What prevents rainbows from being seen as complete circles?
Rainbows are not seen as complete circles due to a combination of two optical phenomena: refraction and reflection. To understand why rainbows appear as arcs instead of full circles, we need to consider the following:
1. Refraction: When sunlight passes through water droplets in the air, such as raindrops, it undergoes refraction. Refraction occurs when light changes speed as it moves from one medium (e.g., air) to another (e.g., water). As light enters a water droplet, it bends or refracts, causing the different colors that make up white light to separate.
2. Reflection: Once inside the water droplet, light is reflected off the inner surface of the droplet. This reflection causes the light to change direction and exit the droplet while still being refracted.
Now, let's consider how these phenomena affect the formation of rainbows:
- As sunlight enters a water droplet, it refracts and separates into its component colors. This separation is due to the varying wavelengths of different colors of light.
- After the separation, the light is reflected off the inner surface of the droplet.
- Finally, as the reflected light exits the droplet, it refracts again, causing the colors to recombine.
The key point to note is that the angle at which the light exits the droplet determines the direction in which it reaches an observer's eyes. For this reason, only a limited range of angles allows the observer to see the light and, consequently, the rainbow.
In the case of rainbows, the observer usually stands on the ground, which restricts their line of sight to an upper half-circle above the horizon. This means that you can only see the top half of the circular rainbow, resulting in the familiar arc shape. The arc is also formed because the light from different raindrops reaches the observer at slightly different angles.
It's worth noting that under certain conditions, such as when observing a rainbow from an elevated position, such as an airplane or a mountain peak, a full circle can indeed be seen. However, this is relatively rare and requires specific circumstances.
In conclusion, the combination of refraction, reflection, and the observer's position restricts our view of a rainbow, preventing it from appearing as a complete circle.