Dictionary tag: S

Refraction in a raindrop

An important optical effect that explains how raindrops produce rainbows is refraction.

Refraction refers to the way that electromagnetic radiation (light) changes speed and direction as it travels across the interface between one transparent medium and another.

  • As light travels from a fast medium such as air to a slow medium such as water it bends toward the normal and slows down.
  • As light passes from a slower medium such as water to a faster medium such as air it bends away from the normal and speeds up.
  • In a diagram illustrating optical phenomena like refraction or reflection in a raindrop, the normal is a line drawn from the surface of a raindrop to its centre.
  • The speed at which light travels through a given medium is expressed by its refractive index (also called the index of refraction).
  • If we want to know in which direction light will bend at the boundary between transparent media we need to know:
    • Which is the faster, less optically dense (rare) medium with the smaller refractive index.
    • Which is the slower, more optically dense medium with the higher refractive index.
  • The degree to which refraction causes light to change direction is dealt with by Snell’s law.
  • Snell’s law considers the relationship between the angle of incidence, the angle of refraction and the refractive indices (plural of index) of the media on both sides of the boundary. If three of the four variables are known, then Snell’s law can calculate the fourth.
More about refraction in a raindrop
  • Light rays (streams of photons) undergo refraction twice when they encounter a raindrop, once as they enter, then again as they leave.
  • Once inside a raindrop, a given photon may reflect off the inside surface of a raindrop several times, but on each refraction, some light crosses the boundary back and undergoes refraction as it escapes into the surrounding air.
  • Some photons never escape, instead, they are absorbed when they strike electrons within a raindrop, releasing heat that can causes evaporation.

Saturation

Saturation refers to the perceived difference between one colour and another in terms of its purity and vividness. The hue of a vivid colour appears to be at full strength and can leave an after-image of its complementary colour as an observer looks away.

  • A saturated colour is a unique spectral colour produced by a single wavelength (or a narrow band of wavelengths) of light.
  • A fully saturated colour (100%) is the purest version of a hue and appears bright and vibrant.
  • Unsaturated colours (0-10%) can appear:
    • Misty or milky the nearer they are to white.
    • Dull and washed out as their hue disappears leaving achromatic grey tones.
About saturation & wavelength
  • Saturation is one of the three primary properties of colour, alongside hue and brightness.
    • A colour looks saturated when made by a single or a small range of wavelengths.
    • A colour made by one wavelength of light is often referred to as a pure spectral colour.
    • Unsaturated colours appear faded due to a wider range of wavelengths.
    • Saturation is linked to the complexity of light.
Light complexity
  • Light complexity, linked to saturation, refers to the quantity and range of wavelengths of light used to create a colour.
    • Spectral colours are simple because they consist of just one wavelength of light.
    • Bands of colour are relatively simple because they are composed of a continuous range of wavelengths.
    • Non-spectral colours can be produced from a mix of many wavelengths from different parts of the spectrum, making them the most complex.
  • In reality, colours are often produced by complex combinations of wavelengths.
  • The greater the number and spread of wavelengths across the visible spectrum present in a colour, the lower the saturation.
  • The human eye can perceive millions of different colours due to the complex interactions of wavelengths and the eye’s colour receptors.
About saturation and colour models
About the HSB colour model

The HSB colour model is an additive colour model used to mix light (subtractive colour models are used to mix pigments and inks).

  • The main difference between the HSB colour model and the RGB colour model is how colours are represented and managed in software and applications.
  • The HSB model represents colours based on hue, saturation, and brightness, whereas the RGB model mixes red, green, and blue light to create colours.
  • HSB is popular because it provides a user-friendly way to select and modify colours when using applications like Adobe Creative Cloud for design, photography, or web development.
  • On HSB colour wheels, saturation typically increases from the centre towards the edge.

In the HSB colour model:

  • Hue refers to the perceived difference between colours and is usually described using names such as red, yellow, green, or blue.
    • Hue can be measured as a location on an HSB colour wheel and expressed as a degree between 0 and 360.
  • Saturation refers to the vividness of a colour compared to an unsaturated colour.
    • Saturation is measured between a fully saturated colour (100%) and an unsaturated colour (0%)that appear either:
      • Dull and washed out until all colour disappears, leaving only a monochromatic grey tone (0%).
      • Misty or milky the nearer they are to white.
    • On many HSB colour wheels, saturation decreases from the edge to the centre.
  • Brightness refers to the perceived difference in the appearance of colours under ideal sunlit conditions compared to poor lighting conditions where a hue’s vitality is lost.
    • Brightness can be measured as a percentage from 100% to 0%.
    • As the brightness of a fully saturated hue decreases, it appears progressively darker and achromatic.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Saturation & colour

