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How does gravitational redshift show that gravity affects light?

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gravitational lens — gravity A galactic cluster, about five billion light-years away, produces a tremendous gravitational field that “bends” light around it. This lens produces multiple copies of a blue galaxy about twice as distant. Four images are visible in a circle surrounding the lens; a fifth is visible near the center of the picture, which was taken by the Hubble Space Telescope.

Gravitational redshift is a phenomenon predicted by general relativity that demonstrates how gravity influences light. Because the speed of light remains constant, light cannot lose energy by slowing down when it escapes a gravitational field. Instead, it must expend energy to work against that field, which it does by decreasing its frequency.12

Since the energy of light is defined by the equation $E = h ν$ (where $h$ is Planck’s constant and $ν$ is frequency), a decrease in frequency corresponds to a shift toward longer, redder wavelengths—hence the term "redshift".1

This effect confirms that light is subject to gravitational forces, effectively behaving as if it possesses an equivalent mass of $m = h ν / c^{2}$ . This principle has been verified through observations of white dwarf stars and precise laboratory experiments, such as the 1960 measurement of gamma rays by Robert Pound and Glen Rebka.21