Formation of Rainbows
Rainbows are formed by the reflection, refraction and the dispersion of light. Water droplets have a different refractive index as compared to air. As a result, the water droplets in the air would act like prisms. Sunlight consists of a spectrum of colours and the water droplets disperses these colours.
n ≈ 1.333
(average refractive index across the visible spectrum)
In addition to that, water droplets behave similarly to mirrors. Light undergoes internal reflection and the angle of the resultant beam is between 40° to 42° from the incident beam.
Light from the sun is reflected in all directions by the water droplets. An observer looking at a rainbow can be said to be located at the apex of a cone. A rainbow would appear as a ring. However, due to limited vision caused by the horizon, the rainbow would usually appear to be semicircle in shape instead.
Rainbows are formed by the reflection, refraction and the dispersion of light. Water droplets have a different refractive index as compared to air. As a result, the water droplets in the air would act like prisms. Sunlight consists of a spectrum of colours and the water droplets disperses these colours when the sunlight hits the droplets.
The formation of indoor rainbow is similar to the formation of outdoor rainbow. A light source and a water source are the fundamental to creating a rainbow. LED, tungsten and HMI are common light sources.
LED as Light Source
LEDs are made by a process called MOCVD (Metal organic chemical vapour decomposition). Coloured LED lights are not single wavelength emitters. They emit light over a band of wavelength. They have active region that is usually GaN based and doped with a metal such as In (Indium), which give a blue colour and Sb (arsenic), which gives a red colour. A typical LED white light is obtained by a mixture of the Red, Green and Blue LED mixture. Hence, if the mixing of colours is not perfect, there will be a shift in the colour of light emitted from the LED light source.
Currently the most popular method of producing LED bulbs is not by the convectional mixture of red, green and blue LED, but just mixing blue and yellow light to produce white light. Based on CIE diagram, we can predict the amount of blue or yellow to add to get a desired colour.
Tungsten as a Light Source
The tungsten filament of a vacuum incandescent lamp is heated to temperatures where visible light is emitted by resistance heating. The filament acts as an electrical resistor, which dissipates power proportional to the voltage applied, times the current through the filament. Tungsten halogen lamps on the other hand are ideal light source as they are able to provide a broad band of spectral radiation ranging from the ultraviolet through the visible and into the infrared region, unlike the LED which is only able to produce a few selected regions of wavelength of visible light. However, tungsten bulbs are inefficient as the evaporated tungsten particles tent to deposit on the glass and reduce the output of the light.
Hydrargyrum medium-arc iodide (HMI) as light source
HMI Lamps uses an arc lamp instead of an incandescent bulb to produce light. HMI lights require a ballast, an electronic or a magnetic device which provides the ignition pulse and regulates the arc. This causes a flickering effect. Electronic ballast has a flicker free mode. This mode converts the sine wave of the A.C. power supply into square waves so that the arc does not extinguish at any point of the cycle. HMI are 2 to 5 times more efficient than incandescent light and they only operate at around 5600k, daylight temperature.