About saturation & wavelength
  • Saturation is one of the three primary properties of colour, alongside hue and brightness.
    • A colour looks saturated when made by a single or a small range of wavelengths.
    • A colour made by one wavelength of light is often referred to as a pure spectral colour.
    • Unsaturated colours appear faded due to a wider range of wavelengths.
    • Saturation is linked to the complexity of light.
Light complexity
  • Light complexity, linked to saturation, refers to the quantity and range of wavelengths of light used to create a colour.
    • Spectral colours are simple because they consist of just one wavelength of light.
    • Bands of colour are relatively simple because they are composed of a continuous range of wavelengths.
    • Non-spectral colours can be produced from a mix of many wavelengths from different parts of the spectrum, making them the most complex.
  • In reality, colours are often produced by complex combinations of wavelengths.
  • The greater the number and spread of wavelengths across the visible spectrum present in a colour, the lower the saturation.
  • The human eye can perceive millions of different colours due to the complex interactions of wavelengths and the eye’s colour receptors.

Saturation & colour models

About saturation and colour models

Scattering

Scattering takes place when streams of photons (or waves of light) are deflected in different directions.  In this resource, the term is used to refer to the different forms of deviation produced by diffusion, dispersion, interference patterns, reflection and refraction as well as by the composition and surface properties of different media.

Regular scattering
  • When light of a particular wavelength strikes the surface and enters a raindrop its subsequent path depends upon the point of impact, the refractive indices of air and water and the surface properties of the droplet.
  • For incident rays of a single wavelength striking the surface of a single droplet at different points,  it is the different angles at which they enter the droplet that are the chief determinant of the way they scatter as they exit the droplet. In this case.
  • For incident rays of a white light striking the surface of a single droplet at different points, it is the combined effects of the different angles at which they enter the droplet along with the effects of chromatic dispersion (causing the separation of white light into spectral colours) that determine the form of scattering.
  • Chromatic dispersion refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.
  • Regular scattering is not random and obeys the law of reflection and refraction (Snell’s law).
Random scattering
  • In optics, diffusion results from any material that scatters light during transmission or reflection producing softened effects without sharp detail.
  • Objects produce diffuse reflections when light bounces off a rough or uneven surface and scatters in all directions.
  • Transparent and translucent materials transmit diffuse light unless their surfaces are perfectly flat and their interiors are free of foreign material.
  • All objects obey the law of reflection on a microscopic level, but if the irregularities on the surface of an object are larger than the wavelength of light, the light undergoes diffusion.
  • A reflection that is free of the effects of diffusion is called a specular reflection.
  • In the case of raindrops, random scattering can result from:
    • Atmospheric conditions affecting incident sunlight.
    • Turbulence distorting the shape of raindrops.
    • Light being reflected off the surface of multiple raindrops, one after another, before reaching an observer.

Definition

Explanation

Summary

About sections (temp)

Related diagrams

References

Scattering

Scattering occurs when parallel streams of photons (light waves) encounter an obstacle and change direction.
In this context, scattering refers to various forms of deviation linked to diffusion, dispersion, interference, reflection, and refraction, as well as the composition and surface properties of different materials.

Scattering does not take place:

  • When parallel rays of light reflect off a smooth, flat surface like a mirror, producing a distortion-free reflection.
  • When parallel rays of light reflect off a smooth convex surface (although the reflection appears magnified).
  • When parallel rays of light reflect off a smooth concave surface (although the reflection typically appears smaller and inverted).
  • When parallel rays of light pass through translucent materials containing dissolved substances like dyes.
About regular scattering
  • Regular scattering of light refers to the predictable deviation and deflection of light rays when they pass through or reflect off smooth and transparent surfaces.
  • Regular scattering takes place when:
    • Parallel rays of light reflect off a curved surface like a convex or concave mirror. In this case, regular scattering creates a magnified or reduced distortion-free reflection.
    • Parallel rays of incident light are deflected by objects like raindrops and prisms that have smooth surfaces and transparent interiors.
  • Regular scattering is frequently accompanied by chromatic dispersion, which separates white light into its constituent spectral colours.
  • Chromatic dispersion refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength produce a rainbow effect.
  • On a microscopic level, all types of scattering follow the laws of reflection and refraction (Snell’s law).

