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# Physics 202 Week 9 Lecture and Ch. 18 Book notes PHYS 202

Marketplace > University of Oregon > Physics 2 > PHYS 202 > Physics 202 Week 9 Lecture and Ch 18 Book notes
Melissa
UO
GPA 3.26

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Ch. 18 notes of the book as well as week 9 lectures
COURSE
General Physics >4
PROF.
Jenkins T
TYPE
Class Notes
PAGES
7
WORDS
CONCEPTS
mirrors, concave, convex
KARMA
25 ?

## Popular in Physics 2

This 7 page Class Notes was uploaded by Melissa on Sunday March 6, 2016. The Class Notes belongs to PHYS 202 at University of Oregon taught by Jenkins T in Fall 2015. Since its upload, it has received 26 views. For similar materials see General Physics >4 in Physics 2 at University of Oregon.

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Date Created: 03/06/16
• 18.1 the Ray model of light ⿞Ignores diﬀraction ⿞Light ray is a line in direction along which light energy is ﬂowing ‣ Not a physical entity or thing ‣ Travel in straight lines ‣ Can cross over each other ‣ Travels forever unless it interacts with matter ‣ Object is a source of light rays ‣ Eye sees by focusing a bundle of rays ‣ Applicable if size of objects or openings is much greater than the wavelength ⿞>1mm ‣ within a material, light can be scattered or absorbed ‣ At an interference between two materials, light can be reﬂected, refracted or both ⿞The propagation of light ‣ If we block the paths where light arrives completely out of phase, then all the light arrives in phase so you would only expect to see a much briber spot than if the light only traveled the straight line path • Happens in a diﬀraction grating ⿞Seeing objects ‣ In order for our eyes to see an object, Rays from that object must enter the eye ‣ A light beam is a bundle of light rays ‣ We see a point on an object when a bundle of diverging light rays from the point enters our eyes ‣ The optical system sees the object at the point from which the rays diverge ‣ Point source: emits rays in every direction so some of the rays will enter the eye no matter where it is located ‣ Extended source: a point source so as long as it is not blocked, it will be seen from every point ‣ Non- luminous objects reﬂect rays that strike them • Paper, skin, glass • Diﬀuse refraction-reﬂect incident light in every direction • Scattering: single rays are broken into weaker rays that leave in all directions ⿞Sky is blue bc air molecules scatter blue light more than other colors ⿞Clouds are white bc the water droplets in them scatter all colors of light equally ⿞Scattered light allows us to read a book under a lamplight ⿞Shadows ‣ In a point source: objects intercept some of the rays which cause a dark spot to result behind it while the other rays travel around it to illuminate other parts of the screen ‣ In an extended source: each point source casts its own shadow but the shDoq region is not sharp because each point overlaps • 18.2 Reﬂection ⿞On a shiny surface: specular reﬂection ⿞ ⿞When a ray is incident on a surface, part of the ray is reﬂected from the surface and part is transmitted through the surface where it is refracted or absorbed ⿞The normal is the line perpendicular to the surface at point where Ray strikes the surface ⿞Angles are measured from the normal ⿞Law of reﬂection ‣ Incident Ray and the reﬂected Ray are both in the same plane which is perpendicular to the surface ‣ Angle of reﬂection=angle of incidence ⿞Diﬀuse reﬂection ‣ Reﬂection of rays oﬀ an object that is not shiny ‣ Reﬂected rays leave in multiple diﬀerent directions ‣ Occurs on the rough surfaces for which irregularities in the surface are greater than the wavelength of the light ‣ Has diﬀerent normals so the angle of reﬂection diﬀers,causing the light to scatter ‣ Eyes see the reﬂected light at all angles ⿞Plane mirror ‣ Virtual images occur because no rays actually leave the from point,P or the darkness behind the mirror( diverge from pt not on object) ‣ Image distance is equal to the object distance • • How eyes see the images ⿞Rays from Each point on the burst spread out in all directions and strike every point on the mirror but only a few enter your eye ⿞Rays from points P and Q