# Gravitational Lensing and Redshift

hi there, I was just at a seminar in which the speaker was talking about using gravitational lensing to investigate galaxy clusters

my question is pretty simple: is there any effect on the redshift of the image of an object which is "lensed" by the gravitational lensing phenomenon? I would imagine that it would not, because all that's going on is bending of the path that the light is taking. However, I am not very well educated on the topic, and perhaps there are things that I am not taking into account.

Chronos
Gold Member
A photon is blue shifted as it enters a gravitational field, then redshifted by the same amount as it exits. Intervening matter [i.e., a lensing body] has no effect.

A photon is blue shifted as it enters a gravitational field, then redshifted by the same amount as it exits. Intervening matter [i.e., a lensing body] has no effect.

Is this because the force of the gravity on the photon as it enters the gravitational field 'pulls' the photon inward, thus shortening the wavelength and blueshifting it? And as it leaves the gravitational field, the gravity is pulling on it, which causes the wavelength to increase, thus redshifting it?

Drakkith
Staff Emeritus
Is this because the force of the gravity on the photon as it enters the gravitational field 'pulls' the photon inward, thus shortening the wavelength and blueshifting it? And as it leaves the gravitational field, the gravity is pulling on it, which causes the wavelength to increase, thus redshifting it?

Pretty much, yes.

A photon is blue shifted as it enters a gravitational field, then redshifted by the same amount as it exits. Intervening matter [i.e., a lensing body] has no effect.

thanks

m k
Gold Member
Is there enough space in local group that lensed image through one can have measurable difference of cosmological redshift compared to its sister image through "clean" space?