122 Review May 12 2003 Lising Reviewing for the final Some of these ideas are definitions or assumptions of the model those are the foothold ideas we use as the basis for reasoning Other ideas are reasoned results when it s a reasoned result you should be able to explain where it came from how it arises from the definitions and assumptions of the model and you should be able to respond to conflicting lines of reasoning I m not going to list all the results just a few core examples Memorize foothold ideas use them to figure out these and other results These ideas below are the new ideas since the last midterm For the final you are also responsible for the stuff on both previous review sheets Light lenses and mirrors continued Reasoned result technique to find images with lenses and curved mirrors 3 rays 1 Light that hits the lens parallel to the centerline bends through the focal point or as if it were going through the focal point 2 Light coming from the focal point or toward the focal point comes out parallel to the centerline 3 Light that hits the very center of the lens goes straight through Light that hits the center of the mirror bounces back at equal angles as if it hit a vertical mirror Definition To focus an image with a lens or mirror means getting it so that the screen is at the image location so that all rays from a single point on the object come to a single point on the screen Make sure you are distinguishing between focusing and focal points Definition The focal length f is the distance from the focal point to the screen Result For a thin lens or a thin mirror the image distance and object distance are related by the thin lens thin mirror formula 1 d i 1 d o 1 f For convex mirrors and diverging lenses f is a negative number Also you need to remember where these formulas came from so you can figure out what positive or negative distances mean Result More curved mirrors lenses have smaller focal lengths than less curved ones Light wave vs particle models Foothold Speed of light in air c 3 x 10 8 m s Foothold Speed of light in a different medium vwave c n n index of refraction Light as waves gives this not light as particles Results If n increases decreases and f stays the same or things would pile up This gives Snell s Law n1sin 1 n2sin 2 If Snell s law is unsolvable there is no refraction only reflections This is called Total internal reflection Definition Light rays from the particle model are the lines perpendicular to the wavefronts The wavefronts are the lines along which the peaks of the wave are at any moment the lines you saw in the ripple tank See the lines in drawings below The blue lines are the wavefronts and the green lines are the light rays your were working with in the particle model Waves traveling in one direction like a flashlight or laser rays peak peak trough 1 only peaks drawn Waves traveling out in all directions from a single point like a point source or light scattering when it hits a non mirror 122 Review May 12 2003 Lising Foothold At an aperture waves will get blocked just like the rays The waves that go through either go through continuing in straight lines if the aperture is big compared to the wavelength or spread out like a point source if the aperture is small close to wavelength size The particle model predicts the first of these but the wave model predicts both Larger opening than wavelengthstraight lines continue Small opening compared to wavelength like a point source on other side Result When you see more bright spots than apertures like the pattern of bright spots we saw in lab this can be explained by interference thinking of light as waves Definitions Constructive interference when waves at a given point are adding in amplitude positive or negative Examples are peaks and peaks overlapping or troughs and troughs Destructive interference is when waves at a given point are canceling peaks and troughs for example Result Constructive interference points are louder brighter etc and the displacement at those points oscillates between big peaks and big troughs Destructive interference points are softer dimmer etc and not much disturbance at all is happening there Foothold Light exerts a very small force on an object when it hits it Particle works wave doesn t for this Result Light is like both waves and particles Usually both models work but sometimes only one does A more sophisticated model of electric field waves made by wiggling charges predicts everything but it s too weird to have good intuitions about so we students and real physicists both end up using both the regular waves and particle models and keeping track of what works where The only difference is that a physicist or advanced student can explain WHY using the field waves model the other two models work sometimes and not others Definition Light comes in little bundles or packets of waves We call these light particles photons They have no mass because they are just little packets of field waves But they do have Light Color the electromagnetic spectrum and electromagnetic radiation Foothold The frequency of light determines the color Result The wavelength also determines the color in a given medium since the wavelength and frequency are related by the speed of light in that medium We always see through the same medium so we can talk about visible frequencies or visible wavelengths Only a small range of frequencies are visible 2 122 Review May 12 2003 Lising Result The energy of each photon is determined by the frequency Def Ephoton hf h is a constant joules Hz that is very very small because each photon has only a tiny bit of energy The brightness of the light amplitude of the wave is equivalent to the number of photons Result The higher the energy the more dangerous the photon X rays and g rays have a tiny wavelength and can penetrate deeply into tissue meaning they re less likely to be absorbed and pass through the skin But when they do get absorbed they are very damaging since they can knock electrons off atoms and break molecular bonds Mass energy and Radioactive decay Basically you re responsible for knowing this stuff only superficially Foothold Every particle has energy like a potential energy stored in its mass The amount of energy is given by the famous equation attributed to Einstein Emass mc2 where c is the speed of light as given above Check for yourself to make sure the units work but you are not responsible for any mass energy calculations Mass doesn t have
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