What characteristic of light changes when it passes through a lens?

When light passes through a lens, its speed changes due to the interaction with the lens material. Light travels at different speeds in different media; it moves fastest in a vacuum and slower in materials like glass or plastic. As light transitions from air into the lens, the speed decreases because the optical density of the lens material is greater than that of air. This change in speed is critical as it allows the lens to bend or refract the light, enabling it to focus or spread out beams of light.

The key point here is the phenomenon of refraction, which occurs due to the change in speed of light when entering a medium of different density. This refraction is what allows lenses to form images, whether they are convex or concave.

Other properties of light, such as frequency, remain constant when light enters a different medium. The wavelength of light does change in accordance with the speed change, but the frequency is the defining characteristic that stays the same. Intensity may vary due to absorption or other factors, but it is not a characteristic of light that changes simply by passing through a lens.