## Fast medium

Light travels through different media such as air, glass or water at different speeds.  A fast medium is one through which it passes through more quickly than others.

• Light travels through a vacuum at 299,792 kilometres per second.
• Light travels through other media at lower speeds.
• In some cases, it travels at a speed which is near the speed of light (the speed at which light travels through a vacuum) and in other cases, it travels much more slowly.
• It is useful to know whether a medium is fast or slow to predict what will happen when light crosses the boundary between one medium and another.
• so:
• If light crosses the boundary from a medium in which it travels fast into a material in which it travels more slowly, then it will bend towards the normal.
• If light crosses the boundary from a medium in which it travels slowly into a material in which it travels more quickly, then the light ray will bend away from the normal.
• In optics, the normal is a line drawn in a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.

https://en.wikipedia.org/wiki/Refraction

## Fast medium

Light travels through different media such as air, glass or water at different speeds.  A fast medium is one through which it passes through more quickly than others.

• Light travels through a vacuum at 299,792 kilometres per second.
• Light travels through other media at lower speeds.
• In some cases, it travels at a speed which is near the speed of light (the speed at which light travels through a vacuum) and in other cases, it travels much more slowly.
• It is useful to know whether a medium is fast or slow to predict what will happen when light crosses the boundary between one medium and another.
• so:
• If light crosses the boundary from a medium in which it travels fast into a material in which it travels more slowly, then it will bend towards the normal.
• If light crosses the boundary from a medium in which it travels slowly into a material in which it travels more quickly, then the light ray will bend away from the normal.
• In optics, the normal is a line drawn in a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.

## Fundamental Force

There are four fundamental forces that account for all the forms of pulling and pushing between things.

• Whenever there is a push-pull interaction between two objects, forces are being applied to each of them. When the interaction ceases, the two objects no longer experience the force and their momentum continues uninterrupted.
• On a macro-scale wherever there is a concentration of stuff, in planets, suns or galaxies, that is where most of the push-pulls happen.
• Everything everywhere is in motion.
• Nothing in the Universe is stationary unless its temperature is reduced to absolute zero. But nothing can be cooled to a temperature of exactly absolute zero.
• Motion applies to things like objects, bodies, matter, particles, radiation and space-time. We also refer to the motion of images, shapes and boundaries. So motion signifies a change in the position of the elements of a physical system. An object’s motion, and so its momentum stays the same unless a force acts on it.

The four fundamental forces of nature are:

• Gravitational force: Gravity is the phenomenon that causes things with mass or energy to gravitate towards one another. Planets, stars, galaxies, and even light are all affected by gravity. The effect of gravity on small things like human beings when in the vicinity of something big like a planet is obvious. It is the Moon’s gravity that causes ocean tides on Earth. Gravity accounts for physical objects having weight. Gravity has an infinite range, although its effects become weaker as objects get further away from one another.
• Weak Nuclear force: In nuclear physics and particle physics, the weak nuclear force explains the interaction between subatomic particles that is responsible for the radioactive decay of atoms. The weak nuclear force doesn’t affect electromagnetic radiation.
• Strong Nuclear force: The strong nuclear force holds matter together. It binds the sub-atomic particles, protons and neutrons, that form the nucleus of an atom. Whilst repulsive electromagnetic forces push them apart, the attractive nuclear force is strong enough to overcome them at short range. The range at work here is measured in femtometres. The nuclear force plays an essential role in storing energy that is used in nuclear power and nuclear weapons.
• Electromagnetic force: The electromagnetic force is the force that occurs between electrically charged particles, such as electrons, and is described as either a positive or negative charge. Objects with opposite charges produce an attractive force between them, while objects with the same charge produce a repulsive force. The electromagnetic force is carried by photons in the form of electric and magnetic fields that propagate at the speed of light.

https://en.wikipedia.org/wiki/Force

## Force

Forces bind things together and push things apart.

• The push-pull interactions between things are described as the interplay of forces.
• Forces explain how anything interacts with anything else in the whole of the natural world.
• Forces produce motion and can cause an object with mass to change velocity.
• Changes in velocity include causing things to start moving from a state of rest, to accelerate or slow down.
• Quarks and leptons, fundamental particles present in all forms of matter, are bound together by fundamental forces.
• There are four fundamental forces that account for all the forms of pulling and pushing between things in the Universe.

https://en.wikipedia.org/wiki/Force

## Fovea

The entire surface of the retina contains nerve cells, but there is a small portion with a diameter of approximately 0.25 mm at the centre of the macula called the fovea centralis where the concentration of cones is greatest.

• This region is the optimal location for the formation of image detail.
• The eyes constantly rotate in their sockets to focus images of objects of interest as precisely as possible at this location.

## Frequency

The frequency of electromagnetic radiation (light) refers to the number of wave-cycles of an electromagnetic wave that pass a given point in a given amount of time.

• The frequency of a wave should not be confused with the speed at which the wave travels or the distance it travels.
• The term frequency refers to the measurement of the frequency of wave-cycles that pass a given point in a given amount of time.
• Frequency is measured in Hertz (Hz). One Hertz is one wave-cycle per second.
• The wavelength and frequency of light are closely related. the higher the frequency, the shorter the wavelength.
• The amount of energy transported by a light wave increases with the frequency of oscillations and as the length of each oscillation decreases.

https://en.wikipedia.org/wiki/Frequency