Electromagnetic Waves & Interference of light

1. Electromagnetic Waves & Interference of light The discovery of electromagnetic waves, led us to the understanding of visible light (the light we see in all the colors of the rainbow) as a type of electromagnetic wave. Other types of EM waves exist, for example radio waves that you can use to listen to music in your car or to “look” at the far reaches of the cosmos (top right). Infrared waves are emitted by warm blooded animals. A “thermogram” (bottom right), which uses infrared radiation, can help us identify areas with irregular temperature variations, as in the inflammation near the abdomen seen in the image.

2. Producing an electromagnetic wave • A traveling electromagnetic wave produced by an ac generator attached to an antenna.

3. E and B field direction when V is positive. • Positive charges are in the top. • Current flows up. • RHR2 the B field is into the page. • In general: • E and B will be perpendicular to each other. • They are both perpendicular to the direction of propagation. • Point fingers along E • Curl fingers in the direction of B • Thumb will point in propagation direction.

4. An electromagnetic wave. • Electric and Magnetic Fields are perpendicular. • Oscillating E field produces a B field and • Oscillating B field produces an E field… • Wave only needs the E and B fields, it can travel in vacuum! • Speed of electromagnetic waves:

5. Light as an electromagnetic wave • c=constant • c=lf, so larger frequency means smaller wavelength • Visible light wavelengths • Roses are 700nm, Violets are 400nm… • What are the frequencies?

6. Electromagnetic Spectrum • Radio waves • f=106-109 Hz • Alternating Current (AC) circuits in metal antennas • Molecules and accelerated electrons in space.

7. Electromagnetic Spectrum • Microwaves • f=109-1012 Hz • Long distance phone conversations (cell phones). • Cooking (absorption of microwaves by water molecules). • RADAR • Highest frequency produces by purely electronic devices.

8. Electromagnetic Spectrum • Infrared • f=1012-4.3x1014 Hz • Can feel as heat on skin. • Some animals, such as the pit viper, can detect infrared light. • Allows to see warm blooded animals in the dark. • Emitted by warm bodies: produced by vibrations and rotations of molecules.

9. Electromagnetic Spectrum • Visible • f=4.3x1014-7.5x1014 Hz • Light that we can see. • Each frequency is interpreted by our nervous system as a different color. • Produced by electrons changing their positions within an atom • Quantum mechanics, atomic level.

10. Electromagnetic Spectrum • Ultraviolet • f=7.5x1014-1017 Hz • Skin is sensitive to UV • Sun tans (in low doses) • Skin cancer (in high doses) • Sun produces UV light, Ozone in stratosphere absorbs most of it • Reducing ozone in the stratosphere can have unwelcome consequences! • Bees are sensitive to UV, and flowers look different in UV. • The flowers help the bees to “bullseye” the pollen! • UV absorbing chemicals in plants, help bees and fend off herbivores.

11. Electromagnetic Spectrum • X-rays • f=1017-1020 Hz • Generated by the rapid deceleration of high-speed electrons. • Can pass through soft tissue • Do not pass through denser material, such as bone. • Can used as “anti-smuggling” diagnostic.

12. Electromagnetic Spectrum • g-rays • f>1020 • Generated by rearrangement of protons and neutrons in nucleus of an atom. • Also, generated by collision of particle of matter with particle of antimatter: • annihilation! • produces radiation, no “particle” with mass is left. • Supernova explosions • Gamma-ray bursts • Some of the most violent events in the universe • STILL DON’T KNOW THEIR SOURCE. • Highly penetrating and destructive to cells. • Used to treat cancer • Used to kill microorganisms in food.

13. Superposition

14. Interference: Two antennae • Point P0: midway between the antennas • the waves travel the same distance • they interfere constructively. • At point P1 the distance l2 is greater than the distance l1 by one wavelength; • P1 is also a point of constructive interference. • At Q1 the distance l2 is greater than the distance l1 by half a wavelength • waves interfere destructively

15. Young’s Two Slit Experiment • The first screen produces a small source of light that illuminates the two slits, S1 and S2. • After passing through these slits the light spreads out into an interference pattern • alternating bright and dark fringes on a distant screen.

16. Two slits act as two circular sources • According to Huygens’s principle, each of the two slits in Young’s experiment acts as a source of light waves propagating outward in all forward directions. • Light from the two sources can overlap • Result: Interference pattern.

17. Pathlength from each slit to a screen • Light propagating from two slits to a distant screen along parallel paths; • Paths make an angle q relative to the normal to the slits. • The difference in path length: d sin q • d is the slit separation.

18. Bright and dark fringes • Bright fringes • Path difference is a “whole number” of wavelengths. • Dark fringes • Path difference is a “whole number plus a half” wavelength.

19. Example 35.1 Two slit interference • Two slits with a separation of 0.200 mm create an interference pattern on a screen 1.00 m away. • If the third bright fringe above the central fringe is a linear distance of 9.49 from it, what is the wavelength of light used in the experiment?

20. Example 35.2 Broadcast Pattern • A radio station operates at a frequency of 1500 KHz. • Two identical vertical dipole antennae 400 m apart are used, and they oscillate in phase. • For distances greater than 400 m, in what direction is the intensity greatest in the resulting radiation pattern?