Yet the greatest concentration of outgoing rays is found at these 40-42 degree angles of deviation. Other paths are dependent upon the location of the sun in the sky and the subsequent trajectory of the incoming rays towards the droplet. Some of the paths are dependent upon which part of the droplet the incident rays contact. There are a multitude of paths by which the original ray can pass through a droplet and subsequently angle towards the ground. As shown in the diagram, the red light refracts out of the droplet at a steeper angle toward an observer on the ground. 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. The angle of deviation between the incoming light rays from the sun and the refracted rays directed to the observer's eyes is approximately 42 degrees for the 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. The shorter wavelength blue and violet light refract a slightly greater amount than the longer wavelength red light. 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. 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. 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. Upon refracting twice and reflecting once, the light ray is dispersed and bent downward towards an observer on earth's surface. A light ray from the sun enters the droplet with a slight downward trajectory. The diagram at the right depicts such a path. 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. Each path is characterized by this bending towards and away from the normal. There are countless paths by which light rays from the sun can pass through a drop. The droplet causes a deviation in the path of light as it enters and exits the drop. And upon exiting the droplet, light speeds up and bends away from the normal. The decrease in speed upon entry of light into a water droplet causes a bending of the path of light towards the normal. Light waves refract when they cross over the boundary from one medium to another. The water represents a medium with a different optical density than the surrounding air. To understand these questions, we will need to draw upon our understanding of refraction, internal reflection and dispersion.Ī collection of suspended water droplets in the atmosphere serves as a refractor of light. But just exactly how do the droplets of water disperse and reflect the light? And why does the pattern always appear as ROYGBIV from top to bottom? These are the questions that we will seek to understand on this page of The Physics Classroom Tutorial. The net effect of the vast array of droplets is that a circular arc of ROYGBIV is seen across the sky. As you sight into the sky, wavelengths of light associated with a specific color arrive at your eye from the collection of droplets. Each individual droplet of water acts as a tiny prism that both disperses the light and reflects it back to your eye. 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. A rainbow is an excellent demonstration of the dispersion of light and one more piece of evidence that visible light is composed of a spectrum of wavelengths, each associated with a distinct color. One of nature's most splendid masterpieces is the rainbow.
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