Radiant energy and electromagnetic radiation are two terms that refer to the same concept. Both refer to the propagation of energy through space in the form of waves. These waves appear as oscillating electric and magnetic fields, the fundamental feature of electromagnetic radiation.
- Electromagnetic radiation can be viewed as either electromagnetic waves or a stream of photons. These two perspectives are not mutually exclusive but rather complementary, as explained by the concept of wave-particle duality.
- Wave Perspective: From this viewpoint, electromagnetic radiation is thought of as waves of oscillating electric and magnetic fields traversing through space. The energy of the radiation is distributed across the wave, with its intensity related to the amplitude and frequency of the wave.
- Photon Perspective: Electromagnetic radiation can also be thought of as a succession of massless particles known as photons. Each photon carries a discrete quantum of energy, referred to as photon energy, directly related to the radiation’s frequency. This perspective acknowledges the particle-like behaviour of electromagnetic radiation.
- The concept of wave-particle duality which has been developed in Quantum Field Theory, reconciles these contrasting viewpoints by claiming that electromagnetic radiation exhibits both wave-like and particle-like attributes. This duality has been experimentally verified.
Radiation is energy that comes from a source and travels through space at the speed of light.
- Radiant energy has an electric field, and a magnetic field and may be described in terms of electromagnetic waves or in terms of bundles of photons travelling in a wave-like pattern.
- Visible light is a form of radiation often described in terms of either electromagnetic waves or photons.
- Types of radiation with the highest energy include ultraviolet radiation, x-rays, and gamma rays.
- When x-rays or gamma-rays interact with atoms, they can remove electrons which destabilises them and make them radioactive.
- Radioactivity is the spontaneous release of energy from an unstable atom as it returns to a stable state.
- Ionizing Radiation is the energy that comes out of a radioactive atom.
Radiometry is the study of how light, carried by electromagnetic waves made up of particles called photons, travels through space. It involves measuring and analysing the energy (radiant energy) of these waves and their component particles.
- Radiometry studies the properties of electromagnetic radiation such as intensity, spectral distribution and polarization, and how light interacts with matter (absorption, reflection, and scattering).
- Electromagnetic radiation and the electromagnetic energy it transports can be described in terms of waves.
- Electromagnetic radiation (radiant energy) includes all wavelengths of light from radio waves to gamma rays.
- Electromagnetic radiation can be described in terms of photons and their properties.
- Energy: Photons have energy that depends on their frequency or wavelength. Higher-frequency photons have more energy than lower-frequency photons.
- Number: The number of photons in a given electromagnetic radiation depends on its intensity. Higher-intensity radiation has more photons than lower-intensity radiation.
- Direction: Photons travel in straight lines, but their direction can be changed by interacting with matter.
- Polarization: Photons can be polarized, which means that their electric and magnetic fields oscillate in a particular direction.
- Speed: Photons travel at the speed of light, which is approximately 299,792,458 meters per second in a vacuum.
A rainbow is an optical effect produced by illuminated droplets of water. Rainbows are caused by reflection, refraction (bending) and dispersion (spreading out) of light in individual droplets and result in the appearance of an arc of spectral colours.
- Atmospheric rainbows only appear when weather conditions are ideal and an observer is in the right place at the right time.
- Waterfalls, lawn sprinklers and other things that produce air-borne water droplets can produce a rainbow.
- An atmospheric rainbow is formed from countless individual droplets each of which reflects and refracts a tiny coloured image of the Sun towards the observer.
- As white light passes through water droplets, refraction causes the light to disperse and separate into the different colours seen by an observer.
- If the sun is behind an observer then the rainbow will appear in front of them.
- When a rainbow is produced by sunlight, the angles between the sun, each droplet and the observer determine which ones will form part of the rainbow, the colour each droplet will produce and the sequence in which they appear.
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The term rainbow angle is often paired with rainbow ray to measure the angle at which light is deflected back towards an observer as it passes through a raindrop.
- At lightcolourvision.org the term rainbow angle is avoided but is treated as being synonymous with the angle of deflection.
