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In the last section of Einstein's paper of general relativity (1916), he deduced the curvature of light when passing massive objects. It says from Huygens' principle, the light rays must suffer a curvature of −∂γ/∂n at some point in the path (γ is the velocity). The Huygens' principle is about the propagation and interference of wave, how is it related with light rays' curvature?

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Huygens' principle relates with the curvature of light because it describes how the wavefronts of light propagate and how they can be affected by external factors such as a gravitational field.

Huygens' principle states that every point on a wavefront can be considered as a source of secondary wavelets that spread out in all directions. The new wavefront at a later time is the envelope of these secondary wavelets. In the case of light, the wavefronts can be thought of as the path followed by individual photons.

A massive object has a non-zero energy density and pressure, and this energy density and pressure cause the fabric of space-time to warp or curve. This effect is known as "gravitational warping" or "gravitational lensing". According to general relativity, the presence of mass or energy causes space-time to be curved, and this curvature in turn affects the path of any object moving through it. When light passes by a massive object, its wavefronts curve which can be described mathematically by −∂γ/∂n. Einstein field equations relate the curvature of space-time to the energy and momentum of matter and radiation within it.

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  • $\begingroup$ Thank you for your help, but why we can describe the curvature mathematically by −∂γ/∂n. How can we get it? $\endgroup$
    – user353731
    Jan 27 at 3:35
  • $\begingroup$ @user353731Answer to that is in the picture you uploaded of book page in question. If you have confusion abt any particular issue, lmk $\endgroup$
    – Haris
    Jan 27 at 3:49

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