Believe Not Only Your Eyes But Other Senses Too, Sometimes They Falter

We tend to rely on our eyes as reliable witnesses of events taking place before us. In contrast to the information conveyed by other senses like ear or nose, we trust what we see. There are glitches in this reasoning, of course. The curved mirrors in the hall of mirrors of a fair present us with our distorted images, which we know to be far from reality. Travellers across hot deserts see illusions of water reservoirs reflecting images of trees. These mirages are caused by the bending of light as it travels down from the source towards the sandy base, the bending being caused by refraction of light travelling through successively hotter, and hence less dense, layers of air. This bending ultimately leads to reflection of light and hence to the illusory presence of water.

In short, the proverbial statement that light travels in a straight line seems to have occasional contradictions. Light can bend in special circumstances leading to sights that we would not ordinarily expect to see. And the person who needs to be very careful about this situation is the astronomer.
All his evidence collected through telescopes is brought by light. If that light somehow got bent, the evidence would be distorted. Fortunately, there are very few agents known to cause such distortion through bending. Amongst those the most likely culprit would be gravity.

Isaac Newton, the originator of the law of gravitation, had indeed wondered if gravity could bend light. Will a light ray passing close to a massive body feel the gravitational attraction of that body? If it did, the light ray would bend round the body; and if it didn’t, it would continue in a straight line oblivious of the proximity of that body. Newton had no observational or experimental evidence for either possibility. As he refused to speculate, he left this problem as one of his unresolved queries.

In 1915, some two and a half centuries later, his query was answered by Einstein who had his own theory of gravity, which clearly predicted that light rays are bent by gravitational pulls. But how to verify that claim? Fortunately, there appeared on the scene Arthur Stanley Eddington, an astronomer who was one of the few scientists who could understand Einstein’s theory. Eddington proposed an experiment: photograph a star when its light ray graze the Sun and compare it with the image when the Sun is nowhere near the ray from the star. The star image should appear shifted in the former case if light ray was bent by the gravity of the Sun. The expected effect was very small, about 2,000th part of the degree used to measure angles.