Polarization of Light in a Raindrop
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This is one of a set of almost 40 diagrams exploring Rainbows.
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Description
POLARIZATION OF LIGHT IN A RAINDROP
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About the diagram
About rainbows and light
Rainbows result from light encountering raindrops in the presence of an observer. The phenomenon of rainbows offers many clues as to the nature of light.
- Light is a form of radiation, a type of energy that travels in the form of electromagnetic waves and can also be described as a flow of particle-like ‘wave-packets’, called photons.
- Radiation, electromagnetic waves and photons are all concepts that are interchangeable with the more general concept of light.
Theories of light
There are four principal theories that underpin our understanding of the physical properties of light as it relates to rainbows:
- Wave theory – the idea that light is transmitted from luminous bodies in an undulatory wave-like motion.
- Particle theory – the idea that the constitution and properties of light can be described in terms of the interactions of elementary particles.
- Electromagnetic theory – the classical theory of electromagnetism that describes light as coupled electric and magnetic fields, transporting energy as it propagates through space as a wave. The energy is stored in its electric and magnetic fields and can be measured in terms of its intensity.
- Quantum theory – explains the interactions of light with matter (atoms, molecules etc.) and describes light as consisting of discrete packets of energy, photons. Quantum physics suggests that electromagnetic radiation behaves more like a classical wave at lower frequencies and more like a classical particle at higher frequencies, but never completely loses all the qualities of one or the other.
These theories tell us things about the properties of light
- Light is electromagnetic radiation, the force carrier of radiant energy.
- Whilst it carries energy and has momentum, light has no mass and so is not matter.
- Light is the result of the interaction and oscillation of electric and magnetic fields.
- Light is a microscopic phenomenon that needs macroscopic metaphors such as waves and particles to describe it.
- Once emitted at its source, light can propagate indefinitely through a vacuum in a straight line at the speed of light (299,792,458 metres a second) but can be deflected by gravity.
- In any specific instance, light can be described in terms of the inter-relationship of its wavelength, frequency and energy.
- Light slows down and is deflected as it propagates through air, water, glass and other transparent media as photons interact with matter.
Phenomena associated with light include:
- Absorption
- Diffraction
- Dispersion
- Interference
- Photoelectric effect
- Polarization
- Reflection
- Refraction
- Scattering
- Transmission
Some facts about electromagnetic waves
- An electromagnetic wave carries electromagnetic radiation.
- Electromagnetic radiation is measured in terms of the amount of electromagnetic energy carried by an electromagnetic wave.
- Electromagnetic waves can be imagined as synchronised oscillations of electric and magnetic fields propagating at the speed of light in a vacuum.
- The kinetic energy carried by electromagnetic waves is often simply called radiant energy or light.
- Electromagnetic waves are similar to other types of waves in so far as they can be measured in terms of wavelength, frequency and amplitude.
- Other terms for the amplitude of light are intensity and brightness.
- Another term for the speed at which light travels is its velocity.
- We can feel electromagnetic waves release energy when sunlight warms our skin.
- The position of an electromagnetic wave within the electromagnetic spectrum can be identified by its frequency, wavelength or energy.
Some facts about photons
- Photons are the elementary building blocks and so the smallest unit used to describe light.
- Photons are the carriers of electromagnetic force and travel in harmonic waves.
- Photons are zero mass bosons.
- Photons have no electric charge.
- The amount of energy a photon carries can make it behave like a wave or a particle. This is called the “wave-particle duality” of light.
Facts about the electromagnetic spectrum
- Visible light is just one tiny part of the electromagnetic spectrum.
- Our eyes only respond to the visible light which we see as colours between red and violet.
- The electromagnetic spectrum includes, in order of increasing frequency and decreasing wavelength: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.
- The size of the longest wavelengths is unknown but the shortest is believed to be in the vicinity of the Planck length (approximately 1.6 x 1035 meters).
About rainbows and the polarization of light
Polarization of electromagnetic waves refers to the geometrical orientation of their oscillations.
