Measure the angle of refraction in each container and use Snell's law to calculate the index of refraction to determine the speed of light in each medium.
Chocolate and marshmallows play an essential role in a simple home experiment that enables students to calculate the speed of light. Fill a large microwave-safe tray with a single layer of marshmallows. Remove your microwave's turntable and place the mashmallow tray on high for 30 seconds. Identify the marshmallow valleys and hills formed by the uneven heating of the microwave as the tray heats up. Immediately remove the tray from the microwave and set a chocolate chip on each marshmallow peak. Allow the pan to cool before measuring the distance between each chocolate chip from those immediately surrounding it. Find the average of these measurements and convert it to meters. This distance is half a wavelength, so it is necessary to multiply the average by 2,450,000,000 Hz (the speed at which microwaves travel). The final answer should be somewhere near 299,000,000 kilometers per second, which is the approximate speed of light.