Let’s consider two cases of regular scattering in more detail:

  • When parallel rays of light with a single wavelength strike and enter an object like a raindrop or prism, their path depends on the initial point of impact, the refractive indices of air and water, and the object’s surface properties.
  • When parallel rays of incident light with a single wavelength meet the curved surface of a transparent medium at various points, the different angles at which they strike the surface and experience deflection mainly determine how they scatter as they exit the medium.
About random scattering
Random scattering
  • Random scattering refers to the scattering of light rays in various directions when they encounter irregularities or imperfections on a surface.
  • Random scattering takes place when a material reflects or transmits light rays in multiple directions.
  • Random scattering can produce reflections that appear soft, lack distinct detail, or even result in no reflection at all.
  • When light passes through sheets of glass with irregular yet smooth surfaces, it distorts the view of the world beyond. Random scattering is responsible wherever the image appears blurry and confused.
  • A reflection that is free of the effects of random scattering is called a specular reflection. Mirrors generally produce specular reflections.
Diffuse light
  • Diffuse light involves the random scattering of light in all directions when it encounters a rough or uneven surface.
  • Diffuse light is produced when it bounces off rough or uneven surfaces, scattering light in every direction.
  • Diffuse light can be the result of the overall structure and composition of the medium, such as when light is transmitted through the interior of a medium that:
    • Contains foreign material
    • Contains suspended particles of different sizes
    • Has an irregular interior structure
    • Has variations in density
    • Absorbs light and then re-emits it
  • Translucent materials containing dissolved substances such as dyes don’t cause random scattering.
  • On a microscopic scale, all objects adhere to the law of reflection; however, when surface irregularities are larger than the wavelength of light, the light undergoes scattering leading to diffusion.
About scattering in raindrops

Here are three related descriptions of what causes scattering when visible light strikes a raindrop:

  • When light of a specific wavelength strikes the surface and enters a raindrop its subsequent path depends upon the point of impact, and the refractive indices of water and air.
  • When rays of light of a single wavelength strike a raindrop at different points, scattering is primarily determined by the angles at which they enter the droplet.
  • The interaction between refraction and chromatic dispersion gives rise to the appearance of rainbow colours when parallel white light rays strike various points on the surface of a raindrop.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Scattering

Scattering takes place when streams of photons (or waves of light) are deflected in different directions.  In this resource, the term is used to refer to the different forms of deviation produced by diffusion, dispersion, interference patterns, reflection and refraction as well as by the composition and surface properties of different media.

Regular scattering
  • When light of a particular wavelength strikes the surface and enters a raindrop its subsequent path depends upon the point of impact, the refractive indices of air and water and the surface properties of the droplet.
  • For incident rays of a single wavelength striking the surface of a single droplet at different points,  it is the different angles at which they enter the droplet that are the chief determinant of the way they scatter as they exit the droplet. In this case.
  • For incident rays of a white light striking the surface of a single droplet at different points, it is the combined effects of the different angles at which they enter the droplet along with the effects of chromatic dispersion (causing the separation of white light into spectral colours) that determine the form of scattering.
  • Chromatic dispersion refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength become visible to a human observer.
  • Regular scattering is not random and obeys the law of reflection and refraction (Snell’s law).
Random scattering
  • In optics, diffusion results from any material that scatters light during transmission or reflection producing softened effects without sharp detail.
  • Objects produce diffuse reflections when light bounces off a rough or uneven surface and scatters in all directions.
  • Transparent and translucent materials transmit diffuse light unless their surfaces are perfectly flat and their interiors are free of foreign material.
  • All objects obey the law of reflection on a microscopic level, but if the irregularities on the surface of an object are larger than the wavelength of light, the light undergoes diffusion.
  • A reflection that is free of the effects of diffusion is called a specular reflection.
  • In the case of raindrops, random scattering can result from:
    • Atmospheric conditions affecting incident sunlight.
    • Turbulence distorting the shape of raindrops.
    • Light being reflected off the surface of multiple raindrops, one after another, before reaching an observer.

Scattering: physics

About scattering in physics
Rayleigh scattering
Mie scattering
Non-selective scattering
Tyndall effect
    • Tyndall effect is another phenomenon related to scattering, where light is scattered by colloidal particles, causing them to become visible in a transparent medium.
    • Colloidal particles are small solid particles or liquid droplets that are dispersed within a medium, typically a liquid or a gas.

