Hands On Physics


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(For previous installments of the story, look at the introduction and inital investigations ) Thomas Jefferson loved to write about the mild climate of Virginia and its azure skies. Were he to return today, he would notice barely any difference in temperature, rain and snow. We know this because Jefferson kept very detailed daily records of the weather at Monticello from 1810 to 1816. But he would notice a difference in the sky, which today is cloudier and hazier than in Jefferson's time.

When dust, smoke, pollen or tiny droplets of water float in the air, the sky becomes hazy. Outdoor photographers sometimes like haze because it diffuses sunlight, thus softening its glare. Some plants that grow in shady places like haze because it provides light they would otherwise not receive.
But haze also has other effects. Since haze scatters some sunlight back into space, many scientists think that increased haze over the Northern Hemisphere has caused a slight cooling effect. On a hazy summer day, people with light skin can receive a sunburn even when in the shade. This happens when most of the Sun's ultraviolet radiation is received from the hazy sky instead of the direct sun. In some regions haze can be so thick that shadows are faint or even invisible.

Haze also obscures visibility. Over the past few decades, scenic vistas in many regions of the United States have been obscured by haze originating from air pollution created by large power plants and increased automobile traffic.


The TERC VHS-1 Sun photometer detects a narrow range of wavelengths without using a filter [2, 3]. The result is an instrument which costs only a fraction of the price of the most inexpensive filter instruments [3]. The key to the VHS is its use of a common light-emitting diode (LED) as a detector of light [2]. LEDs are designed to make, not detect, light. But it happens that they detect light quite well. Even better, unlike detectors designed specifically to detect light, LEDs detect a relatively narrow band of wavelengths. For example, an LED that emits greenish-yellow light at a peak wavelength of about 555 nanometers (nm) detects green light at a peak wavelength of about 525 nm over a spectral width of about 50 nm.

The main function of the VHS Sun Photometer is measuring haze. But it also has other uses. Here's what you will be able to do with this versatile instrument:

The core activity of this unit is the construction of a Sun Photometer.

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