Question

Why does the rainbow form an arc? Why does it not go straight?

Answer 1

A rainbow is defined as an arch in the sky made up of various hues generated when the sun's rays are refracted by raindrops, or any show of various colors formed by the spectrum of light, or any display of brilliant or colorful items.
A rainbow is formed when a rainy day clears up and the sun's rays catch on water drops in the atmosphere, forming a red, orange, yellow, green, blue, indigo, and violet arch. A rainbow of colors can be seen in a grocery store produce aisle with many different colored fruits and vegetables.

Complete step by step solution:
When sunlight strikes a raindrop, the red-light waves are bent at a $42$-degree angle from the sun's initial direction. Violet light waves that are shorter are only bent at a $40$-degree angle. Because of this symmetry, the raindrop seems to be symmetrical when viewed along a line of sight that forms a $40$- to $42$-degree angle with the sun's direction. As a result, we see the rainbow as an arc rather than a straight line.
Explanation:
Refraction, or the bending of light as it moves from one medium to another, generates rainbows as sunlight passes through raindrops. This is similar to wheeling a shopping cart along the edge of a parking lot: if one of the wheels rolls off the pavement onto adjacent grass, the cart will begin to turn toward the grass. This is because wheels on the pavement can roll quicker than wheels on the grass.
When sunlight strikes a raindrop, it does not travel as quickly through the water as it does through the atmosphere, thus it bends somewhat. The light then reverses direction when it exits the raindrop and returns to its previous speed in the air. Light is refracted through a raindrop and into our eyes when it strikes the rain at the proper angle, resulting in a rainbow.
But how does "white" sunshine produce a rainbow of colors? Sunlight, sometimes known as "white" light, is made up of continuous bands of various colored light, including red, orange, yellow, green, blue, indigo, and violet. Each color has a unique wavelength, or frequency, that refracts somewhat differently while passing through different mediums. As a result, when white light is transmitted through a specific media, it can be broken up into its component colors. A prism, for example, can produce rainbows because, like a raindrop, the glass bends the different hues of light at slightly different angles.
Longer wavelengths are bent at larger angles than shorter wavelengths, thus they are bent less. When sunlight strikes a raindrop, the red-light waves are bent at a $42$-degree angle from the sun's initial direction. Violet light waves that are shorter are only bent at a $40$-degree angle. Between these two, the other colored light frequencies are bent at an angle. This explains why rainbows appear as a continuous band of colors with red at the top and violet at the bottom.
Because a raindrop is spherical, its effect on sunlight is symmetrical around an imaginary axis linking the drop's center and the sun. Because of this symmetry, the various colors of light will be seen as long as the raindrop is viewed along a line of sight that makes a $40$- to $42$- degree angle with the direction of the sun. A rainbow is thus a circle with an angular radius of $42$degrees, centered on the point precisely opposite the sun from the observer—the so-called antisolar point.

Note: The earth comes in the way and prevents us from seeing the whole circle. We can see more of the circle as the sun gets closer to the horizon. We'd witness a full semicircle of a rainbow right around sunset, with the arch's tip $42$ degrees above the horizon. A rainbow is less visible above the horizon the higher the sun is in the sky.