Sunlight passing through raindrops causes rainbows via a process called refraction, which is the bending of light as it passes from one medium to another. This is analagous to pushing a shopping cart at the edge of a parking lot: This is because the wheels moving on the pavement are able to roll faster than the wheels on the grass.
A rainbow is an excellent demonstration of the dispersion of light and one more piece of evidence that visible light How do rainbows form composed of a spectrum of wavelengthseach associated with a distinct color.
To view a rainbow, your back must be to the sun as you look at an approximately 40 degree angle above the ground into a region of the atmosphere with suspended droplets of water or even a light mist.
Each individual droplet of water acts as a tiny prism that both disperses the light and reflects it back to your eye. As you sight into the sky, wavelengths of light associated with a specific color arrive at your eye from the collection of droplets.
But just exactly how do the droplets of water disperse and reflect the light? These are the questions that we will seek to understand on this page of The Physics Classroom Tutorial. To understand these questions, we will need to draw upon our understanding of refractioninternal reflection and dispersion.
The Path of Light Through a Droplet A collection of suspended water droplets in the atmosphere serves as a refractor of light. The water represents a medium with a different optical density than the surrounding air.
Light waves refract when they cross over the boundary from one medium to another. The decrease in speed upon entry of light into a water droplet causes a bending of the path of light towards the normal. And upon exiting the droplet, light speeds up and bends away from the normal.
The droplet causes a deviation in the path of light as it enters and exits the drop. There are countless paths by which light rays from the sun can pass through a drop. Each path is characterized by this bending towards and away from the normal.
One path of great significance in the discussion of rainbows is the path in which light refracts into the droplet, internally reflects, and then refracts out of the droplet.
The diagram at the right depicts such a path. A light ray from the sun enters the droplet with a slight downward trajectory. Other entry locations into the droplet may result in similar paths or even in light continuing through the droplet and out the opposite side without significant internal reflection.
But for the entry location shown in the diagram at the right, there is an optimal concentration of light exiting the airborne droplet at an angle towards the ground.
As in the case of the refraction of light through prisms with nonparallel sides, the refraction of light at two boundaries of the droplet results in the dispersion of light into a spectrum of colors.
The shorter wavelength blue and violet light refract a slightly greater amount than the longer wavelength red light. Since the boundaries are not parallel to each other, the double refraction results in a distinct separation of the sunlight into its component colors.
Because of the tendency of shorter wavelength blue light to refract more than red light, its angle of deviation from the original sun rays is approximately 40 degrees. As shown in the diagram, the red light refracts out of the droplet at a steeper angle toward an observer on the ground.
There are a multitude of paths by which the original ray can pass through a droplet and subsequently angle towards the ground. Some of the paths are dependent upon which part of the droplet the incident rays contact. Other paths are dependent upon the location of the sun in the sky and the subsequent trajectory of the incoming rays towards the droplet.
Yet the greatest concentration of outgoing rays is found at these degree angles of deviation. At these angles, the dispersed light is bright enough to result in a rainbow display in the sky. Now that we understand the path of light through an individual droplet, we can approach the topic of how the rainbow forms.
The Formation of the Rainbow A rainbow is most often viewed as a circular arc in the sky.One path of great significance in the discussion of rainbows is the path in which light refracts into the droplet, internally reflects, and then refracts out of the droplet.
The diagram at the right depicts such a path. A light ray from the sun enters the droplet with a slight downward trajectory. These droplets actually form a circular arc.
Rainbows form a complete circle, however only half is visible. The horizon only Picture from the web site of the Department of Physics and Astronomy, Arizona State University allows us to see half of the rainbow circle, so we see just an arc.
Jun 29, · Light refracting and reflecting through and from a raindrop. This splits the visible light spectrum into its respective color which we then see as the timberdesignmag.com: Resolved.
Check out our fun rainbow facts for kids and enjoy a range of interesting information about rainbows.
Learn about different types of rainbows, how rainbows form, what colors can we see in a rainbow, and much more. Read on and have fun learning everything you’ve ever wanted to know about rainbows.
Scientific American is the essential guide to the most awe-inspiring advances in science and technology, explaining how they change our understanding of the world and shape our lives. Rainbows appear in seven colors because water droplets break sunlight into the seven colors of the spectrum.
You get the same result when sunlight passes through a prism. The water droplets in the atmosphere act as prisms, though the traces of light are very complex.