Wave

A wave can be thought of as a disturbance that travels through a medium from one location to another location. Waves are produced as energy is transmitted through a medium such as air or water. Electricity produces microscopic waves that travel through a conductor such as a copper wire.

About waves
  • Waves propagate through a medium as atoms and molecules bump into each other producing a domino affect.
  • Waves share common features such as amplitude, crests, direction of travel, frequency and wavelength.
About waves in water
  • If you throw a stone into a pond it produces a series of ripples (waves) that spread out in concentric circles before crashing against obstacles.
  • Seen from a boat at sea, waves form as the wind and tide apply forces that disturb the water.
  • In general terms:
    • The frequency of waves in water can be counted as individual waves rise to a crest at any given point.
    • Wavelength can be calculated by looking at the distance from the crest of one wave to the crest of the next.
    • Amplitude can be measured by looking at the distance from the top of a crest of a wave to the bottom of the next trough.
    • When looking at waves on water it is easy to see in what direction they are travelling.
    • The energy carried by waves at the beach is obvious when you go for a swim and are thrown head-over-heels.
Electromagnetic waves
  • Electromagnetic waves are invisible either because they are too small to see or because our eyes don’t respond to them:
    • The wavelength of some radio waves can be measured in metres but our eyes are not tuned to see them.
    • When we see colours around us electromagnetic waves are entering our eyes but their amplitude, frequency and wavelength are too small to see.
    • Whilst we may not be able to see electromagnetic waves we may be able to sense them as heat or feel a buzz in a wire.
  • Electromagnetic waves can vary in size so much that their wavelength may need to be measured in kilometres or trillions of picometres (1012) .
  • The frequency of electromagnetic waves may be as infrequent as 1 per second (1 hz per second) or as frequent as a quadrillion per second (1015).
References
  • https://en.wikipedia.org/wiki/Wave

Wave-cycle

A wave-cycle refers to the path of a wave measured from any point through the course of a single oscillation to the same point on the next oscillation.

  • Imagine a wave-cycle as a series of points marked on the path of the wave between one crest and the next.
  • All electromagnetic waves share features such as crests, troughs, oscillations, wavelength, frequency, amplitude, direction of travel.
  • Whilst a wave-cycle is the path from one point on a wave during a single oscillation to the same point on completion of that oscillation, wavelength is a measurement of the same phenomenon along the axis of the wave.

http://www.imagewheel.org/knowledge-base/wave-cycles/

Wave-cycle

A wave-cycle refers to the path of a wave measured from any point through the course of a single oscillation to the same point on the next oscillation.

  • Imagine a wave-cycle as a series of points marked on the path of the wave between one crest and the next.
  • All electromagnetic waves share features such as crests, troughs, oscillations, wavelength, frequency, amplitude, direction of travel.
  • As a wave oscillates (vibrates), it can be viewed as a series of individual oscillations measured from one crest to the next crest, one trough to the next trough, or from the start of a wave-cycle to the next starting point.
  • Whilst wave-cycle refers to the path from one point on a wave during a single oscillation to the same point on completion of that oscillation, wavelength is a measurement of the same phenomenon along the axis of the wave.

http://www.imagewheel.org/knowledge-base/wave-cycles/

Wave-particle duality

Wave–particle duality is the concept in quantum mechanics that every particle can be partly described in terms of particles, but also in terms of waves.

  • The dual wave-like and particle-like nature of light is known as the wave-particle duality.
    • Electromagnetic radiation is often described in terms of waves. However, the energy imparted by these waves is absorbed at single locations the way particles are absorbed.
    • The absorbed energy of an electromagnetic wave is called a photon and represents the quanta of light.
    • When a wave of light is absorbed as photons, the energy of the wave collapses to specific locations, and these locations are where the photons “arrive”. This is called the wave function collapse.
  • Albert 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 do.

https://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality

Wavelength

Wavelength is a measurement from any point on the path of a wave to the same point on its next oscillation. The measurement is made parallel to the centre-line of the wave.

