1. Chapter 3 - Radiation

2. Section 3.1 Information from the Skies

3. LIGHT AND RADIATION
Given the impossibility of traveling to such remote parts of the universe, how do astronomers know anything about objects far from Earth? How do we obtain detailed information about any planet, star, or galaxy too distant for a personal visit or any kind of controlled experiment? The answer is that we use the laws of physics, as we know them here on Earth, to interpret the electromagnetic radiation emitted by these objects.

4. Radiation is any way in which energy is transmitted through space from one point to another without the need for any physical connection between those two locations.
The term electromagnetic just means that the energy is carried in the form of rapidly fluctuating electric and magnetic fields.
Despite the different names, the words light, rays, radiation, and waves really all refer to the same thing. The names are just historical accidents, reflecting the fact that it took many years for scientists to realize that these apparently very different types of radiation are in reality one and the same physical phenomenon.

5. WAVE MOTION
Simply stated, a wave is a way in which energy is transferred from place to place without physical movement of material from one location to another.
In wave motion, the energy is carried by a disturbance of some sort.
This disturbance, whatever its nature, occurs in a distinctive repeating pattern.
Ripples on the surface of a pond, sound waves in air, and electromagnetic waves in space, despite their many obvious differences, all share this basic defining property.

6. Imagine a twig floating in a pond
Imagine a twig floating in a pond. A pebble thrown into the pond at some distance from the twig disturbs the surface of the water, setting it into up-and-down motion. This disturbance will propagate outward from the point of impact in the form of waves.
When the waves reach the twig, some
of the pebble's energy will be imparted to
it, causing the twig to bob up and down.
In this way, both energy and information —
the fact that the pebble entered the
water—are transferred from the place
where the pebble landed to the location
of the twig. We could tell that a pebble
(or, at least, some object) had entered the
water just by observing the twig.
With a little additional physics, we could
even estimate the pebble's energy.

7. We characterize waves not only by the speed with which they move but also by the length of their cycle.
How many meters does it take for the wave to repeat itself at a given moment in time? This is its wavelength, defined as the length of an individual wave cycle. The wavelength can be measured as the distance between two adjacent wave crests, two adjacent wave troughs, or any other two similar points on adjacent wave cycles.
The maximum departure of the wave from the undisturbed state—still air or the flat pond surface—is called its amplitude.

8. 1 wave frequency = wave period
If a wave moves at high speed, then the number of crests or cycles passing any given point per unit time—the wave's frequency—is high. Conversely, if a wave moves slowly, with only a few crests passing per unit time, we say that it has a low frequency. The frequency of a wave is just 1 divided by the wave's period: 1
wave frequency =
_______________
wave period

9. Wavelength and wave frequency are inversely related
Wavelength and wave frequency are inversely related. Doubling the frequency halves the wavelength, halving the frequency doubles the wavelength, and so on. This inverse relationship is easily understood. For a given wave speed, if the wave crests are close together, then more of them pass by a given point each second; when the crests are far apart, few of them pass by per unit time. The wave speed is simply the product of the wavelength and the frequency:

10. DIFFRACTION AND INTERFERENCE
Light exhibits two key properties that are characteristic of all forms of wave motion: diffraction and interference. Diffraction is the deflection, or "bending," of a wave as it passes a corner or moves through a narrow gap.
Figure 3.4 Diffraction of a light wave. (a) If radiation were composed of rays or particles moving in perfectly straight lines, no bending would occur as a beam of light passed through a circular hole in a barrier, and the outline of the hole, projected onto a screen, would have perfectly sharp edges. (b) In fact, light is diffracted through an angle that depends on the ratio of the wavelength of the wave to the size of the gap. The result is that the outline of the hole becomes "fuzzy," as shown in this actual photograph of the diffraction pattern produced by a small circular opening.

11. Interference is the ability of two or more waves to reinforce or cancel each other.
Figure 3.5 Interference of two identical waves: (a) constructive and (b) destructive. In constructive interference, the two waves (of amplitude a) reinforce each other to produce a larger-amplitude wave of the same wavelength. However, in destructive interference, the two waves exactly cancel out.

12. Section 3.2 - What is in Waves

13. Electrons are said to carry a negative charge, whereas protons carry an equal and opposite positive charge. Just as a massive object exerts a gravitational force on any other massive body (as we saw in Chapter 2), an electrically charged particle exerts an electrical force on every other charged particle in the universe.
Buildup of electrical charge (a net imbalance of positive over negative, or vice versa) is what causes "static cling" on your clothes when you take them out of a hot clothes dryer, or the shock you sometimes feel when you touch a metal door frame on a particularly dry day.

14. Figure 3.6 (a) Particles carrying like electrical charges repel one another, whereas particles carrying unlike charges attract. (b) A charged particle is surrounded by an electric field, which determines the particle's influence on other charged particles. We represent the field as a series of field lines. (c) If a charged particle begins to vibrate back and forth, its electric field changes. The resulting disturbance travels through space as a wave.

15. ELECTROMAGNETIC WAVES
The laws of physics tell us that a magnetic field must accompany every changing electric field. Magnetic fields govern the influence of magnetized objects on one another, much as electric fields govern interactions between charged particles. The fact that a compass needle always points to magnetic north is the result of the interaction between the magnetized needle and Earth's magnetic field.
Figure 3.7 Earth's magnetic field interacts with a magnetic compass needle, causing the needle to become aligned with the field.

16. The disturbance produced by our moving charge actually consists of vibrating electric and magnetic fields, always oriented perpendicular to one another and moving together through space. These fields do not exist as independent entities; rather, they are different aspects of a single physical phenomenon: electromagnetism. Together, they constitute an electromagnetic wave that carries energy and information from one part of the universe to another.
Figure 3.8 Electric and magnetic fields vibrate perpendicular to each other. Together they form an electromagnetic wave that moves through space at the speed of light.

17. Now consider a real cosmic object—a star, say
Now consider a real cosmic object—a star, say. When some of its charged contents move around, their electric fields change, and we can detect that change. The resulting electromagnetic ripples travel outward in waves, requiring no material medium in which to travel. Small charged particles, either in our eyes or in our experimental equipment, eventually respond to the electromagnetic field changes by vibrating in tune with the received radiation. This response is how we detect the radiation—how we see.

18. Both theory and experiment tell us that all electromagnetic waves move at a very specific speed—the speed of light (always denoted by the letter c).
We will round this value off to c= 3.00   105 km/s. This is an extremely high speed. In the time needed to snap your fingers (about a tenth of a second) light can travel three quarters of the way around our planet! If the currently known laws of physics are correct, then the speed of light is the fastest speed possible.

19. The speed of light is very large, but it is still finite
The speed of light is very large, but it is still finite. That is, light does not travel instantaneously from place to place. This fact has some interesting consequences for our study of distant objects. It takes time—often lots of time—for light to travel through space. The light we see from the nearest large galaxy—the Andromeda Galaxy, shown in Figure 3.1—left that object about 3 million years ago—around the time our first human ancestors appeared on planet Earth. We can know nothing about this galaxy as it exists today. For all we know, it may no longer even exist! Only our descendants, 3 million years into the future, will know if it exists now. So as we study objects in the cosmos, remember that the light now seen left those objects long ago. We can never observe the universe as it is—only as it was.