Path of Rays Through a Raindrop

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.

Yes! Light travels faster in air than in water.

Deviation measures the degree to which raindrops cause sunlight to change direction in the process of its refraction and reflection back towards an observer. The position of raindrops in the sky and the amount of deviation determine whether the light will be visible to an observer.

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

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.

Internal reflection occurs when light travelling through a medium, such as water or glass, reaches the boundary with another medium, like air, and a portion of the light reflects back into the original medium. This happens regardless of the angle of incidence, as long as the light encounters the boundary between the two media.

• Internal reflection is a common phenomenon not only for visible light but for all types of electromagnetic radiation. For internal reflection to occur, the refractive index of the second medium must be lower than that of the first medium. This means internal reflection happens when light moves from a denser medium, such as water or glass, to a less dense medium, like air, but not when light moves from air to glass or water.
• In everyday situations, light is typically both refracted and reflected at the boundary between water or glass and air, often due to irregularities on the surface. If the angle at which light strikes this boundary is less than the critical angle, the light is refracted as it crosses into the second medium.
• When light strikes the boundary exactly at the critical angle, it neither fully reflects nor refracts but travels along the boundary between the two media. However, if the angle of incidence exceeds the critical angle, the light will undergo total internal reflection, meaning no light passes through, and all of it is reflected back into the original medium.
• The critical angle is the specific angle of incidence, measured with respect to the normal (a line perpendicular to the boundary), above which total internal reflection occurs.
• In ray diagrams, the normal is an imaginary line drawn perpendicular to the boundary between two media, and the angle of refraction is measured between the refracted ray and the normal. If the boundary is curved, the normal is drawn perpendicular to the curve at the point of incidence.

In the field of optics, dispersion is shorthand for chromatic dispersion which refers to the way that light, under certain conditions, separates into its component wavelengths, enabling the colours corresponding with each wavelength to become visible to a human observer.

• Chromatic dispersion refers to the dispersion of light according to its wavelength or colour.
• Chromatic dispersion is the result of the relationship between wavelength and refractive index.
• When light travels from one medium (such as air) to another (such as glass or water) each wavelength is refracted differently, causing the separation of white light into its constituent colours.
• When light undergoes refraction each wavelength changes direction by a different amount. In the case of white light, the separate wavelengths fan out into distinct bands of colour with red on one side and violet on the other.
• Familiar examples of chromatic dispersion are when white light strikes a prism or raindrops and a rainbow of colours becomes visible to an observer.

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.

A light source is a natural or man-made object that emits one or more wavelengths of light.

• The Sun is the most important light source in our lives and emits every wavelength of light in the visible spectrum.
• Celestial sources of light include other stars, comets and meteors.
• Other natural sources of light include lightning, volcanoes and forest fires.
• There are also bio-luminescent light sources including some species of fish and insects as well as types of bacteria and algae.
• Man-made light sources of the most simple type include natural tars and resins, wax candles, lamps that burn oil, fats or paraffin and gas lamps.
• Modern man-made light sources include tungsten light sources. These are a type of incandescent source which means they radiate light when electricity is used to heat a filament inside a glass bulb.
• Halogen bulbs are more efficient and long-lasting versions of incandescent tungsten lamps and produce a very uniform bright light throughout the bulb’s lifetime.
• Fluorescent lights are non-incandescent sources of light. They generally work by passing electricity through a glass tube of gas such as mercury, neon, argon or xenon instead of a filament. These lamps are very efficient at emitting visible light, produce less waste heat, and typically last much longer than incandescent lamps.
• An LED (Light Emitting Diode) is an electroluminescent light source. It produces light by passing an electrical charge across the junction of a semiconductor.
• Made-made lights can emit a single wavelength, bands of wavelengths or combinations of wavelengths.
• An LED light typically emits a single colour of light which is composed of a very narrow range of wavelengths.

Total internal reflection occurs when light travelling through a denser medium strikes a boundary with a less dense medium at an angle exceeding a specific critical angle. As a result, all the light is reflected back into the denser medium, and no light transmits into the second medium.

• Total Internal reflection only takes place when the first medium (where the light originates) is denser than the second medium.
• The critical angle is the angle of incidence above which total internal reflection occurs.
• The critical angle is measured with respect to the normal.
  • The normal is an imaginary line drawn in a ray diagram perpendicular to, so at a right angle to (90⁰), to the boundary between two media.