Polarization restricts the orientation of the oscillations of the electric field of electromagnetic waves to a single plane from the point of view of an observer. This phenomenon is known as plane polarization.
- Plane polarization filters out all the waves where the electric field is not orientated with the plane from the point of view of an observer.
- To visualize plane polarization, imagine trying to push a large sheet of card through a window fitted with close-fitting vertical bars.
- Only by aligning the card with the slots between the bars can it pass through. Align the card at any other angle and its path is blocked.
- Now substitute the alignment of the electric field of an electromagnetic wave for the sheet of card, and plane polarization for the bars on the window.
- Polarizing lenses used in sunglasses rely on plane polarization. The polarizing plane is orientated horizontally and cuts out glare by blocking vertically aligned waves.
- Plane polarization is one of the optical effects that account for the appearance of rainbows.
- It is the position of each raindrop on the arc of a rainbow, with respect to the observer, that determines the angle of the polarizing plane.
- Rainbows are typically 96% polarized.
Let’s take this one step at a time
- Rainbows form in the presence of sunlight, raindrops and an observer, and involve a combination of refraction, reflection and chromatic dispersion.
- It is during reflection off the back of a droplet that light becomes polarized with respect to an observer.
- The rear hemisphere of a raindrop forms a concave mirror in which an observer sees a tiny reflection of the Sun.
- As a rainbow forms, an image of the Sun forms in each and every raindrop and the ones in exactly the right place at the right time become visible to the observer.
- The light reflected towards an observer is polarized on a plane bisecting each droplet and at a tangent to the arc of the rainbow.
- The rear hemisphere of a raindrop is best thought of as the half of the raindrop opposite the observer and with the Sun at its centre.
- Now recall that to see yourself in a normal flat mirrored surface it has to be aligned perpendicular to your eyes. Get it right and you see yourself right in the middle. If it’s not perpendicular, then you see your image off-centre because the mirror is not aligned with your eyes on either the horizontal or vertical planes.
- The Sun appears right in the centre of every raindrop from the point of view of an observer only if it is in exactly the right position in the sky at the right time. In all other cases, the light is scattered in other directions.
- Only rays that strike at the point where the horizontal and vertical planes intersect are reflected towards the observer. Rays that strike to the left or right or above/below the centre-point miss the observer.
- The correct alignment of a raindrop involves the vertical axis of the rear hemisphere being at exactly 900 with respect to the observer. In the case of a primary rainbow, the horizontal axis is titled downwards by approx. 20.50.
Some key terms
The visible part of the electromagnetic spectrum is called the visible spectrum.
- The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
- As light travels through the air it is invisible to our eyes.
- Human beings don’t see wavelengths of light, but they do see the spectral colours that correspond with each wavelength and 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.
- The visible spectrum is often divided into named colours, though any division of this kind is somewhat arbitrary.
- Traditional colours referred to in English include red, orange, yellow, green, blue, and violet.
ROYGBV are the initials for the sequence of colours that make up the visible spectrum: red, orange, yellow, green, blue, and violet.
- The visible spectrum refers to the range of colours visible to the human eye.
- White light, when passed through a prism, separates into a sequence of individual colours corresponding with ROYGBV which is the range of colours visible to the human eye.
- White light separates into ROYGBV because different wavelengths of light bend at slightly different angles as they enter and exit the prism.
- ROYGBV helps us remember the order of these spectral colours starting from the longest wavelength (red) to the shortest (violet).
- A rainbow spans the continuous range of spectral colours that make up the visible spectrum.
- The visible spectrum is the small band of wavelengths within the electromagnetic spectrum that corresponds with all the different colours we see in the world.
- The fact that we see the distinct bands of colour in a rainbow is an artefact of human colour vision.
Polarization of electromagnetic waves refers to the direction in which they oscillate, perpendicular to the direction of the wave’s propagation.
- Polarization can be induced in light waves by various means, such as reflection, refraction, and scattering.
