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What is Physics?. First of all, Physics is a Science. So our first question should be: What is a Science?. Science. What is a science? Physics is a science. Biology is a science. Is Psychology a science? Is Political Science a science? Is English a science?

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What is Physics?


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    1. What is Physics? First of all, Physics is a Science. So our first question should be: What is a Science?

    2. Science • What is a science? • Physics is a science. Biology is a science. • Is Psychology a science? • Is Political Science a science? • Is English a science? • What makes a field of inquiry into a science?

    3. Scientific Method What makes a field of inquiry into a science? • Any field that employs the scientific method can be called a science. • So what is the Scientific Method? • What are the “steps” to this “method”?

    4. Scientific Method • 1. Define the “problem”: what are you studying? • 2. Gather information (data). • 3. Hypothesize (try to make “sense” of the data by trying to guess why it works or what law it seems to obey). This hypothesis should suggest how other things should work. So this leads to the need to: • 4. TEST, but this is really gathering more information (really, back to step 2).

    5. Scientific Method Note one thing about step 3: the predictive power of the hypothesis gives us something else to look for. We are in essence trying to extend our common sense to areas in which we initially have little common sense.

    6. Scientific Method Fascinating Question Is the scientific method really a never ending loop, or do we ever reach “THE TRUTH”?

    7. Scientific Method Is the scientific method really a never ending loop, or do we ever reach “THE TRUTH”? Consider: can we “observe” or “measure” perfectly? If not, then since observations are not perfect, can we perfectly test our theories? If not, can we ever be “CERTAIN” that we’ve reached the whole “TRUTH” ?

    8. Scientific Method If we can’t get to “THE TRUTH”, then why do it at all? We can make better and better observations, so we should be able to know that we are getting closer and closer to “THE TRUTH”. Is it possible to get “close enough”? Look at our applications (engineering): is our current understanding “good enough” to make air conditioners?

    9. Physics Now Physics is a science, but so are Chemistry and Biology. How does Physics differ from these others? It differs in the first step of the method: what it studies. Physics tries to find out how things work at the most basic level. This entails looking at: space, time, motion (how location in space changes with time), forces (causes of motion), and the concept of energy.

    10. Scientific Method and Light To try to show the scientific method in action, we’ll look at light.

    11. Light What is it?

    12. Light • What is it? Moving energy • There are two basic ways that energy can move from one place to another: particles can carry the energy, or the energy can propogate in waves. • Can light be explained as a wave or as a particle?

    13. Light • What is it?Moving energy • Wave or particle? How do we decide?

    14. Light • What is it? Moving energy • Wave or particle? How do we decide? • If a wave, what is waving? (waving even in a vacuum?)

    15. Light • What is it? Moving energy • Wave or particle? How do we decide? • If a wave, what is waving? (waving even in a vacuum?) Electric & Magnetic Fields

    16. Properties of Light • speed of light • colors • reflection • refraction (bending) • shadows • energy theory • absorption of light • emission of light

    17. Property 1: Speed of Light • particle (photon) prediction?

    18. Property 1: Speed of Light • particle (photon) ? no prediction • wave (E&M) prediction?

    19. Property 1: Speed of Light • particle (photon) ? no prediction • wave (E&M) ? For a wave on a string, we can start from Newton’s Second Law and get a wave equation that leads to the relation: vphase = [T/] (speed of wave depends on parameters of the string the wave travels on - T is tension in the string and  is the mass density of the string)

    20. Property 1: Speed of Light • particle (photon) ? no prediction • wave (E&M) ?Maxwell’s Eqs. In a similar way to the wave on a string, we can get a wave equation from Maxwell’s Eqs for Electromagnetism. This predicts: vphase = [1/oo] where the o and o are the electric and magnetic properties of vacuum.

    21. Property 1: Speed of Light • particle (photon) ? no prediction • wave (E&M) ? Maxwell’s Eqs. in vacuum: v = [1 / {o o}]1/2 where o = 1/{4k} = 1 / {4 * 9x109 Nt-m2/Coul2} o = 4 * 1x10-7 T-s /Coul v= [4*9x109 / 4*1x10-7 ]1/2 = 3 x 108 m/s = c

    22. Property 1: Speed of Light • particle (photon) ? no prediction • wave (E&M) ? Maxwell’s Eqs. in material, vphase = [1/oo]  = Ko , where K>1; and   o ; so v < c According to the wave theory, light should move slower in material than in vacuum.

    23. Property 1: Speed of Light • particle (photon) ? no prediction • wave (E&M) ? in vacuum, v = c; in material, v < c we’ll come back to this when we look at refraction.

