Education

Visible light is electromagnetic radiation with wavelengths roughly between 400 nm (violet) and 700 nm (red). Wavelength determines the color we perceive. Every wavelength also has a corresponding frequency — the number of wave cycles per second — related by f = c / λ, where c is the speed of light. Shorter wavelengths mean higher frequencies. Drag the slider to see how color and frequency change across the visible spectrum.

Note: The colors shown may not exactly match the wavelength stated; this demo is intended to illustrate the visible spectrum.

When light crosses the boundary between two media with different refractive indices, it changes direction — a phenomenon called refraction. Snell's Law relates the angle of incidence (θ₁) and the angle of refraction (θ₂) to the refractive indices of the two media: n₁ sin(θ₁) = n₂ sin(θ₂). A higher refractive index means light travels more slowly in that medium and bends toward the normal. When light passes from a denser medium to a less dense one (n₁ > n₂), there exists a critical angle beyond which all light is reflected back — total internal reflection — the principle behind fiber optics and prisms.

A positive lens bends parallel (collimated) rays toward a single point called the focus. The distance from the lens to that point is the focal length. This bending is a direct consequence of Snell's law: as each ray crosses from air into the denser glass, it refracts toward the normal, and when it exits the far side it refracts again. Because the lens surfaces are curved, rays striking farther from the center meet the surface at steeper angles and are bent more strongly, causing all parallel rays to converge at the same focal point.

Note: this diagram uses a paraxial approximation and ignores aberrations such as spherical aberration and chromatic aberration.

Light enters through a Schmidt corrector plate (which corrects spherical aberration), hits the primary mirror at the back, reflects to the secondary mirror, then passes through a hole in the primary behind the tube.

Astronomers locate objects on the celestial sphere — an imaginary sphere surrounding Earth — using two coordinates. Declination (DEC) works like latitude: it measures degrees north (+) or south (−) of the celestial equator. Right Ascension (RA) works like longitude but is measured in hours (0 h – 24 h), counting eastward from the vernal equinox — where the Sun crosses the equator each March. Hours are used because Earth's rotation makes the sky appear to turn once every 24 hours, so 1 h of RA = 15° of arc.

The gold arc shows RA measured along the equator from the vernal equinox; the magenta arc shows DEC measured from the equator along the meridian. Planet positions computed via JPL Horizons for Tucson, AZ (32.22° N, 110.97° W) on 2026-May-05 at 12:00 MST (19:00 UTC) (Design Day!!!).

Polar alignment means pointing an equatorial mount's rotation axis at Earth's rotational pole (near Polaris in the north, or near Sigma Octantis in the south). When that axis is lined up correctly, the mount can track the sky using mostly one motion in Right Ascension (RA), matching Earth's rotation.

This is especially important for long-exposure imaging and time-lapses. Poor polar alignment causes stars to drift and rotate across the frame (field rotation), which leads to streaked stars, soft detail, and jumpy playback over time.

Test your knowledge of the night sky! Can you identify these famous constellations? Each pattern represents actual star positions.