enter your eye after reﬂecting from diﬀerent areas of the mirror which is why you can't see the full image of an object ⿞Images are considered real if light rays diverge from a point not on the object • image formation by plane mirrors ⿞Image formed is virtual, upright,vans same size as the object behind the mirror and reversed left to right ⿞Specular reﬂection: reﬂection from a smooth surface ‣ Eye must be in the correct position to see reﬂected light ‣ Parallel rays remain parallel afterwards • 18.3 Refraction ⿞2 things happen when a light Ray crosses boundary between air and glass ‣ Part of the light reﬂects from boundary which obeys law of reﬂection ‣ Part of the light continues into the second medium • Transmitted rather than reﬂected • Transmitted Ray changes direction as it crosses the boundary ⿞ ⿞Snell's law: ‣ N is determined by how much a light Ray is bent when crossing the boundary between two diﬀerent media ‣ Angle 1 is the incident medium ‣ Angle 2 is the refracting medium ‣ When a Ray is transmitted into a material with higher index of refraction , it bends to make a smaller angle with the normal: higher index, lower angle ‣ When a Ray is transmitted into a material with a lower index of refraction, it bends to make a larger angle with the normal: lower index, higher angle ⿞Total Internal Reﬂection ‣ As the angle of incidence increases, transmission gets weaker ‣ Where 100% of the light reﬂects ‣ Critical angle : • Reached when angle 2 is 90* • The refracted light disappears at the critical angle ‣ If ⿞Fiber Optics ‣ Sometimes called light pipes ‣ angle of incidence is smaller than the critical angle when they reach the ﬂat end of the ﬁber so they refract without diﬃculty ‣ Depend on total internal reﬂection and are able to transmit light signals with very small losses ‣ Small scratches on the lens however can alter the rays angle of incidence and allow for leakage of light • 18.4 Image formation by refraction ⿞Line through object and perpendicular to the boundary is called the optical axis ⿞Image distance is found through : ‣ Looking for the virtual image • 18.5 thin lenses: Ray tracing ⿞Lens is a transparent material that uses refraction of light rays at curved surfaces to form an image ⿞Converging lenses: cause the rays to refract toward the optical axis ‣ Thicker in the center than at the edges ‣ ‣ Optical axis is also known as the focal point ‣ Distance of the focal point from the lens is called the focal length of the lens • There are two focal points, one on each side of the lens ‣ all refraction occurs as the rays cross the lens plane, and all distances are measured from the lens plate ‣ two sides of the lens are nearly parallel and thus that the rays are displaced rather than bent, so displacement becomes zero • • • • • • • • All rays from a point on the object that strike the lens help to form an image • 3 special rays similar to images above ⿞ ‣ Found in magnifying glasses ‣ Real image: occurs when object is outside of the focal point, making it not only real but inverted as well ‣ Magniﬁcation • ‣ Virtual image • Always upright • Always deﬁne the image distance to be negative for a virtual image ⿞Diverging lenses ‣ Causes rays to refract away from the axis ‣ Also has two focal points ‣ Found in cameras, eyepieces, and eyeglasses • 18.6 Image formation with spherical mirrors: type of image depends on the distance ⿞Concave mirrors like a converging mirror ‣ Parallel rays reﬂect oﬀ the shiny front surface of the mirror and pass through a single point on the optical axis ‣ Law of reﬂection claims that the incoming Ray will reﬂect at the same angle, but on opposite side of the optical axis ‣ Images are real but inverted ‣ ⿞Convex mirrors ‣ Parallel rays that reﬂect oﬀ its surface appear to have come from a point behind the mirror ‣ Image is upright but smaller than what it actually looks like, virtual ‣ • 18.7 The Thin Lens Equation ⿞ • -Light rays are always in front of the mirror so distances in front are positive and real, those behind are negative and virtual -concave has positive focal length, convex has negative Positive magniﬁcation means image is upright; negative means inverted

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