The Rainbow: A Physics Phenomenon

Irmary Ortiz Carlo

What is a rainbow? A rainbow can be defined as an optical and meteorological phenomenon. This phenomenon causes a spectrum of light that appear in the sky when the Sun shines onto droplets of moisture in the Earth’s atmosphere. In other words, a rainbow is an arc of spectral colors that appears in the sky opposite to the sun as a result of the refractive dispersion of sunlight in drops of rain or mist.

It takes the form of a multicolored arc. The arc that is formed is red on the outer part and violet on the inner part. A rainbow spans a continuous spectrum of different colors. The discrete bands are an effect of the human color vision. We can see that the rainbow has a sequence of colors. That sequence is red, orange, yellow, green, blue, indigo and violet.

We can see a rainbow in places like waterfalls or fountains. This effect can be artificially created. If you disperse water droplets into the air during a sunny day, you can create an artificial rainbow. It is very difficult for a person to take a picture of the complete semi-circle of a rainbow in one frame. This would require an angle of view of 84 degrees. For a 35 mm camera, a lens with a focal length of 19 mm or less wide-angle lens would be required to obtain the picture of the complete semi-circle.

You want to know how a rainbow is created? Well, the first step is: the light is refracted as it enters the surface of the raindrop, then is reflected off the back of the drop, and finally is refracted again as it goes out of the drop. The effect is that the incoming light is reflected back over a different range of angles. The most intense light is at an angle of 40–42 degrees. The angle is independent of the size of the drop. But that angle does depend on its refractive index. The seawater has a higher refractive index than the rain water refractive index. That’s why the radius of a 'rainbow' in sea spray is smaller than a true rainbow. The amount by which light is refracted depends on its wavelength, and also depends on its color. Blue light, as we know, has shorter wavelength and that’s why is refracted at a greater angle than the red light. But due to the reflection of light rays from the back part of the droplet, the blue light emerges from the droplet at a shorter angle to the original incident white light ray than the red light do.

A rainbow doesn’t appear at a specific location in the sky. Its apparent position depends on the location of the person that it is observing it and also depends on the position of the sun at that moment. All raindrops refract and reflect the light of the Sun in the same way, but only the light from particular raindrops reaches the spectator eye. This light is what constitutes the rainbow for the person that is observing it. The position of a rainbow in the sky is always in the contrary direction of the Sun with respect to the person that is observing it. The interior of the rainbow is always slightly brighter than the exterior part.

The Persian physicist, Ibn al-Haytham, tries to give a scientific explanation for the rainbow. In his work, he explained the formation of a rainbow as an image, which forms at a concave mirror. If the rays of light coming from a light source that is farther, reflect to any point on axis of the mirror, they form concentric circles in that particular point. When it is supposed that the sun is the light source, the eye of the spectator as a point on the axis of the concave mirror and a cloud as a reflecting surface, then it can be seeing the concentric circles are forming on the axis.

Rene Descartes also made some experiments. He experimented with passing rays of light through a large glass sphere that was filled with water. By taking measurements of the angles that the rays emerged, he concluded that the primary bow was caused by a single internal reflection inside the raindrop. Also he concluded that a secondary bow could be caused by two internal reflections. Descartes supported the conclusion he made with a derivation of the law of refraction and he correctly calculated the angles for the two bows. Rene Descartes explanation about the colors of a rainbow was based on a traditional theory that says that colors were produced by a modification or alteration of white light.

Also Isaac Newton made some experiments about this topic. He demonstrated that white light was composed of the light of all the colors of the rainbow. Newton uses a glass prism to separate into the full spectrum of colors, rejecting the traditional theory that the colors were produced by a modification or alteration of white light. This well known scientist also demonstrate that red light gets refracted less than the blue light. That led to the first scientific explanation of the biggest features of a rainbow.

In conclusion, as you can see a rainbow is a complete physics phenomenon. It is something that involves a lot of physics terms. Many scientists tried to explain it in different ways, but all that ways involves the physics.