Hello, This has been bugging me for some time now, so I would be interested to see what I have been missing so far. Imagine a single ray of light (made up of many photons) hitting a perfectly non-absorbing (for this wavelength of light) spherical dielectric object, which has finite mass. The outside medium is a vacuum - i.e. where the photons are coming from - and there is no influence of gravity, friction etc. The object has some refractive index that is not 1, and the ray hits it at some angle that is not zero with respect to its axis of symmetry (i.e. it doesn't go through the centre of the sphere). The ray enters and exits the object (although a single interaction is enough to demonstrate my point). The result would be a change in direction of the ray with respect to its original path, and therefore a change in momentum. A force is required to do this (one exerted by the object's reactionary electromagnetic field, caused by the oscillating electrons in the atoms it consists of). There must also be an opposite force exerted on the particle, which would then start to move in that direction. As far as I am aware, the scattered field of the ray, whilst having had its direction changed, will have the exact same magnitude of momentum, and therefore the exact same energy as it did before the interaction with the object. However, the object has has work done on it, therefore it seems to me like energy has been created from nowhere, which cannot be the case. If anyone has any insight as to what I have not thought of in this scenario, I would greatly appreciate the help! Thanks in advance for your time!