# How is speed of light constant if it bends? [duplicate]

Gravitational lensing causes light to be bent.If light changes direction how can it be travelling with constant speed?Where am I wrong.Thanks for any help.

• See my answer to this question and the comments therein: physics.stackexchange.com/questions/219690/… Dec 3, 2015 at 15:25
• @tmwilson26 Ok I understand but when light is bending due to gravitational pull,I feel that the resultant (vector) force would cause the motion of the light and it would definitely be less than the original... Dec 3, 2015 at 15:31
• You have to be careful how you treat light in this context. It bends because of the curvature of space and maintains a constant speed in all reference frames. Just because the direction has changed, doesn't mean that the speed has changed. Dec 3, 2015 at 15:35
• Speed is a scalar. Speed and direction together form a vector. Dec 3, 2015 at 16:02
• tatan : see this answer, follow the references and note what Einstein said: "As a simple geometric consideration shows, the curvature of light rays occurs only in spaces where the speed of light is spatially variable". Light bends because the speed of light varies. Also note this question. John Rennie said "if I measure the speed of light at some point that is distant from me I will generally get a value different from c". Dec 3, 2015 at 20:50