5.1 The Behavior of Light Flashcards
The visible light and other radiation we receive from the stars and planets is generated by processes at the ………… level—by changes in the way the parts of an …….. interact and move.
atomic / atom
Thus, to appreciate how light is generated, we must explore how atoms work.
…………, as used in this book, is a general term for waves (including light waves) that ………….. outward from a source.
Radiation / radiate
The scientist who played a role in this field comparable to Newton’s role in the study of gravity was physicist ………. ………… …………., born and educated in Scotland. Inspired by a number of ingenious experiments that showed an intimate relationship between electricity and magnetism, …………. developed a theory that describes both electricity and magnetism with only a small number of elegant equations. It is this theory that gives us important insights into the nature and behavior of light.
James Clerk Maxwell
Maxwell
Maxwell’s theory deals with these electric charges and their effects, especially when they are moving. In the vicinity of an electron charge, another charge feels a force of …………….or …………….
opposite charges ……………; like charges ………….
attraction or repulsion:
attract / repel.
In the nucleus (central part) of every atom are protons, which are positively charged; outside the nucleus are electrons, which have a negative charge.
Maxwell’s theory
When charges are not in motion, we observe only this electric attraction or repulsion. If charges are in motion, however (as they are inside every atom and in a wire carrying a current), then we measure another force called ……………..
magnetism.
Experiments with electric charges demonstrated that magnetism was the result of moving charged particles. Sometimes, the motion is clear, as in the coils of heavy wire that make an industrial electromagnet. Other times, it is more subtle, as in the kind of magnet you buy in a hardware store, in which many of the electrons inside the atoms are spinning in roughly the same direction; it is the …………… of their motion that causes the material to become magnetic.
alignment
Physicists use the word ………. to describe the action of forces that one object exerts on other distant objects. For example, we say the Sun produces a gravitational field that controls Earth’s orbit, even though the Sun and Earth do not come directly into contact. Using this terminology, we can say that stationary electric charges produce electric …………., and moving electric charges also produce magnetic …………….
field
Actually, the relationship between electric and magnetic phenomena is even more profound. Experiments showed that changing magnetic fields could produce electric currents (and thus changing electric fields), and changing electric currents could in turn produce changing magnetic fields. So once begun, electric and magnetic field changes could continue to trigger each other.
R 1
Maxwell analyzed what would happen if electric charges were …………. (moving constantly back and forth) and found that the resulting pattern of electric and magnetic fields would spread out and travel rapidly through space. Something similar happens when a raindrop strikes the surface of water or a frog jumps into a pond. The disturbance moves outward and creates a pattern we call a wave in the water
oscillating
You might, at first, think that there must be very few situations in nature where electric charges oscillate, but this is not at all the case. As we shall see, atoms and molecules (which consist of charged particles) oscillate back and forth all the time. The resulting electromagnetic disturbances are among the most common phenomena in the universe.
Maxwell was able to calculate the speed at which an electromagnetic disturbance moves through space; he found that it is equal to the speed of light, which had been measured experimentally. On that basis, he speculated that light was one form of a family of possible electromagnetic disturbances called ……….. ………
electromagnetic radiation
When light (reflected from the pages of an astronomy textbook, for example) enters a human eye, its changing electric and magnetic fields stimulate nerve endings, which then transmit the information contained in these changing fields to the brain.
But electromagnetic waves do not require water or air: the fields generate each other and so can move through a vacuum (such as outer space). This was such a disturbing idea to nineteenth-century scientists that they actually made up a substance to fill all of space—one for which there was not a single shred of evidence—just so light waves could have something to travel through: they called it the ……………
aether
The nice thing about a wave is that it is a repeating phenomenon. Whether it is the up-and-down motion of a water wave or the changing electric and magnetic fields in a wave of light, the pattern of disturbance repeats in a cyclical way. Thus, any wave motion can be characterized by a series of ………….. and ………..
crests and trough
Moving from one crest through a trough to the next crest completes one cycle. The horizontal length covered by one cycle is called the ……………
wavelength
For visible light, our eyes perceive different ………….. as different colors: red, for example, is the longest visible ……………., and violet is the shortest.
The main colors of visible light from longest to shortest …………… can be remembered using the mnemonic ……………—for _R_ed, _O_range, _Y_ellow, _G_reen, _B_lue, _I_ndigo, and _V_iolet.
wavelengths / wavelength / wavelength
ROY G BIV
We can also characterize different waves by their ………………, the number of wave cycles that pass by per second. If you count 10 crests moving by each second, for example, then the frequency is 10 cycles per second (cps).
frequency
The increase in the area that the light must cover is proportional to the ………….. of the distance that the light has traveled.
square
If we stand twice as far from the source, our eyes will intercept two-squared (2 × 2), or four times less light. If we stand 10 times farther from the source, we get 10-squared, or 100 times less light.