Scattering: Raindrops

About scattering in raindrops

Here are three related descriptions of what causes scattering when visible light strikes a raindrop:

  • When light of a specific wavelength strikes the surface and enters a raindrop its subsequent path depends upon the point of impact, and the refractive indices of water and air.
  • When rays of light of a single wavelength strike a raindrop at different points, scattering is primarily determined by the angles at which they enter the droplet.
  • The interaction between refraction and chromatic dispersion gives rise to the appearance of rainbow colours when parallel white light rays strike various points on the surface of a raindrop.

Scattering: Random

About random scattering
Random scattering
  • Random scattering refers to the scattering of light rays in various directions when they encounter irregularities or imperfections on a surface.
  • Random scattering takes place when a material reflects or transmits light rays in multiple directions.
  • Random scattering can produce reflections that appear soft, lack distinct detail, or even result in no reflection at all.
  • When light passes through sheets of glass with irregular yet smooth surfaces, it distorts the view of the world beyond. Random scattering is responsible wherever the image appears blurry and confused.
  • A reflection that is free of the effects of random scattering is called a specular reflection. Mirrors generally produce specular reflections.
Diffuse light
  • Diffuse light involves the random scattering of light in all directions when it encounters a rough or uneven surface.
  • Diffuse light is produced when it bounces off rough or uneven surfaces, scattering light in every direction.
  • Diffuse light can be the result of the overall structure and composition of the medium, such as when light is transmitted through the interior of a medium that:
    • Contains foreign material
    • Contains suspended particles of different sizes
    • Has an irregular interior structure
    • Has variations in density
    • Absorbs light and then re-emits it
  • Translucent materials containing dissolved substances such as dyes don’t cause random scattering.
  • On a microscopic scale, all objects adhere to the law of reflection; however, when surface irregularities are larger than the wavelength of light, the light undergoes scattering leading to diffusion.

Scotopic curve

A scotopic curve is a graphical representation of the sensitivity of the human eye to light under low-light conditions, such as at night or in very dimly lit environments.

  • A scotopic curve shows the minimum amount of light required for the human eye to detect a stimulus at different wavelengths of light. The curve is based on the response of the eye’s rod cells, which are responsible for detecting light in low-light conditions.
  • Unlike the photopic curve, which peaks at around 555 nanometres (green-yellow light), the scotopic curve peaks at around 507 nanometres (blue-green light). This means that in low-light conditions, our eyes are most sensitive to blue-green light.
  • Scotopic and photopic curves have different units of measurement.
    • A photopic curve uses units of luminous flux, which is a measure of the total amount of visible light emitted by a source.
    • A scotopic curve, on the other hand, uses units of luminous intensity, which is a measure of the brightness of a light source per unit of solid angle.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Secondary colour

A secondary colour is a colour made by mixing two primary colours in a given colour space. The colour space may be produced by an additive colour model that involves mixing different wavelengths of light or by a subtractive colour model that involves mixing pigments or dyes.

Secondary colour

A secondary colour is made by mixing two primary colours in a specific colour space. The colour space can be from an additive colour model using different light wavelengths or a subtractive model using pigments or dyes.

  • Secondary colours in additive colour models differ from spectral colours in a rainbow.
  • The RGB colour model can create a vast range of colours.
  • Because the RGB colour model involves adding different wavelengths of light together (additive colour), the resulting colour often appears lighter to a viewer than its components.
  • When all three primary (or secondary) colours are mixed together in equal proportions, the result is white.
  • In subtractive colour models, like the CMYK model used for printing, the primary colours are cyan (C), magenta (M), and yellow (Y) and black (K).
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Secondary rainbow

A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction and dispersion of light in individual droplets and results in the appearance of an arc of spectral colours.

A secondary rainbow is formed when sunlight is refracted and reflected twice by water droplets in the air. The colours of a secondary rainbow are always in the reverse order of a primary rainbow, with violet on the outside and red on the inside.

