how rainbow works
A rainbow is a meteorological phenomenon that occurs when sunlight is refracted (bent) and internally reflected inside raindrops in the atmosphere. The interaction of light with water droplets results in the formation of a spectrum of colors in the sky. The physics behind the formation of a rainbow involves several key optical principles!
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Refraction: Refraction is the bending of light as it passes from one medium (e.g., air) into another medium with a different optical density (e.g., a water droplet). When sunlight enters a raindrop, its speed and direction change due to the change in the refractive index between air and water. This bending of light is wavelength-dependent, meaning that different colors of light are bent by different amounts.
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Dispersion: Dispersion is the separation of light into its component colors based on their wavelengths. When white sunlight enters a raindrop, it is composed of a spectrum of colors ranging from violet (shortest wavelength) to red (longest wavelength). The different colors are bent by varying degrees during refraction, causing them to spread out.
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Internal Reflection: After refraction, the light undergoes internal reflection off the inner surface of the raindrop. This reflection occurs because the light strikes the inside surface of the droplet at an angle greater than the critical angle for total internal reflection.
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Secondary Refraction: Following internal reflection, the light exits the raindrop through another refraction. This refraction further separates the colors and sends them on different paths as they leave the droplet.
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Spectral Dispersion: As the separated colors exit the raindrop, they are dispersed into a circular pattern or arc. This dispersion is due to the continuous range of angles at which the colors exit the droplet.
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Formation of the Rainbow: The circular pattern of colors created by the dispersion inside the raindrop forms a complete circle. However, only a semicircle is visible to an observer on the ground because the ground blocks the lower half. The center of the circle, known as the "antisolar point," is directly opposite the position of the Sun in the sky.
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Double Rainbow: Occasionally, a secondary and fainter rainbow can be observed outside the primary rainbow. This is called a double rainbow. It is formed by an additional internal reflection within the raindrop, resulting in a second circle of colors with reversed order.
The colors of the primary rainbow (ROYGBIV - red, orange, yellow, green, blue, indigo, and violet) are the result of the dispersion of sunlight, with each color corresponding to a specific wavelength. The specific angle at which each color exits the raindrop determines its position in the rainbow.
The size of the rainbow depends on the size of the raindrops and the angle of sunlight. Smaller droplets produce a larger rainbow, while larger droplets create a smaller one. The height of the Sun in the sky also affects the rainbow's size and position.
Why Rainbow is full circle or semicircle?
The circular or semicircular shape of a rainbow is due to the way light is refracted (bent) and reflected inside raindrops in the atmosphere. Raindrops are typically spherical, but they can be somewhat flattened due to air resistance as they fall. Spherical raindrops tend to produce circular rainbows, while slightly flattened or distorted raindrops can result in semicircular rainbows.