Physics Glossary – O

Ockham’s Razor

A premise in the philosophy of science due to William of Ockham which states that “entities should not be multiplied unnecessarily.” This is commonly interpreted to mean that the simplest of a set of competing viable theories is preferable.

Olber’s Paradox

If the universe has an infinite amount of stars then the sky should be infinitely bright. But many stars have been born, lived, and died already. Basically at any one time there are not enough stars active to fill the space of the Universe with enough radiation to light the night sky so therefore the sky is dark at night.

Optical Activity

A material is optically active if it rotates the plane of polarisation of any light transmitted through it. It is a property found in sugars and some other naturally occurring compounds and was first observed by the French physicist D. Arago, in 1811. There are two types of optically active materials; those that rotate the plane of polarization to the right (clockwise as one looks along the beam of light) are called dextrorotatory (right-handed), and those that rotate the plane to the left (counterclockwise) are called levorotatory (left-handed). A device for measuring optical activity is called a polarimeter.

Oort Cloud

The Oort cloud is a postulated cloud of comets situated about 50,000 to 100,000 AU from the sun. Although no direct observations have been made of such a cloud, it is believed to be the source of most or all comets entering the inner solar system (some short-period comets may come from the Kuiper belt), based on observations of the orbits of comets.


The oscilloscope is an electronic instrument widely used in making electrical measurements. The main component of the oscilloscope is the cathode ray tube (CRT). The CRT is a vacuum tube in which electrons are accelerated and deflected under the influence of electric field. The electrons are deflected in various directions by two sets of plate placed at right angle to each other in the neck of the tube. Signal for the horizontal deflection plate (X-axis) is generated by the scope.

Order of Magnitude

An estimate of the size of something expressed to the nearest power of ten for example the radius of atom is 10-15, this is an estimate.

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