Tryout TOEFL ITP Reading Comprehension Part 3
TOEFL Reading Test
Passage 1: The Process of Rainbow Formation
A rainbow is one of the most fascinating atmospheric optical phenomena. It occurs as a result of the complex interaction between sunlight and water droplets in the atmosphere. After rainfall, the air is often filled with tiny suspended water droplets, creating ideal conditions for a rainbow to form. Under certain circumstances, sunlight striking these water droplets undergoes several optical processes, namely refraction, internal reflection, and subsequent refraction as it exits the droplet.
The process of rainbow formation begins when sunlight enters the water droplet and undergoes its first refraction. This occurs because of the change in medium from air to water, which has a higher refractive index. As a result of this refraction, white light, which is composed of various wavelengths, starts to separate into its component colors. This phenomenon is known as light dispersion, where each color of light is refracted at a different angle according to its wavelength.
After the initial refraction, the light that has been dispersed is internally reflected off the inner surface of the droplet. This reflection causes the light to remain inside the droplet temporarily. Subsequently, the light undergoes a second refraction as it exits the droplet and enters the air. During this process, the separation of colors becomes more distinct, with each color exiting at different angles.
As a result of this refraction and reflection, a spectrum of colors is visible to the observer. Red, which has the longest wavelength, appears on the outer edge of the rainbow, while violet, with the shortest wavelength, appears on the inner edge. The common sequence of colors observed in a rainbow is red, orange, yellow, green, blue, indigo, and violet. Each color appears due to light exiting the droplet at a specific angle, typically around 42 degrees from the direction of the incoming sunlight.
A rainbow usually appears as a semi-circle because of this angular arrangement. When the observer is at a higher vantage point, such as in an airplane, a full circular rainbow may be visible. In addition to primary rainbows, there are secondary rainbows that sometimes appear outside the primary rainbow. A secondary rainbow is formed due to two internal reflections within the water droplet, resulting in a fainter spectrum with the color sequence reversed.
The phenomenon of the rainbow not only represents natural beauty but also illustrates fundamental principles of optical physics. Through an understanding of refraction, reflection, and light dispersion, we can appreciate how the simple combination of sunlight and water droplets can create such a visually stunning display. A rainbow serves as a tangible reminder of how natural laws can generate extraordinary beauty in the world around us.
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