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

question 85

Multiple Choice

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.