Physics

Bending of waves around the edge of an obstacle. When light strikes an opaque body, for instance, a shadow forms on the side of the body that is shielded from the light source. Ordinarily light travels in straight lines through a uniform, transparent medium, but those light waves that just pass the edges of the opaque body are bent, or deflected.

Visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light. The wavelengths of visible light range from about 350 or 400 nm to about 750 or 800 nm.

The study of light and of instruments using light. The subject has a long history. Mirrors were used by the ancient Egyptians c.2000 BC, and Greek philosophers developed simple theories of light (the law of reflection being stated by Euclid in 300 BC).

Return of a wave from a surface that it strikes into the medium through which it has traveled. The general principles governing the reflection of light and sound are similar, for both normally travel in straight lines and both are wave phenomena.

In physics, deflection of a wave on passing obliquely from one transparent medium into a second medium in which its speed is different, as the passage of a light ray from air into glass. Other forms of electromagnetic radiation, in addition to light waves, can be refracted, as can sound waves.

Arrangement or display of light or other form of radiation separated according to wavelength, frequency, energy, or some other property. Beams of charged particles can be separated into a spectrum according to mass in a mass spectrometer.

Phenomena involving both electric and magnetic fields, and the study of such phenomena. The first indication of a link between electricity and magnetism was shown by Hans Christian Ørsted, who demonstrated that an electrical current caused the deflection of a compass needle (1819).

Phenomenon in which the response of a physical system to an external influence depends not only on the present magnitude of that influence but also on the previous history of the system.

Any object that exhibits magnetic properties is called a magnet. Every magnet has two points, or poles, where most of its strength is concentrated; these are designated as a north-seeking pole, or north pole, and a south-seeking pole, or south pole, because a suspended magnet tends to orient itself along a north-south line.

Device for detecting charged particles and other radiation by means of tracks of bubbles left in a chamber filled with liquid hydrogen or other liquefied gas. It was invented in 1952 by Donald Glaser.

property of matter that gives rise to all electrical phenomena (see electricity). The basic unit of charge, usually denoted by e, is that on the proton or the electron; that on the proton is designated as positive (+e) and that on the electron is designated as negative (-e).

Transfer of heat or electricity through a substance, resulting from a difference in temperature between different parts of the substance, in the case of heat, or from a difference in electric potential, in the case of electricity.

All phenomena caused by electric charge. There are two types of electricity: static and current. Electric charge is caused by an excess or deficit of electrons in a substance, and an electric current is the movement of charge through a material.

Abnormally high electrical conductivity of certain substances. The phenomenon was discovered in 1911 by Kamerlingh Onnes, who found that the resistance of mercury dropped suddenly to zero at a temperature of about 4.2 degrees Kelvin.