Refraction

Refraction is the deflection or bending of electromagnetic waves when they pass from one kind of transparent medium into another. The index of refraction of a material is the ratio of the speed of light in a vacuum to the speed of light in the material. Electromagnetic waves passing from one medium into another of a differing index of refraction will be bent in their direction of travel. In 1621, Dutch physicist Willebrord Snell (1591-1626), determined the angular relationships of light passing from one transparent medium to another.

Air and glass have different indices of refraction. Therefore, the path of electromagnetic waves moving from air to glass at an angle will be bent toward the perpendicular as they travel into the glass. Likewise, the path will be bent to the same extent away from the perpendicular when they exit the other side of glass (Figure 1).

Refraction is responsible for many useful devices which bend light in carefully determined ways, from eyeglasses to refracting telescope lenses.

Refraction can cause illusions. This pencil, in Figure 2, appears to be discontinuous at the boundary of air and water. Spacecraft may appear to be in different locations in the sky than they really are.

Electromagnetic waves entering Earth's atmosphere from space are bent by refraction. Atmospheric refraction is greatest for signals near the horizon where they come in at the lowest angle. The apparent altitude of the signal source can be on the order of half a degree higher than its true height. As Earth rotates and the object gains altitude, the refraction effect reduces, becoming zero at the zenith (directly overhead). Refraction's effect on the Sun adds about 5 minutes of time to the daylight at equatorial latitudes, since it appears higher in the sky than it actually is.

If the signal from a spacecraft goes through the atmosphere of another planet, the signals leaving the spacecraft will be bent by the atmosphere of that planet. This bending will cause the apparent occultation, that is, going behind the planet, to occur later than otherwise expected, and to exit from occultation prior to when otherwise expected. Ground processing of the received signals reveals the extent of atmospheric bending, and also of absorption at specific frequencies and other modifications. These provide a basis for inferring the composition and structure of a planet's atmosphere.

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Paulo Nuin: Climategate: How vital to our understanding of climate change are the data from the United Kingdom?

Paulo Nuin
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That's the real question, isn't it? - Mr. Gunn

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Afghanistan, Vietnam and the Question of Expansion [The Primate Diaries]



Bill Moyer's Journal - LBJ's Road to War, Part 2
November 20, 2009

Part 1 / Part 2

It is interesting to note that the suggestion I made earlier about creating "shovel ready" projects in Afghanistan was one of the key approaches that Johnson originally considered but was unable to adopt forty years ago. The reference made to a "Vietnamese New Deal" as a means to end the civil war was made impossible thanks to the US support for a series of corrupt client regimes in South Vietnam. Today, the Hamid Karzai government has made Afghanistan "the fifth most corrupt country in the world" according to the United States Agency for International Development. As they concluded in their March, 2009 report "Assessment of Corruption in Afghanistan" (pdf here):

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Encephalon-79: Call for submissions

The Mouse Trap will be hosting Encephalon, the premier brain and mind carnival for all things related to psychology/neuosceince on 7th December i.e. upcoming Monday. Do send in your submissions to encephalon[DOT]host[AT]gmail[DOT]com or directly to me using contact form/mail to editor[AT]the-mouse-trap[DOT]com.
Looking forward to your rocking submissions to make a memorable 2009 year-end encephalon carnival that [...]

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