  • Wavelength can be measured from any point on a wave. To avoid confusion, it is best to measure wavelength from the top of a crest to the top of the next crest, or from the bottom of a trough to the bottom of the next trough so that the measurement is of the length of a single complete oscillation.
  • The wavelength of an electromagnetic wave is measured in metres.
  • Each type of electromagnetic radiation, such as radio waves, visible light and gamma waves,  forms a band of wavelengths on the electromagnetic spectrum.
  • The greater the energy, the larger the frequency and the shorter (smaller) the wavelength. Given the relationship between wavelength and frequency — the higher the frequency, the shorter the wavelength — it follows that short wavelengths are more energetic than long wavelengths.
  • The visible part of the electromagnetic spectrum is composed of the range of wavelengths that correspond with all the different colours we see in the world.
  • Human beings don’t see wavelengths of visible light, but they do see the spectral colours that correspond with each wavelength and the other colours produced when different wavelengths are combined.
  • The visible spectrum includes all the spectral colours between red and violet and each is produced by a single wavelength of light.
  • The wavelength of visible light is measured in nanometres.
  • The wavelength of visible light is measured in nanometres. There are 1,000,000,000 nanometres in a metre.
  • The visible spectrum is often divided into named colours, though any division is somewhat arbitrary.
  • Traditional colour names in English include red, orange, yellow, green, blue, and violet. But the visible spectrum is, in fact, continuous, and the human eye can distinguish many thousands of intermediary spectral colours.
  • Wavelengths corresponding with the colours of the visible spectrum are usually measured in nanometres. There are therefore 300 different colours between 400 nanometres (violet) and 700 nanometres (red). But if picometres are used instead, then there are 300,000 different wavelengths each of which corresponds with a different colour.
  • The perceived colour (hue) of a light stimulus depends on its wavelength.
  • A colour produced by a single wavelength is called a pure spectral colour.
  • Light is rarely of a single wavelength. Light is usually a mixture of several different wavelengths.
  • The greater number of spectral colours associated with a light source, the lower the saturation, so light of mixed wavelengths produces duller more neutral colours.

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

Wavelength

Wavelength is a measurement from any point on the path of a wave to the same point on its next oscillation. The measurement is made parallel to the centre-line of the wave.

White light

White light is the name given to visible light that contains all wavelengths of the visible spectrum at equal intensities.

  • As light travels through a vacuum or a medium it is described as white light if it contains all the wavelengths of visible light.
  • As light travels through the air it is invisible to our eyes.
  • When we look around we see through the air because it is very transparent and light passes through it.
  • The term white light doesn’t mean light is white as it travels through the air.
  • One situation in which light becomes visible is when it reflects off the surface of an object.
  • When white light strikes a neutral coloured object and all wavelengths are reflected then it appears white to an observer.

White light

White light is the name given to visible light that contains all wavelengths of the visible spectrum at equal intensities.

  • The sun emits white light because sunlight contains equal amounts of all of the wavelengths of the visible spectrum.
  • As light travels through a vacuum or a medium it is described as white light if it contains all the wavelengths of visible light.
  • As light travels through the air it is invisible to our eyes.
  • The term white light has two meanings. It can refer to:
    •  A mixture of all the wavelengths of visible light travelling through space without thinking about its observation.
    • What an observer sees when all the colours that make up the visible spectrum strike a white or neutral coloured surface.
  • The human eye also sees white when the wavelengths of light corresponding with the three primary colours red, green and blue (RGB) are projected onto a neutrally coloured surface.
  • Light is only visible as it is emitted by an object such as an electrical filament or when it strikes an object.
  • White light appears coloured when some wavelengths of light are reflected by the surface of an object but others are absorbed.
  • Artificial light sources typically emit light with an uneven distribution of wavelengths or intensities.
  • Whilst there is no single, unique specification of “white light”, there is indeed a unique specification of “white object”, or, more specifically, “white surface”.
  • White is the lightest possible colour.
  • White is an achromatic colour, meaning a colour without hue.

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

Why the sky is blue?

Perhaps the most common of atmospheric effects, the blueness of the sky, is caused by the way sunlight is scattered by tiny particles of gas and dust as it travels through the atmosphere.

The sky is blue because more photons corresponding with blue reach an observer than any other colour.

