Rainbows appear curved because sunlight shines into raindrops. The sunlight then leaves the raindrop at a different angle than when it first entered, resulting in a curved shape.
Rainbows are caused by the reflection of light in raindrops, which creates a spectrum of lights. Sunlight enters the raindrop on the side closest to the sun and bends to a different angle. It then bounces off the surface of the raindrop and bends again on its way out the other side. All the angles in the raindrop must be perfect for the raindrop to emit sunlight. Only the raindrops that create the same angles are part of the rainbow. Rainbows always appear at the same angle but the height of the rainbow depends on the position of the sun in the sky.