Is a machine which uses a moving belt to accumulate very high charges on a hollow metal globe. The potential differences achieved in modern Van de Graaff generators can be up to 5 megavolts; applications for these exist with high voltage X-ray tubes and atom splitting experiments.

Is a derived SI unity of radioactivity. 1 Bq one atom per second will decay. Named after Antoine Henri Becquerel.

The lowest possible temperature possible – at this temperature there is no movement or vibration in the atoms. The temperature is 0 K or -273C

The rate of change of velocity measured in m/s/s.

Is a difference in the position of a star due to the motion of the earth.

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.

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.

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 is the most common vision problem. It’s caused by an irregularly shaped cornea and results in a mismatched focussing horizontally and vertically.

The mean Sun-Earth distance, 1 AU = 149,600,000 km = 92,957,000 miles.