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Lesson 3.6
Lesson 3.5 |
Lesson 3.6 Reflection Overview This lesson deals with virtual images, plane and curved mirrors and laws of reflection. On completion of this lesson, you should be able to use ray diagrams to describe the images formed by plane and curved mirrors. You should also be able to use the MINI LAB
CHOICE OF ACTIVITIES
Plane mirror A mirror with a plane or flat surface. Curved mirror A mirror with a curved surface. The surface is usually spherical or can be thought of as being part of a sphere. Concave mirror A curved mirror in which the reflecting surface forms part of the inside of a hollow sphere. Convex mirror A curved mirror in which the reflecting surface forms part of the outside of a sphere. Regular reflection When the reflecting surface is highly polished and regular, parallel rays of light striking the surface will be reflected in such a way the they are still parallel after reflection. Reflections from a mirror are regular whereas reflections from a rough surface are diffuse. Specular reflection Regular reflection is sometimes called specular reflection. Diffuse reflection Rough surfaces can sometimes reflect as much (or even more) light than a mirror or highly polished surface. The surface roughness however causes rays of light to be reflected in a variety of directions. Parallel rays of light striking a rough surface will be reflected in such a way that they are no longer parallel after reflection and they are thus reflected diffusely. Object For all diagrams showing reflection or refraction of light, the object is assumed to be the source of the light Incident ray The incident ray on a ray diagram represents that path followed by a light ray before reflection The point of incidence The point on the surface of the reflecting surface where the incident ray meets the surface. The normal A line at right angles to the surface through a chosen point. Generally this is at the point of incidence The angle of incidence The angle between the incident ray and the normal The angle of reflection The angle between the reflected ray and the normal
Rays and Burning Boats One of the earliest records of the use of light rays and reflectors relates to the siege of Syracuse by the Roman general Marcellus in 212 B.C. Archimedes arranged a series of mirrors so that the sun's rays were focused on Marcellus' fleet, reducing his barges to ashes. Smoke and mirrors have long been used to impress and deceive people. There are still people who attribute mystical powers to mirrors. Mirrors and reflectors are so much part of our everyday lives that it’s worth remembering that these are simple objects that are governed by very simple laws. Plane Mirrors and Reflection Mirrors are solid objects with highly polished, smooth surfaces. A plane mirror, as opposed to a curved mirror, is flat. It’s reflecting surface exists in one plane only. Reflection is the change in direction of a wave when it bounces off a boundary. When light reaches the surface of a medium that is denser than the medium it is traveling in, some of the light is invariably reflected and some of the light is invariably absorbed. If the material is opaque, very little will be transmitted. If the surface of the more-dense medium is smooth and flat, regular reflection will occur. If the surface is rough, diffuse reflection will occur. Regular reflection When regular reflection occurs, parallel rays of light reflected from a surface remain parallel after reflection. This is sometimes called specular reflection. Diffuse reflection When light is reflected diffusely, parallel rays of light reflected from the surface travel in different directions and the light is scattered. Line of Sight Light travels in straight lines unless it is reflected, refracted or diffracted. In looking at images, the image will appear to be where light could reach the eye by traveling in a straight line. The virtual image behind a mirror gives the impression that the mirror is transparent and that light is traveling in straight lines from the image to the eye. Real and Virtual Images A real image can be projected onto a solid surface. Real images are where they appear to be. Photographs and images projected onto a screen are real images. Virtual images (e.g. rainbows and images behind a mirror) are not where they appear to be. Light reflected from a mirror appears to come from behind the mirror and gives the impression that the object is behind the mirror. Virtual images in a plane mirror appear to be as far behind the reflecting surface as they are in front of it. Virtual images in a plane mirror also appear to be the same size as the object and do not appear to be upside down. The image is however laterally inverted. What appears to be on the left side will be what is actually on the right side. For example, when viewing your face, your left ear will appear to be your right ear etc. Laws of Reflection Laws of reflection:
Ray Diagrams Rays represent the paths that light waves follow after leaving a source and interacting with objects and materials in their path. Beams can be regarded as bundles of electromagnetic waves traveling in the same direction. Unless the beam is a laser beam, the beam would tend to diverge with time and distance. Beams and rays are sometimes used interchangeably in describing the behavior of radiation but beams should be regarded as occupying more volume than rays. Ray diagrams illustrate the paths that rays and beams follow as they interact with objects and materials. These are usually 2-dimensional diagrams that use lines and arrows to illustrate the paths followed by typical rays. Partial Reflection When light or radiation reaches the boundary of a new medium, some or all of the radiation may be reflected. In practice, roughly 2% of light is reflected when it shines perpendicularly on still water and about 4% of light is reflected when it shines perpendicularly on glass. Curved Mirrors Curved mirrors have reflecting surfaces that form part of a sphere. Convex mirrors have reflecting surfaces that represent part of the outside of a sphere. Concave mirrors have reflecting surfaces that represent part of the inner surface of a hollow sphere. The Center of curvature represents the point at the center of the sphere of which the curved mirror is part. The pole is the center of the curved mirror. The radius of curvature is the distance from the center of curvature to the pole – or any point on the reflecting surface. The principal axis is a straight line running through the center of curvature and the pole. The principal focus or focal point is a special point halfway between the center of curvature and the pole and is situated on the principal axis. The focal length is the distance between the principal focus and the pole. Convex mirrors create virtual images that appear to be smaller than the object being viewed. If an image of a bright object (like a candle flame) is projected onto a screen by reflection from a convex mirror, it will be unfocused and appear to be much larger than the object. Convex mirrors are useful for viewing objects that would be outside of the field of vision provided by a plane mirror. For example, convex mirrors are sometimes mounted at the intersection between a driveway and a road so that motorists leaving the driveway can detect traffic in the road before leaving the driveway. Shopkeepers use convex mirrors to view areas of the store that could be susceptible to pilferage. Concave mirrors create virtual images that appear to be larger than the object being viewed. Real, focused images that are smaller and brighter than the object can be created by concave mirrors on a screen in front of the mirror. Rules of Reflection for Curved Mirrors
Mirror Equations The mirror formula gives the relationship between the focal length of a curved mirror, the distance of an object from the center of the mirror and the distance of it’s image from the same point. A sign convention is used to indicate distances of objects and real images as positive. Distances of virtual images are negative. (1/f = 1/v + 1/u) Linear magnification is the ratio of the height of an image formed by a mirror to the height of the object (Linear magnification = height of image / height of object) Spherical aberration In practice, all rays of light parallel to the principal axis are not reflected such that they pass through the focal point of a concave mirror. Rays parallel to, but further from the principal axis than those near to the axis are reflected in such a way that the miss the principal focus by a greater degree.
Review Questions
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