In outer space, the Sun forms a blinding disk of white light set against a completely black sky. The only other light is produced by stars and planets (etc.) that appear as precise white dots against a black background. The sharpness of each of these distant objects results from the fact that photons travel through the vacuum of space in straight lines from their source to an observer’s eyes. In the absence of gas and dust, there is nothing to scatter or diffuse light into different colours and no surfaces for it to mirror or reflect off.

All of this changes when sunlight enters the atmosphere. Here, the majority of photons do not travel in straight lines because the air is formed of gases, vapours and dust and each and every particle represents a tiny obstacle that refracts and reflects light. Each time a photon encounters an obstacle both its speed and direction of travel change resulting in dispersion and scattering. The outcome is that, from horizon to horizon, the sky is full of light travelling in every possible direction and it reaches an observer from every corner.

The following factors help to account for why blue photons reach an observer in the greatest numbers:

  • The sky around the Sun is intensely white in colour because vast numbers of photons of all wavelengths make the journey from Sun to an observer in an almost straight line.
  • In every other area of the sky, light has to bend towards an observer if they are to see colour. It is this scattering of light that fills the sky with diffuse light throughout the day.
  • Longer wavelengths of light (red, yellow, orange and green) are too big to be affected by tiny molecules of dust and water in the atmosphere so scatter the least so few are redirected towards an observer.
  • Shorter wavelengths (blue and violet) are just the right size to interact with obstacles in the atmosphere. These collisions scatter light in every possible direction including towards an observer.
  • Because blue is relative intense compared with violet in normal conditions and in the absence of the longer wavelengths the sky appears blue.
  • However, there is a whole band of wavelengths corresponding with what we simply call blue. As a result, different atmospheric conditions fill the sky with an enormous variety of distinctly different blues during the course of the day.

Why the sky is sometimes red

If we understand why the sky is usually blue it’s easier to understand why it can be filled with reds and pinks at sunrise and sunset.
 

Let’s review why the sky is blue
  • In most weather conditions, the Sun and the area around it appear intensely white to an observer because vast numbers of photons of every wavelength make the journey from Sun to their eyes in an almost straight line.
  • The Sun, and the area around it, appears white because it contains a mixture of all wavelengths of light (white light).
  • In every other area of the sky, sunlight is striking billions of particles that make up the atmosphere and scattering in every possible direction.
  • If it were not for this scattering (deflection of light in all directions), the sky would be as black as night. In reality, an observer is bathed in light arriving from every direction and the sky, as a result, appears to be full of diffuse light.
  • Not all wavelengths of light behave in the same way when scattered by the small particles that make up the atmosphere.
  • Longer wavelengths of light (red, yellow, orange and green) are too big to be affected by tiny molecules of dust and water so scatter the least.
  • Shorter wavelengths (blue and violet) are just the right size and are affected by reflection, refraction and scattering as they strike successions of particles. It is these collisions that direct light in every possible direction including towards an observer.
  • Because human eyes are more sensitive to blue than violet, in most atmospheric conditions, and in the absence of the longer wavelengths, the sky appears blue.
  • A wide band of wavelengths corresponds with what we often describe as blue. As a result, the sky is filled with an enormous variety of distinctly different blues during the course of every day.
Why the sky is sometimes red
  • A red sky suggests an atmosphere loaded with dust or moisture and that the Sun is near the horizon.
  • In the morning and evening, photons must travel much further through the atmosphere than at mid-day.
  • Assuming the air above our heads is around 20 km, the total distance light travels increases fivefold to around 500 km when the Sun is on the horizon.
  • Remember that:
    • Longer wavelengths of light (red, yellow, orange and green) are too big to be affected by tiny molecules of dust and water so scatter the least.
    • Shorter wavelengths (blue and violet) are just the right size and are affected by reflection, refraction and scattering as they strike successions of particles.
  • In the right weather conditions, light travelling horizontally through the atmosphere undergoes so much scattering that no yellow, green, blue or violet wavelengths remain.
  • In these conditions, the light that reaches us, illuminating the sky and clouds and reflecting off every surface around us, is composed of wavelengths that bath the world in red and orange.