- The angle of deflection (rainbow angle) is measured at the point where the path of an incidence ray and the path of the same ray after it exits a raindrop towards the observer can be shown to intersect.
- To make the incident and exiting ray intersect in a ray-tracing diagram the incident ray is extended forwards in a straight line beyond the raindrop. The ray exiting the droplet towards the observer is then extended backwards until both intersect. The angle of deflection (rainbow angle) lies between the two.
- The angle of deflection (rainbow angle), for any ray that is contributing directly to the arcs of a primary rainbow, is always between approx. 40.70 and 42.40.
Viewing angle, angular distance and angle of deflection
- The term viewing angle refers to the number of degrees through which an observer must move their eyes or turn their head to see a specific colour within the arcs of a rainbow.
- The term angular distance refers to the same measurement when shown in side elevation on a diagram.
- The angle of deflection measures the degree to which a ray striking a raindrop is bent back on itself in the process of refraction and reflection towards an observer.
- The term rainbow rays refers to the path taken by the deflected ray that produces the most intense colour experience for any particular wavelength of light passing through a raindrop.
- The term angle of deviation measures the degree to which the path of a light ray is bent back by a raindrop in the course of refraction and reflection towards an observer.
- In any particular example of a ray of light passing through a raindrop, the angle of deviation and the angle of deflection are directly related to one another and together add up to 1800.
- The angle of deviation is always equal to 1800 minus the angle of deflection. So clearly the angle of deflection is always equal to 1800 minus the angle of deviation.
- In any particular example, the angle of deflection is always the same as the viewing angle because the incident rays of light that form a rainbow are all approaching on a trajectory running parallel with the rainbow axis.
The rainbow axis is an imaginary straight line that connects the light source, observer and anti-solar point.
- The centre of a rainbow is always on its axis.
- The centre of a rainbow always corresponds with the anti-solar point.
- When drawing a diagram showing the axis of a rainbow, the Sun and anti-solar point, are at opposite ends with the observer between them.
- From an observer’s point of view, the rainbow axis is an imaginary line that they look along towards the centre of a rainbow.
Rainbow colour refers to the colours seen in rainbows and other situations where visible light separates into its component wavelengths and the corresponding hues become visible to the human eye.
- Rainbow colour (also called spectral colour) is a colour model.
- A colour model is a theory of colour that establishes terms, definitions, rules and conventions for understanding and describing colours and their relationships with one another.
- A spectral colour is a colour evoked in normal human vision by a single wavelength of visible light (or by a narrow spread of adjacent wavelengths).
- When all the spectral colours are mixed together in equal amounts and at equal intensities, they produce white light.
- In order of wavelength, the rainbow colours (ROYGBV) are red (longest visible wavelength), orange, yellow, green, blue and violet (shortest visible wavelength).
- It is the sensitivity of the human eye to this small part of the electromagnetic spectrum that results in our perception of colour.
- Whilst the visible spectrum and its spectral colours are determined by wavelength (and corresponding frequency), it is our eyes and brains that interpret these differences in electromagnetic radiation and produce colour perceptions.
- Naming rainbow colours is a matter more closely related to the relationship between perception and language than anything to do with physics or optics.
- Even commonplace colour names associated with rainbows such as yellow or blue defy easy definition. These names are concepts related to subjective impressions.
- Modern portrayals of rainbows show six colours – ROYGBV. This leaves out other colours such as cyan and indigo.
- Atmospheric rainbows actually contain millions of spectral colours. Measured in nanometres there are around 400 colours between red and violet, measured in picometres there are 400,000.
Rainbow colours are the colours seen in rainbows and in other situations where visible light separates into its different wavelengths and the spectral colours corresponding with each wavelength become visible to the human eye.
- The rainbow colours (ROYGBV) in order of wavelength are red (longest wavelength), orange, yellow, green, blue and violet (shortest wavelength).
- It is the sensitivity of the human eye to this small part of the electromagnetic spectrum that results in our perception of colour.
