1. Waves
Unit 3.2

2. Waves of radiation differ fundamentally from water waves, sound waves, or any other waves that travel through a material medium. Radiation needs no such medium.
Waves

3. When light travels from a distant galaxy, or from any other cosmic object, it moves through the virtual vacuum of space.
Waves

4. The ability of light to travel through empty space was once a great mystery but now this concept is explained by the cornerstone of modern physics.
Waves

5. To understand more about the nature of light, consider for a moment an electrically charged particle, such as an electron or a proton.
Charged Particles

6. Like mass, electrical charge is a fundamental property of matter
Like mass, electrical charge is a fundamental property of matter. Electrons and protons are elementary particles that carry the basic unit of charge.
Charged Particles

7. Just as a massive object exerts a gravitational force on every other massive body, an electrically charged particle exerts an electrical force on every other charged particle in the universe.
Charged Particles

8. Unlike the gravitational force, which is always attractive, electrical forces can be either attractive or repulsive. Like charges form a repulsive force while unlike charges form an attractive force.
Charged Particles

9. How is the electrical force transmitted through space
How is the electrical force transmitted through space? Extending outward in all directions from any charged particle is an electric field.
Charged Particles

10. The electrical field determines the electrical force exerted by the particle on all other charged particles in the universe.
Charged Particle

11. The strength of the electric field decreases with increasing distance from the charge according to the inverse square law.
Charged Particles

12. Now suppose our particle begins to vibrate, perhaps because it becomes heated or collides with some other particle.
Charged Particles

13. Its changing position causes its associated electric field to change, and this changing field in turn causes the electrical force exerted on other charges to vary.
Charged Particles

14. Thus, information about the particle’s state of motion is transmitted through space via a changing electric field. This disturbance in the particle’s electric field travels through space as a wave.
Charged Particles

15. Electromagnetic Waves
The laws of physics tell us that magnetic field must accompany every changing electric field. Magnetic fields govern the influence of magnetic objects on one another.
Electromagnetic Waves

16. Electromagnetic Waves
The fact that a compass needle always points to the magnetic north is the result of the interaction between the magnetized needle and Earth’s magnetic field.
Electromagnetic Waves

17. Electromagnetic Waves
Magnetic fields also exert forces on moving electric charges (ie electric currents) – electric meters and motors rely on this basic fact.
Electromagnetic Waves

18. Electromagnetic Waves
Conversely, moving charges create magnetic field. In short, electric and magnetic fields are inextricably linked to one another: a change in either one necessarily creates the other.
Electromagnetic Waves

19. Electromagnetic Waves
Now, let’s consider a real cosmic object, a star. When some of the star’s charged contents move around, their electric fields change, and we can detect that change.
Electromagnetic Waves

20. Electromagnetic Waves
The resulting electromagnetic ripples propagate (travel) outward as waves through space, requiring no material medium in which to move.
Electromagnetic Waves

21. Electromagnetic Waves
Small charge particles, either in our eyes or in our experimental equipment, eventually respond to the electromagnetic field changes by vibrating in tune with the radiation that is received.
Electromagnetic Waves

22. Electromagnetic Waves
Both theory and experiment tell us that all electromagnetic waves moves at a very specific speed – 299, km/s in a vacuum. We will use 3.00 x 108 m/s.
Electromagnetic Waves

23. Electromagnetic Waves
If the currently known laws of physics are correct, then the speed of light is the fastest speed possible.
Electromagnetic Waves

24. Electromagnetic Waves
The speed of light is very large, but it is still finite. That is, light does not travel instantaneously from place to place.
Electromagnetic Waves

25. Electromagnetic Waves
This fact has some interesting consequences for our study of distant objects. It takes time for light to travel through space.
Electromagnetic Waves

26. Electromagnetic Waves
The light we see from Andromeda Galaxy left it about 2.5 million years ago. We know nothing about this galaxy how it exists today or even if it is still in existence.
Electromagnetic Waves

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