Does Light Weigh Anything?
To get at the answer to this question we must go back to the definition of light. Light is a wave in the ether and contains no particles of matter. It, therefore, does not weigh anything at all.
When men had studied light thoroughly, however, they came to the conclusion that it must have the power of pressure, which, from the standpoint of results, would amount to the same thing as having weight. They reasoned that if you had a perfect balance and let sunlight shine down on one of the sides of the balance, that side should go down under the pressure of light. In their first experiments along this line men failed to show that under such conditions the side of the balance on which the light shone did go down, but by continuous experiments it was proved finally that the light did exert a sufficient pressure to cause the scales to go down, and in effect this is the same as having weight; but this has been found to be a common property of rays of various kinds, including heat, and we, therefore, do not speak of this quality as weight, but as the power of radiating pressure.