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