Gravitational Lensing by a Point Mass

distance to lens in

9

10

light years

1.5

radius of circular image in arcsec

1

mass of lens in

11

10

solar masses

10

Light rays from a source passing a mass are bent due to the gravity of the mass causing the observed image to change in both shape and size. This effect is called gravitational lensing and, among other applications, is used to infer the mass of the lens. This Demonstration displays the image created by a point mass lens. The original source is a disk at a fixed distance of

9

10

light years from the observer.

The observed image depends on the mass

M

of the lens, the distance

D

between the observer and the lens, the size of the source, and the position of the source relative to the lens.

A light ray observed to be at an angle

θ

from the lens is deflected by the angle

α

:

α=4

GM

2

c

1

Dθ

,

where

G

is the gravitational constant and

c

is the speed of light.