Essential Definitions in Physics, Optics, and Cosmology
Fundamental Concepts in Physics
Sound and Wave Propagation
Speed of Sound (Sound Velocity)
The sound velocity is the speed of propagation of sound waves, a type of longitudinal mechanical wave produced by pressure variations of the medium. These pressure variations (captured by the human ear) contribute to the brain’s perception of sound. Sound does not travel through a vacuum because there are no atoms through which the waves can pass. (330 km / s)
Sound
The sensation produced in the ear by the vibration of bodies, which are propagated through elastic media, such as air.
Echo
The repetition of a sound due to the collision of sound waves with an obstacle and their subsequent reflection. The reflected sound is often perceived at a distance, making it weak and sometimes confused.
Light and Optics
Light
Radiation emitted by bodies heated to high temperatures or by excited bodies, which is perceived by the eyes. Light makes physical objects visible. Its speed of propagation in a vacuum is 299,792.5 km/s.
Optical Instruments
- Microscope: An optical instrument consisting of several lenses used to observe very small objects.
- Telescope: An optical instrument used to observe very distant objects, commonly employed in astronomy.
Mirror
A polished surface, especially polished and metallized glass, that reflects light and produces images of objects.
Scrutiny
A converging lens that widens objects (or makes objects appear wider/larger).
Lenses and Their Types
Lenses are categorized based on how they affect light rays:
- Converging Lenses: Lenses that combine (or join) the light beams/rays.
- Diverging Lenses: Lenses that set light rays apart (disperse them).
Lens Shapes
- Convex: Lenses that are thicker in the middle and thinner at the sides.
- Concave: Lenses that are thinner in the middle and thicker on the sides.
Cosmology and Celestial Bodies
The Universe
Generally defined as everything that physically exists: the entirety of space and time, all forms of matter, energy, and momentum, and the laws and physical constants that govern them.
Components of the Cosmos
- Stars: Massive bodies of gas, mostly hydrogen and helium, which emit light, like our Sun.
- Galaxies: Massive accumulations of stars, gas, and dust. There are millions of galaxies in the Universe.
- Quasars: Very distant objects that emit large amounts of energy.
- Pulsars: Sources of radio waves that vibrate with very regular periods.
- Black Holes: Bodies possessing a gravitational field so immense that not even light can escape.
Our Galactic Home: The Milky Way
Also called the Galaxy, it is a cluster of stars shaped like a disc, which includes the Sun and its Solar System. For a terrestrial observer, the disk of the galaxy appears as a faintly luminous band stretching across the night sky, especially visible on clear, moonless summer nights. The Romans called it the “Way of Milk.”
The Solar System
The Solar System is formed by the Sun, nine planets and their satellites, asteroids, comets, meteoroids, and interplanetary dust and gas.
Planets
A planet is considered one of the nine major celestial bodies in orbit around the Sun, shining by reflected light.
Minor Solar System Bodies
Asteroids, comets, and meteoroids are smaller bodies that also orbit the Sun as their primary star; they are not satellites of a planet.
Planetary Systems Beyond Our Own
The Solar System was long considered the only known planetary system, although in 1980, astronomers found a relatively nearby star surrounded by a bundle of orbiting material of indeterminate size or accompanied by objects supposed to be brown dwarfs. Many astronomers believe in the probable existence of many planetary systems of some kind throughout the Universe.