Polarization of Light in a Raindrop

Yes! Light travels faster in air than in water.

Rainbows are at their best early morning and late afternoon when a shower has just passed over and the Sun is illuminating the curtain of raindrops formed on the trailing edge of the falling rain.

Yes! Every wavelength of light is affected to a different degree by the refractive index of a transparent medium and as a result, changes direction by a different amount when passing from air to water or water to air.

It is the small difference in the refractive index of different wavelengths of incident light that causes dispersion and separation of white light into rainbow colours.

The wavelength of incident light decreases as it travels from air into a raindrop because water is an optically slower medium.

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.

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

• Light bends towards the normal and slows down when it moves from a fast medium (like air) to a slower medium (like water).
• Light bends away from the normal and speeds up when it moves from a slow medium (like diamond) to a faster medium (like glass).
• These phenomena are governed by Snell’s law, which describes the relationship between the angles of incidence and refraction.
• The refractive index (index of refraction) of a medium indicates how much the speed and direction of light are altered when travelling in or out of a medium.
• It is calculated by dividing the speed of light in a vacuum by the speed of light in the material.
• Snell’s law relates the angles of incidence and refraction to the refractive indices of the two media involved.
• Snell’s law states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the refractive indices.

Chromatic dispersion is the process where light, under specific conditions, splits into its constituent wavelengths, and the colours linked with each wavelength become visible to a human observer.

• Chromatic dispersion is the result of the connection between wavelength and refractive index..
• When light moves from one medium (like air) to another (like water or glass), each wavelength is influenced to a varying extent based on the refractive index of the involved media. The outcome is that every wavelength changes its direction and speed.
• If the light source emits white light, the individual wavelengths spread out, with red at one end and violet at the other.
• A familiar example of chromatic dispersion is when white light strikes raindrops and a rainbow becomes visible to an observer.

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.

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 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.

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.

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.

 

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.