Is Gravity a Theory or a Law?

1) Pick an object that will not break, dent the floor, cause a mess, or get either of us in trouble. Hold it out in front of you and release it. What happens? It falls, of course. The gravitational attraction between the Earth and the object pulls it towards the ground. But, when we make this experiment, should we be talking about the Law of Gravity or the Theory of Gravity? Actually, we should be talking about both. To understand why, we need to understand the scientific meaning of the words «law» and «theory».

2) In the language of science, the word «law» describes an analytic statement. It gives us a formula that tells us what things will do. For example, Newton's Law of Universal Gravitation tells us that «Every point mass attracts every single point mass by a force pointing along the line intersecting both points. The force is directly proportional to the product of the two masses and inversely proportional to the square of the distance between the point masses». We can use Newton's Law of Universal Gravitation to calculate how strong the gravitational pull is between the Earth and the object you dropped, which would let us calculate its acceleration as it falls, how long it will take to hit the ground, how fast it would be going at impact, how much energy it will take to pick it up again, etc.

3) While the law lets us calculate quite a bit about what happens, notice that it does not tell us anything about why it happens. That is what theories are for. In the language of science, the word «theory» is used to describe an explanation of why and how things happen. For gravity, we use Einstein's Theory of General Relativity to explain why things fall. A theory starts as one or more hypotheses, untested ideas about why something happens. For example, we might propose a hypothesis that the object that you released fell because it was pulled by the Earth's magnetic field. Once we started testing, it would not take long to find out that my hypothesis was not supported by the evidence. Non-magnetic objects fall at the same rate as magnetic objects. Because it was not supported by the evidence, our hypothesis does not gain the status of being a theory. To become a scientific theory, an idea must be thoroughly tested, and must be an accurate and predictive description of the natural world.

4) While laws rarely change, theories change frequently as new evidence is discovered. Instead of being discarded due to new evidence, theories are often revised to include the new evidence in their explanation. The Theory of General Relativity has adapted as new technologies and new evidence have expanded our view of the universe. So, when we are scientifically discussing gravity, we can talk about the law that describes the attraction between two objects, and we can also talk about the theory that describes why the objects attract each other.

(From www.thehappyscientist.com)

The Optical Image

An optical image may be regarded as the apparent reproduction of an object by a lens or mirror system, employing light as a _(1)_. An entire _(2)_ is generally produced simultaneously, as by the lens in a camera, but images may also be generated sequentially _ (3)_, as in a television system or in the radio transmission of pictures across long distances in space. Nevertheless, the final _(4)_ of all images is invariably the human eye, and, whatever means is used to transmit and control the light, the final _(5)_ must either be produced simultaneously or scanned so _(6)_ that the observer’s persistence of vision will give him the mental impression of a complete image covering a finite field of view. For this to be effective the image must be repeated (as in motion pictures) or scanned (as in television) at least 40 times a second to eliminate _(7)_ or any appearance of intermittency.