Does the amount that a gravitational lens bends light depend on wavelength? Also, how do we know that redshifted objects aren't just intrinsically red objects?

Hi Karl,

>But if the gravity lens is bending the light from the distant galaxy,
>similar to the way a lens bends light, how do we know we're not just
>seeing the red end of a defracted light source?

Diffraction happens when light waves bend around an obstacle.
Refraction happens when light waves change direction on moving from
one substance into another, e.g. from air into the glass of a lens.
Waves of different wavelengths, i.e. different colors, are bent by
different amounts.

In the case of a gravitational lens, light waves change direction as
they pass near the foreground galaxies, because the galaxies' mass
bends space nearby. The effect doesn't depend on the light's
wavelength.

The Hubble Space Telescope is only used to find the lensed object.
The object's redshift is actually measured by photographing its
spectrum using the Keck telescope in Hawaii. Spectral lines that
would normally appear in the ultra-violet are seen in the red and
infra-red. This means the object must be so far out across the
expanding universe that it's moving away from us at a good fraction of
the speed of light.

If the object were reddened by refraction instead, then the red
spectral lines would be in their usual places, and the blue ones would
just be missing.

Best wishes,
-Neal.

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