Absolute Temperature
The lowest possible temperature possible – at this temperature there is no movement or vibration in the atoms. The temperature is 0 K or -273 C
Acceleration
The rate of change of velocity measured in m/s/s.
Aberration, Astronomical
Is a difference in the position of a star due to the motion of the earth.
Aberration, Chromatic
Is caused by the dispersion of the lens material, the variation of its refractive index n with the wavelength of light. Since the focal length f of a lens is dependent on n, if follows that different wavelengths of light will be focused to different positions. Chromatic aberration of a lens is seen as fringes of color around the image.
Achromatic lens
A lens which is free from chromatic aberration, this produces a image which has no colour fringes. It is created by combining a crown glass lens and a flint glass lens.
Alpha particle
A type of fast ion emitted by many types of heavy radioactive nuclei, such as uranium. Actually, the nucleus (atom stripped of all electrons) of the gas helium.
Astigmatism
Astigmatism is the most common vision problem. It’s caused by an irregularly shaped cornea and results in a mismatched focussing horizontally and vertically.
Astronomical unit (AU)
The mean Sun-Earth distance, 1 AU = 149,600,000 km = 92,957,000 miles.