A gravitational lens refers to a distribution of matter (such as a cluster of galaxies) between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer. This effect is known as gravitational lensing and the amount of bending is one of the predictions of Albert Einstein's general theory of relativity. (Classical physics also predicts bending of light, but only half that of general relativity's.) Although Orest Khvolson (1924) or Frantisek Link (1936) are sometimes credited as being the first ones to discuss the effect in print, the...

The Crookes radiometer, also known as a light mill, consists of an airtight glass bulb, containing a partial vacuum. Inside are a set of vanes which are mounted on a spindle. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. The reason for the rotation was a cause of much scientific debate in the ten years following the invention of the device, but in 1879 the currently accepted explanation for the rotation was published. Today the device is mainly used in physics education as a d...

Wave–particle duality is the concept that every elementary particle or quantic entity may be partly described in terms not only of particles, but also of waves. It expresses the inability of the classical concepts "particle" or "wave" to fully describe the behavior of quantum-scale objects. As Einstein wrote: "It seems as though we must use sometimes the one theory and sometimes the other, while at times we may use either. We are faced with a new kind of difficulty. We have two contradictory pictures of reality; separately neither of them fully explains the phenomena of light, but together they...