Topics: light that does not pass.

The length of a shadow tells something about the angle between the light and the object producing the shadow. The Sun is at a fairly low, but not extremely low angle for the people of the beach, for example. For a dramatic illustration of this effect, see the National Geographic cover picture of camels in the desert:
http://www.georgesteinmetz.com/

More than two thousand years ago, Eratosthenes, a Greek who lived in Alexandria, used this principle to deduce the radius of the Earth to within a few percent. He knew that at noon on the summer solstice in the town of Syene the Sun was directly overhead and cast no shadow, whereas in Alexandria it did. The difference can be explained if the Earth is round, and the radius of Earth can be computed from the length of the shadow cast by a pole of known length, and the distance between Alexandria and Syene.
http://keyah.asu.edu/lessons/Eratosthenes/KM4.html

The nature of a shadow depends on the nature of the object and the light that it is blocking. For example, our shadow looks different in visible light as compared to X-ray light. Bone is denser than muscle tissue and skin, so it stops more of the X-rays. Fewer X-rays make it to the region of the film that's behind the bone, creating an X-ray shadow image.
http://missionscience.nasa.gov/ems/11_xrays.html

In a similar way, astronomers use distant X-ray sources as tools to "see inside" dark clouds and learn about the elements in those clouds.
http://chandra.harvard.edu/press/10_releases/press_051110.html

Seeing around corners - Diffraction.

The wave-like nature of light affects the way that light casts shadow. For example, it allows light to bend around corners and cause a blur at the edge of the shadow. This effect, called diffraction, can produce unwanted effects, such as the so-called diffraction spikes seen in many optical astronomical images. These spikes are caused by the bending of light around the support structure for the secondary mirror in a reflecting telescope.

Diffraction depends on the wavelength of light, with long wavelengths being diffracted more than short ones. This effect can be seen in "rainbow shadows," and has many practical applications, such as diffraction gratings used to study the structure of molecules, and holograms.

One of the scientific instruments aboard Chandra uses diffraction gratings to produce rainbow shadows on X-rays. The intensity of each X-ray "color" or energy provides information on the X-ray spectrum of the source, such as the elements present in a hot gas cloud produced by a supernova explosion, or around a black hole.
http://chandra.harvard.edu/about/science_instruments.html
http://chandra.harvard.edu/photo/2001/xtej1118/index.html

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