- There are several types of polarization, including circular, elliptical and plane polarization.
- Circular polarization refers to waves that rotate in circles as they propagate, with the electric and magnetic fields perpendicular to each other.
- Elliptical polarization combines linear and circular polarization, in which the wave oscillates in an elliptical pattern.
- Plane polarization (sometimes called linear polarization) refers to waves that oscillate in a single plane, such as waves that are vertically or horizontally polarized.
The spectral colour model represents the range of pure colours that correspond to specific wavelengths of visible light. These colours are called spectral colours because they are not created by mixing other colours but are produced by a single wavelength of light. This model is important because it directly reflects how human vision perceives light that comes from natural sources, like sunlight, which contains a range of wavelengths.
- The spectral colour model is typically represented as a continuous strip, with red at one end (longest wavelength) and violet at the other (shortest wavelength).
- Wavelengths and Colour Perception: In the spectral colour model, each wavelength corresponds to a distinct colour, ranging from red (with the longest wavelength, around 700 nanometres) to violet (with the shortest wavelength, around 400 nanometres). The human eye perceives these colours as pure because they are not the result of mixing other wavelengths.
- Pure Colours: Spectral colours are considered “pure” because they are made up of only one wavelength. This is in contrast to colours produced by mixing light (like in the RGB colour model) or pigments (in the CMY model), where a combination of wavelengths leads to different colours.
- Applications: The spectral colour model is useful in understanding natural light phenomena like rainbows, where each visible colour represents a different part of the light spectrum. It is also applied in fields like optics to describe how the eye responds to light in a precise, measurable way.
- The observer effect is a principle of physics and states that any interaction between a particle and a measuring device will inevitably change the state of the particle. This is because the act of measurement itself imposes a disturbance on the particle’s wave function, which is the mathematical description of its state.
- The concept of observation refers to the act of engaging with an electron or other particle, achieved through measuring its position or momentum.
- In the context of quantum mechanics, observation isn’t a passive undertaking, observation actively alters a particle’s state.
- This means that any kind of interaction with an atom, or with one of its constituent particles, that provides insight into its state results in a change to that state. The act of observation is always intrusive and will always change the state of the object being observed.
- It can be challenging to reconcile this with our daily experience, where we believe we can observe things without inducing any change in them.
A human observer is a person who engages in observation by watching things.
- In the presence of visible light, an observer perceives colour because the retina at the back of the human eye is sensitive to wavelengths of light that fall within the visible part of the electromagnetic spectrum.
- The visual experience of colour is associated with words such as red, blue, yellow, etc.
- The retina’s response to visible light can be described in terms of wavelength, frequency and brightness.
- Other properties of the world around us must be inferred from light patterns.
- An observation can take many forms such as:
- Watching an ocean sunset or the sky at night.
- Studying a baby’s face.
- Exploring something that can’t be seen by collecting data from an instrument or machine.
- Experimenting in a laboratory setting.
Reflection is the process where light rebounds from a surface into the medium it came from, instead of being absorbed by an opaque material or transmitted through a transparent one.
- The three laws of reflection are as follows:
- When light hits a reflective surface, the incoming light, the reflected light, and an imaginary line perpendicular to the surface (called the “normal line”) are all in the same flat area.
- The angle between the incoming light and the normal line is the same as the angle between the reflected light and the normal line. In other words, light bounces off the surface at the same angle as it came in.
- The incoming and reflected light are mirror images of each other when looking along the normal line. If you were to fold the flat area along the normal line, the incoming light would line up with the reflected light.
The visible spectrum is the range of wavelengths of the electromagnetic spectrum that correspond with all the different colours we see in the world.
- As light travels through the air it is invisible to our eyes.
- Human beings don’t see wavelengths of light, but they do see the spectral colours that correspond with each wavelength and 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.
- The visible spectrum is often divided into named colours, though any division of this kind is somewhat arbitrary.
- Traditional colours referred to in English include red, orange, yellow, green, blue, and violet.
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.
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.
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