What are electromagnetic waves?

1. What are electromagnetic waves? • Electricity can be static, like what holds a balloon to the wall or makes your hair stand on end.     • Magnetism can also be static like a refrigerator magnet. • But when they change or move together, they make waves - electromagnetic waves.

2. Electromagnetic Waves Electromagnetic waves are formed when an electric field (shown as orange arrows) couples with a magnetic field (shown as blue arrows). The magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

3. Electromagnetic Waves have different wavelengths • When you listen to the radio, watch TV, or cook dinner in a microwave oven, you are using electromagnetic waves. • Radio waves, television waves, and microwaves are all types of electromagnetic waves. • They differ from each other in wavelength.

4. Wavelength Wavelength can be quantified by measuring the distance between two peaks or two troughs. • The scientific symbol for wavelength is a Greek letter called lambda (λ).

5. Wavelength

6. Amplitude • Amplitude is a measurement of the vertical distance of the wave from the average. • Everywhere else along the wave, amplitudes are less than the maximum. • Bigger ocean waves have larger wave heights or amplitude.

7. Amplitude

8. Frequency • Frequency refers to how many waves are made per time interval usually described as cycles per second. • Usually, we use the unit Hertz to state frequency. • A frequency of 10 cps is noted as a frequency of 10 Hertz. So, one cycle per second is one Hertz. • 1 cps = 1 Hertz • The unit Hertz is abbreviated this way: • 1 Hertz = 1 Hz

9. An animation of wave functions with incrasing frequency

10. The Wave Nature of Light

11. GENERAL PROPERTIES OF ELECTROMAGNETIC RADIATION Note the trends: bluer light has shorter  , higherf, and more energy. Redder light has longer  , lowerf, and less energy.

12. Light Light is a form of electromagnetic radiation, vibrating electric and magnetic fields moving through space.

13. Some Basic Properties

14. Electromagnetic Spectrum

16. Wave Properties of Light • How interference works? • The black wave is the sum of the blue and brown ones. • When the blue one is "in phase" with the brown, they interfere constructively and the black one is larger than either. • When the blue one is "out of phase", they interfere destructively and cancel each other out; the black wave vanishes. 1. Interference

17. Wave Properties Rather than drawing the curvy lines for a wave, sometimes we just draw a straight line for the wave crest seen from the top (by G. Rieke)

18. Interference Animation Here is how the interference animation transfers to this version. (by G. Rieke)

19. Wave Properties of Light 2. Diffraction • When light encounters a barrier, such as a slit, its path bends and it can illuminate areas behind the slit that are larger than the width of the slit. • Here are water waves at a breakwater ( from J. Alward,)

20. Diffraction Animation Diffraction in action .(animation by G. Rieke)

21. Particle properties 1. Discrete energies • Photons have specific, discrete energies. 2. Isolated arrival times

22. Wave-Particle Duality • If we shine photons one at a time into the box below, we will detect them as discrete particles, each one at a specific position against the back surface. • However, if we collect a large number of photons, they will distribute themselves in the "dark" areas and avoid the "light" ones. • The "dark" areas are where the positive wavefronts overlap from the two slits letting each photon through. • Thus, they result from the combination of diffraction and interference. • Perhaps the most curious part is that each photon must pass through both slits! This experiment is called "Young's fringes" after the first scientist to do it.

23. Young's Fringes

24. Double-slit electron diffraction • The French physicist Louis de Broglie proposed in 1924 that electrons also have wave properties such as wavelength and frequency. • Later (1927) the wave nature of electrons was experimentally established by C.J. Davisson and L.H. Germer.  • To explain the idea, to others and themselves, physicists often used a thought experiment, in which Young's double-slit demonstration is repeated with a beam of electrons instead of light.

25. Double-slit electron diffraction Obeying the laws of quantum mechanics, the stream of electrons would split in two, and the smaller streams would interfere with each other, leaving the same kind of light and dark fringes.

26. Photoelectric Effect

27. Light properties are related to the temperature • As objects get hotter, they emit more and at shorter wavelengths. (animation by G. Rieke) • The curves as shown to the left are called blackbody curves – they represent the distribution over wavelength of the energy emitted by a hot object whose surface would appear perfectly black if it were cool. • Many astronomical objects radiate energy almost as though they were blackbodies.