class19lecnotes-1 - Class 19 Wednesday Reading 32.1 32.2...

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24, 2010 Reading: 32.1, 32.2 The Interference of Light Last class we discussed some of the properties of light. In particular. we described what our classroom would look like in each wavelength of the electromagnetic spectrum. It would be good for you to get a sense of a typical wavelength for each type of electromagnetic waves (see page 7. Class 18). In your book you can find this information in section 29.6. We discussed that light, electromagnetic waves, do not need the presence of a medium in order to propagate. They can propagate through vacuum. This is because the wave propagation occurs through the mutual regeneration of electric and magnetic fields off each other via a process called induction. You will study this in your next physics course. Hence, the quantities that oscilJate in light are electric and magnetic fields. Due to this stunning property, light can travel billions oflight years through empty space. We talked about ourselves as light bulbs, emitters of mostly infrared radiation. The heat radiated by your body is transmitted across the classroom to your neighbors, to the seats, etc. This infrared radiation that we call heat is produced by virtue of charges in your body's molecules, charges that vibrate and thus emit electromagnetic radiation. Once set up, this radiation is free to propagate through the room. It would do so, even if we evacuated the classroom of all its air. While my voice could not get carried across the room in vacuum, the heat of our bodies does. This amazing property of light allows us to study our Universe and its fascinating constituent objects. Virtually everything we know about our Universe stems from the study of light. We can infer the composition of a star's atmosphere, or a planet'S surface, the properties of the tenuous medium between the stars, and we can study the beginning of our Universe by studying its light How exactly did light help us find this wealth of infonnation? In order to answer this question, let's go back to the time of Newton. Newton was a strong proponent of the theory that light is made of particles. He argued that if light were a wave, then one should be able to see it undergo diffraction just as sound or water waves do. Diffraction is the spreading of a wave around obstacles as seen in the figure on the next page. It makes it for example possible for sound to spread around the comers, so you can hear it even if you are not in the same room as its source. If, on the other hand, light underwent diffraction, the shadows cast by objects could not be sharp. Newton thus rejected the wave theory of light to which his contemporaries, Huygens and Hooke, adhered. In absence of a theory of electromagnetism, they believed that light-waves propagated through a medium called ether, whose existence was ultimately experimentally disproved in the late Igm century. Today, we understand that light has a dual nature: It sometimes displays a particle nature, and sometimes a wave nature. Phenomena such as the diffraction and the interference
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This note was uploaded on 03/16/2010 for the course PHYS 6B taught by Professor Graham during the Winter '08 term at UCSC.

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class19lecnotes-1 - Class 19 Wednesday Reading 32.1 32.2...

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