Dispersion describes how a prism splits white light into its component wavelengths because the glass bends different colors by different amounts. The refractive index varies with wavelength, so shorter wavelengths bend more than longer ones, and the colors separate to form a spectrum. Absorption would remove some colors, not reveal them as a spread; reflection would bounce light away rather than transmit a spectrum; refraction is the general bending at a boundary, but the wavelength-dependent bending that creates the rainbow all comes from dispersion.

In thin-lens ray tracing, a key rule is that a ray passing through the optical center goes straight without bending. This happens because, in the thin-lens approximation, the two refracting surfaces are so close together that the refractions cancel, so the ray continues along its original path. This undeviated central ray serves as one of the foundational rays used to locate the image, along with the other two standard rays: a ray parallel to the principal axis that refracts through the focal point on the image side, and a ray that passes through the focal point on the object side that exits the lens parallel to the principal axis. By tracing these rays, you can determine where they intersect to find the image position.

Understanding how a converging lens handles rays parallel to the axis. For a convex (converging) lens, a ray that travels parallel to the principal axis is refracted by the lens in such a way that it passes through the focal point on the opposite side of the lens. This is a fundamental rule used in ray diagrams for thin lenses: parallel rays become focused through the far focal point. The same lens would take a ray aimed at the focal point on its near side and bend it to travel parallel to the axis after emerging from the lens. A concave lens would cause the parallel ray to diverge, appearing to come from the near focal point on the same side, not converge to the far focal point. A plane lens would not bring parallel rays to a focus, and a doublet lens is just two lenses combined—it’s the convex lens’s behavior that specifically demonstrates this parallel-to-far-focus property.

Light bends when it moves from one medium to another with a different optical density, changing speed and direction. This change at the boundary is refraction, which explains why the light path curves rather than simply bouncing off the surface, being absorbed, or scattering in many directions. A familiar example is a straw appearing bent in a glass of water—the change in medium causes the light to refract. Note that bending around an object is typically described by diffraction, but among the given options, refraction best fits the idea of the light changing direction at a boundary.