What does gravitational redshift imply for light escaping from a strong gravitational field near a black hole?

• A. Light gains energy and shifts toward shorter wavelengths (blueshift).
• B. Light loses energy and shifts toward longer wavelengths (redshift).
• C. Light's wavelength remains unchanged.
• D. Light increases in speed but wavelength stays the same.

Answer: (B)

Gravitational redshift shows how light loses energy when escaping gravity. In a strong gravitational field, clocks run slower deeper in the gravity well, and a photon's energy is tied to its frequency via E = h f. As photons climb out of a deep gravitational well, they must expend energy against gravity, so their frequency decreases and their wavelength increases (since λ = c/f). This is why light escaping near a black hole is observed with a redshift. The speed of light in vacuum remains locally constant, so saying the speed increases isn’t correct; the observable change is in energy and wavelength. A blueshift would occur for light falling deeper into the gravity well, not for light escaping from it, and a wavelength staying the same would ignore the energy exchange with gravity.