Atreyi SahaPhysics 132Prof. HatchFebruary 5, 2016Lecture #8: Intro to Ray Optics- Exam Reviewo 1 piece of paper both sideso Number 2 Pencil, ID, Calculatoro Multi-bubble - fill in 2 and one is right then 3 points and so ono You can re-do the exam after you take it (75% individual & 25% take-home)- Objects are one of two things (our discussion willbe limited to the visible spectrum)o Self-luminous objects: include things like alight bulb that emits light we can seeo Reflective objects: objects that do not emitlight (i.e. humans, a wall) but we can seesuch objects because they reflect lightwaveso In order to see objects, light must bereflected from the object and go to your eye.If light does not reach your eye, you will beunable to see the object.- Light moves in straight lineso scattered light: we can only see light when it scatters towards our eyes, or is directed towards our eyes. If light does not enter your eye, there is nothing for youto see.o Demo When doing the flashlight demo, you could not see the light being shined on the wall. There were not enough air particles to scatter the light. In contrast, when you spray this fog/air mixture, you can see the light better. There are enough air particles to scatter the light. You see the green laser light reflecting off the particles. We see a line, but are actually looking at the light particles that are colored/reflect green light- Light travels in a nice, straight line unless it interacts with something: i.e. gust in the air or a mirror- We can see reflected light or we can see when light changes direction from travelling in a nice straight line. We can see that change. An example is a mirage. Sunlight will come from the sun, be reflected off of you and enter your eye . Sunlight will also hit surface of the earth but something is going on so that that light will change direction. That changed direction light will come up to our eyeand our eye will perceive the difference.And you think that something is there -since it's grayish/dark, we perceive it tobe water.- When light travels and interacts with a surface, it follows areflection rule. Angle of reflection is equal to the angle of incidence. o We measure our angle from the normalo This is true for all such mirrors (curved or straight) curved mirrors have a local tangent that we can use tocreate the path of the light ray- Distinctions between a ray and a beamo A ray is a single line of lighto A beam is made up of lots of rays and has a spatial size- Specular Reflection vs. Diffuse Reflectiono Specular reflection: reflection off of a smooth surface like a mirror; the arrangement of light will all stay the same; the order remains the same so you can see a reflection/the object.This is the object thatactually reflected the lightand since light is reflectedoff the mirror surface in aordered way, you can see atrue depiction of the object Mirror and water do the same thingo Diffuse reflection: it is the reelection from a surface at which various sections of the surface are at various angles Analogous to a rough surface Each individual light ray will follow the reflection rule, but the reflection will be different based on what surface it hits This ends up scattering the light in different ways You can no longer see what sent the light, the image disappears. What you end up seeing is the surface itselfo Demo Frosty glass (rough surface) - you can't see the light bulb, just lit up frosty glass Clear glass - you can see the light bulb This also happens when it is raining - surface of the road becomes like clear/smooth glass and it becomes hard to see the yellow and white lines on the road because you just end up seeing reflections of cars on the road; use trees as boundaries on the door- Where is the image when the light is reflected from a mirror?o We see the reflected light (blue circle) o We know that light came to our eyes in a straight line so we follow that path back.There are sensors in our eye that allow us to do thiso Where the two light rays intersect, that is where the image is (shown on part a of the image)o For people with one eye, it is hard to tell depth so they must rely on other cueso Calculation relevant comments: In a flat mirror, object distance (Do) = image distance (Di)- What happens to the image when you move an object farther away from a mirror?o The image DOES NOT get smaller. It stays the sameo Try this with full length mirror - put tape where head is and tape where feet areo This is because the mirror and you are thesame height. Light follows the same path, justfarther away. This is why image size stays thesame.- How large does a mirror need to be so you can seeyour whole self?o Doesn't need to be your full heighto You see your head because light must leavethe top of your head --- hit the mirror ---- and then go into your eyeo You need 1/2 of your total height- Index of refraction: it is the ratio between the speed in vacuum and the speed in a mediumo this is because light travels different speeds in different mediao light travels the fastest in a vacuumo n = speed in vacuum/speed in mediumo Light that propagates through the boundary changes speed- If light comes in perpendicular to the surface, it will propagate straight. If light comes in at an angle, it will change direction. It changes direction because the speed changes. Let's say, if light is moving more slowly, it will change direction.o Refraction: light changes direction when it enters a new medium unless it is headed in at a normal
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