The Optics of Rainbows: PTE Listening: Fill in the Blanks practice questions

When sunlight enters a spherical raindrop, it slows down and bends through a process called refraction. This bending causes the white light to into its component wavelengths. Once the separated rays reach the back wall of the water droplet, a portion of the light undergoes internal , bouncing forward toward the front surface. As these rays emerge back into the air, they bend a second time, which increases the angular between the different colours. Observers standing with their backs to the sun perceive this concentrated light as a primary bow, with red appearing along the outer and violet along the inner edge.

Under favourable atmospheric conditions, observers may notice a fainter secondary arc positioned slightly above the main rainbow. This weaker feature occurs because light experiences two internal reflections inside each water droplet before exiting. Because of this additional bounce, the entire colour is reversed, placing red on the inside and violet on the outside. Furthermore, the region of the sky situated between the primary and secondary arcs appears noticeably than the surrounding area. Named after an ancient Greek philosopher who first described it, this dark band is created because droplets in that specific zone cannot light toward the observer at those particular angles, leaving the space largely devoid of rays.

On rare occasions, delicate pastel fringes known as supernumerary bows appear along the inner border of a bright primary rainbow. Geometric optics alone cannot account for these faint bands, requiring researchers to apply the principles of wave instead. When light rays travel along slightly different paths through tiny raindrops of uniform size, the emerging waves periodically reinforce or cancel each other out. This continuous constructive and destructive produces alternating bright and dim bands of colour. The phenomenon is most when raindrops are exceptionally small, usually less than one millimetre in diameter, because larger droplets vary too widely in size to maintain the necessary for visible patterns.