  • A secondary rainbow appears when sunlight is refracted as it enters raindrops, reflects twice off the inside surface, is refracted again as it escapes back into the air, and then travels towards an observer.
  • A secondary rainbow always appears alongside a primary rainbow and forms a larger arc with the colours reversed.
  • A secondary rainbow has violet on the outside and red on the inside of the bow.
  • When both primary and secondary bows are visible they are often referred to as a double rainbow.
  • A secondary rainbow forms at an angle of between approx. 50.40 to 53.40 to its centre as seen from the point of view of the observer.
  • A secondary bow is never as bright as a primary bow because:
    • Light is lost during the second reflection as a proportion escapes through the surface back into the air.
    • A secondary bow is broader than a primary bow because the second reflection allows dispersing wavelengths to spread more widely.
Remember that:
  • The centre of a rainbow is always on an imaginary straight line (the axis of the rainbow) that starts at the centre of the Sun behind you, passes through the back of your head, out through your eyes and extends in a straight line into the distance.
  • The centre-point of a rainbow is sometimes called the anti-solar point. ‘Anti’, because it is opposite the Sun with respect to the observer.
  • The axis of a rainbow is an imaginary line passing through the light source, the eyes of an observer and the centre-point of the bow.
  • The space between a primary and secondary rainbow is called Alexander’s band.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

Summary

References

Secondary rainbow

A secondary rainbow appears when sunlight is refracted as it enters raindrops, reflects twice off the inside surface, is refracted again as it escapes back into the air, and then travels towards an observer.

  • A secondary rainbow always appears alongside a primary rainbow and forms a larger arc with the colours reversed.
  • A secondary rainbow has violet on the outside and red on the inside of the bow.
  • When both primary and secondary bows are visible they are often referred to as a double rainbow.
  • A secondary rainbow forms at an angle of between approx. 50.40 to 53.40 to its centre as seen from the point of view of the observer.
  • A secondary bow is never as bright as a primary bow because:
    • Light is lost during the second reflection as a proportion escapes through the surface back into the air.
    • A secondary bow is broader than a primary bow because the second reflection allows dispersing wavelengths to spread more widely.
Remember that:
  • The axis of a rainbow is an imaginary line passing through the light source, the eyes of an observer and the centre-point of the bow.
  • The space between a primary and secondary rainbow is called Alexander’s band.

Sine

In maths, the sine is a trigonometric function of an angle.

  • The sine of an acute angle is defined in the context of a right-angled triangle.
  • In the context of angles and triangles, “acute” refers to an angle that is greater than 0 degrees and less than 90 degrees.
  • For any given angle, its sine is the ratio of the length of the side opposite that angle to the length of the longest side of the triangle (the hypotenuse).
  • The maths notation for sine is sin.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Sine

In mathematics, the sine is a trigonometric function of an angle.

  • The sine of an acute angle is defined in the context of a right-angle triangle.
  • For any specified angle, it’s sine is the ratio of the length of the side opposite that angle, to the length of the longest side of the triangle (the hypotenuse).
  • The mathematical notation for sine is sin.

Slow medium

Light travels through different media such as air, glass or water at different speeds. A slow medium is one through which it passes more slowly.

  • Light travels through a vacuum at 299,792 kilometres per second.
  • Light travels through other materials at lower speeds. In some materials, it travels at a speed closer to the speed of light in a vacuum, and in others, it travels much more slowly.
  • The speed of light in air is about 299,702 kilometres per second whilst it travels at approximately 123,889 kilometres per second through diamond.
  • It is useful to know whether a material is fast or slow to predict what will happen when light crosses the boundary between one material and another.
    • As 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.
    • As 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.
Light & Refraction
  • The bending of light and accompanying change in speed, when it passes from one medium to another, is called refraction.
  • The amount of bending depends on the difference in the speed of light in the two materials and the angle at which the light enters the new material.
  • The refractive index of a material is a measure of how much light slows down when it enters that material from a vacuum.
Related diagrams

Each diagram below can be viewed on its own page with a full explanation.

References

Snell’s Law Calculator

To calculate the angle of refraction of an incident ray entering a raindrop enter:

  1. The refractive index of air for a ray with wavelength 589.29 nm. n1) = 1.000293
  2. The refractive index of water for ray with wavelength 589.29 nm, (n2) = 1.3333
  3. The Angle of incidence of your ray as it strikes the surface. (θi) = angle between the ray and the normal

The angle of refraction (θ2) will appear in the final box.

Snell’s Law Calculator

https://www.omnicalculator.com/physics/snells-law

Snell’s law calculator

To calculate the angle of refraction of an incident ray entering a raindrop enter:

  1. The refractive index of air for a ray with wavelength 589.29 nm. n1) = 1.000293
  2. The refractive index of water for ray with wavelength 589.29 nm, (n2) = 1.3333
  3. The Angle of incidence of your ray as it strikes the surface. (θi) = angle between the ray and the normal

The angle of refraction (θ2) will appear in the final box.

Snell’s Law Calculator

https://www.omnicalculator.com/physics/snells-law