Why does a primary rainbow form a curved arc rather than a straight band?

Answer: Drops send each color toward your eye at a particular angle from the direction opposite the Sun

Drops send each color toward your eye at a particular angle from the direction opposite the Sun ✓ — Each color reaches your eye from drops lying at a characteristic angle around the direction opposite the Sun. For the red edge of a primary rainbow, this angle is about 42 degrees. Those directions form a circle, but the ground often hides part of it, leaving an arc rather than always an exact semicircle.

Gravity bends sunlight downward into a visible arch — Earth’s gravity does not bend sunlight into an ordinary rainbow. Refraction and internal reflection in raindrops send colors toward your eye at the angles that produce the arc.

Raindrops fall along matching curved paths through the sky — The rainbow’s curve does not trace the paths of falling raindrops. Many different drops send light toward your eye, and the directions from which each color reaches you form a circle.