    24. Property 2: Color • experiment ? • particle (photon) ? • wave (E&M) ?

    25. Property 2: Color experiment ? visible order: • red • orange • yellow • green • blue • violet

    26. Property 2: Color experiment ? invisibleas well as visible total spectrum order: • radio • microwave • IR • visible • UV • x-ray and gamma ray

    27. Property 2: Color particle (photon) ? amount of energy per photon determines “color”

    28. Property 2: Color particle (photon) ?amount of energy among different types: x-ray - most energy;radio - least in visible portion: violet - most energy; red - least

    29. Property 2: Color • particle (photon) ? amount of energy • wave (E&M) ?

    30. Property 2: Color • particle (photon) ? amount of energy • wave (E&M) ? frequency among different types of “light”: low frequency is radio(AM is 500-1500 KHz) high frequency is x-ray & gamma ray in visible spectrum: red is lowest frequency (just above IR) violet is highest frequency (just below UV)

    31. Colors: frequencies & wavelengths (in vacuum) AM radio  1 MHz100’s of m FM radio  100 MHz m’s microwave  10 GHzcm - mm Infrared (IR)1012 - 4x1014Hzmm - 700 nm visible 4x1014 - 7.5x1014700nm -400nm Ultraviolet (UV) 7.5x1014 - 1017400 nm - 1 nm x-ray &  ray > 1017 Hz< 1 nm [This slide will be repeated after we see how we get these values.]

    32. Property 3: Reflection • particle (photon) ? • wave (E&M) ?

    33. Property 3: Reflection • particle (photon) ?bounces “nicely” • wave (E&M) ? bounces “nicely” bounces nicely means: angle incident = angle reflected

    34. Property 4: Refraction experiment ? particle (photon)? wave (E&M) ?

    35. Property 4: Refraction • experiment: objects in water seem closerthan they really are when viewed from air eye air water apparent location real object

    36. Property 4: Refraction • particle (photon) ? incident ray air surface water refracted ray

    37. Property 4: Refraction • particle (photon) ? incident ray vxi = vxr vxi air vyi < vyr vyi surface therefore vi < vr water vxr vyr refracted ray

    38. Property 4: Refraction normal line • wave (E&M) ? incident wave air surface surface water refracted wave normal line

    39. Property 4: Refraction crest of following wave • wave (E&M) ? crest of wave incident wave crest of preceding wave air a a x surface w water w refracted wave normal line

    40. Property 4: Refraction • wave (E&M) ? + = 90o + = 90o crest of wave incident wave crest of preceding wave air a sin() = a /x x surface sin() = w /x w water refracted wave normal line

    41. Property 4: Refraction • wave (E&M) ?Snell’s Law sin(a) = a/x and sin(w) = w/x eliminate x: a/sin(a) = w/sin(w) and use: f = v(or  = v/f) to get f sin(a) / va = f sin(w) / vw NOTE: since w < a, need vw < va which is opposite to the prediction of the particle theory but agrees with wave prediction of Property 1 on speed!

    42. Property 4: Refraction • wave (E&M) ?Snell’s Law nicer form for Snell’s Law: f sin(a) / va = f sin(w) / vw Multiply thru by c/f to get (c/va) sin(a) = (c/vw) sin(w) and use definition of index of refraction: n = c/vto get na sin(a) = nw sin(w) Snell’s Law

    43. Property 4: Refraction • particle (photon) theory: vw > va • wave (E&M) theory: vw < va • experiment ?

    44. Property 4: Refraction • particle (photon) theory:vw > va • wave (E&M) theory: vw < va • experiment: vw < va particle theory fails! wave theory works!

    45. Property 4: Refraction Snell’s Law: n1 sin(1) = n2 sin(2) • NOTE: If n1 > n2(v1< v2), THEN 1 < 2. • NOTE: All 2 values (angles in the faster medium)between 0 & 90 degrees work fine. • NOTE: Not all values of 1(angles in the slower medium) work! Example: If n1 = 1.33, n2 = 1, and 1 = 75o, then 2 = inv sin [n1 sin(1) / n2] = inv sin [1.28] = ERROR

    46. Property 4: Refraction Snell’s Law: n1 sin(1) = n2 sin(2) If n1 sin(1) / n2 > 1 THEN there is NO value of 2 that can satisfy Snell’s law (unless you count imaginary angles!). The math is trying to tell us that there is NO transmitted ray. This is called TOTAL INTERNAL REFLECTION.

    47. Refraction and Thin Lenses Can use refraction to try to control rays of light to go where we want them to go. Let’s see if we can FOCUS light.

    48. Refraction and Thin Lenses What kind of shape do we need to focus light from a point source to a point? lens with some shape for front & back point source of light screen s’ = image distance s = object distance

    49. Refraction and Thin Lenses Let’s try a simple (easy to make) shape: SPHERICAL. Play with the lens that is handed out Does it act like a magnifying glass?

    50. Refraction and Thin Lenses Let’s try a simple (easy to make) shape: SPHERICAL. Play with the lens that is handed out Does it act like a magnifying glass? Does it focus light from the night light?