- The names of rainbow colours are a matter more closely related to the relationship between perception and language than anything to do with physics or scientific accuracy. While the spectrum of light and the colours we see are both determined by wavelength, it’s our eyes and brains that turn these differences in light into the colours we experience.
- In the past, rainbows were sometimes portrayed as having seven colours: red, orange, yellow, green, blue, indigo and violet.
- Modern portrayals of rainbows reduce the number of colours to six spectral colours, ROYGBV.
- In reality, the colours of a rainbow form a continuous spectrum and there are no clear boundaries between one colour and the next.
A ray of light (light ray or just ray) is a common term when talking about how and why rainbows appear.
- The idea that light is made up of rays is so commonplace when describing and explaining rainbows that it is easily taken for granted.
- The idea of light rays is useful when trying to model how light and raindrops produce the rainbow effects seen by an observer.
- Light rays don’t exist in the sense that the term accurately describes a physical property of light. More accurate descriptions use terms like photons or waves.
- Modelling light as rays is a way to discuss and represent the path of light through different media in a simple and easily understandable way.
- When light rays are drawn in a ray-tracing diagram they are represented as straight lines connected at angles to illustrate how light moves and what happens when it encounters different situations and conditions.
- More accurate descriptions of light use terms such as photons or electromagnetic waves.
- Don’t forget that:
- The incident rays of light that contribute to a rainbow seen by an observer are those that approach raindrops parallel with the rainbow axis.
- To understand why incident rays are always parallel with the rainbow axis we need to think in terms of what the observer sees. See: 2.7 Observer’s point of view.
Tiny images of the Sun mirrored in millions of individual raindrops create the impression of bands of colour arching across the sky when an observer sees an atmospheric rainbow.
- Rainbows are formed from tiny indistinguishable dots of light and each one is produced by a water droplet from which an observer manages to catch a glimpse of an image of the Sun.
- It is the precise position of each individual raindrop in the sky that determines:
- Whether or not it is in the range of possible positions that will enable it to reflect an image of the Sun towards the observer.
- The exact spectral colour that it will produce at any moment and over the passage of time as it falls.
- The precise position of each raindrop changes over time as it falls, causing its colour to change from red through to violet. Prior to reflecting red, each raindrop is invisible to an observer. After reflecting violet the amount of light reflected by each raindrop drops off sharply.
- Raindrops reflect and refract the greatest concentration of photons towards an observer from the intense bands of colour within the arcs of a rainbow.
- Raindrops inside the coloured arcs, in the area between the anti-solar point and the inside edge of the violet bow, direct light towards an observer causing this area to appear lighter or brighter than the rest of the sky. Factors that determine the appearance of this area include:
- Lower intensity: Each raindrop reflects far fewer photons in the direction of an observer once they have fallen below the violet band of a rainbow.
- Reduced saturation: The saturation of each rainbow colour reduces sharply as raindrops leave the violet band because they mix with other droplets that are reflecting other colours.
- Any situation where an observer is exposed to a mixture of a wide range of wavelengths in similar proportions produces the impression of white rather than a specific colour.
- Scattering: Light reflected by a raindrop in the direction of an observer may encounter a series of other raindrops on its journey causing random scattering of light in other directions.
Rainbows can be formed by droplets of liquids other than water, or even by a cloud of solid transparent microspheres. The table below shows the viewing angles for primary rainbows produced by a number of different media.
Substance
|
Refractive index
|
Viewing angle
|
| Water |
1.33 |
42.5 |
| Kerosene |
1.39 |
34.5 |
| Carbon tetrachloride |
1.46 |
26.7 |
| Benzene |
1.50 |
22.8 |
| Plate glass |
1.52 |
21.1 |
| Other glass |
1.47 to 1.61 |
25.7 to 14.2 |
Primary rainbow viewing angles for various media
- Materials with an index of refraction of 2.00 or more do not produce primary rainbows.
- Diamonds, for example, do not produce primary rainbows because their index of refraction is 2.42. However, if a diamond is ground into microspheres, it can produce secondary and higher-order rainbows.
Data from https://www.basic-physics.com/rainbows-figuring-their-angles/
