In observations of a double-quasar image produced by gravitational lensing,fluctuations on the intensity of one image appear to be delayed compared to equivalent variations on the other image.Why is this?
A) Rotation of the lensing galaxy has delayed the beam of light on one side compared to the other because of the Doppler effect.
B) One beam has had to pass through material with a greater refractive index than the other beam within the lensing galaxy.
C) The gravitational field of the lensing object slows down the light on one beam compared to the other.
D) The optical path through the gravitational lens producing one image is longer than that for the other image.

Question

Quizplus · Accepted Answer

Gravitational lensing causes the path of light to bend as it passes through a massive object, resulting in two images of the same source being observed. However, due to the bending of the light, the path length for each image is different, resulting in a time delay between the two images. Therefore, the optical path through the gravitational lens producing one image is longer than that for the other image.

In Observations of a Double-Quasar Image Produced by Gravitational Lensing,fluctuations