Physics 102-002 Announcements WebAssign – No Chapter 26 Homework Chapter 28 due next Wednesday Exam 3 Corrections due This Wed, Apr 25 Final Exam May 7, 5:30-7:30, in regular classroom Picture: Image of diffraction of the beam of a Helium-Neon laser when passed through a single slit.

Class Schedule 4/9 Midterm Exam #3 4/11 Chapter 24 Magnetism, (Pg 458-470) 4/16 Chapter 26 Properties of Light 4/18 No class 4/23 Chapter 28 Reflection and Refraction, Part 1 (Pg 530-540) 4/25 Reflection and Refraction, Part 2 (Pg 540-551) 4/30 Midterm Exam #4 5/2 Review and hand back Exam #4 5/7 Final Exam, 5:30 – 7:30 PM

Chapter 28 Reflection and Refraction Reflection and Principle of Least Time Law of Reflection Plane Mirrors Diffuse Reflection Refraction Cause of Refraction Dispersion Rainbows Total Internal Reflection Lenses (and Image Formation by Lenses) Lens Defects Last time

Cause of Refraction Physics Place figure and video In the picture, when the light wave crosses the air-glass boundary there is a change in speed and wavelength. When passing from air into glass, both the speed and the wavelength decrease. Also, the light changes directions as it crosses the air/glass boundary. This bending of the path of light is known as refraction. In other words, it "bends." As seen in the diagram, the wavefront is bent only along the boundary. Once the wavefront passes across the boundary, it travels in a straight line. You can draw a ray perpendicular to the wavefront; this ray represents the direction which the light wave is traveling. Notice that the ray is a straight line inside of each of the two media, but bends at the boundary. Remember the analogy with the tractor and how it starts to turn as a wheel moves from pavement onto the grass? The light wave bends for the same reason. In the diagram, as the right edge of the wave front moves into the “slower” glass, the wave slows down and bends. This bending of light across an interface between two media is what causes the fish to appear nearer to the surface than it actually is. Physics Place figure and video

Dispersion Different wavelengths of light travel through dense media (anything other than a vacuum) at different speeds. In general, the higher the frequency, the slower the speed of the light wave. This means that, for visible light, violet light travels more slowly through a given medium than red light. The colors in between (orange, yellow, green, blue, and indigo) travel at intermediate speeds. Because different colors of light travel at different speeds in transparent materials, they refract by different amounts and become separated. This property is called “dispersion”. Rainbows Dispersion is the property of light that makes a prism separate white light into the colors of the rainbow. Descartes, in 1637, was the first to propose an answer to why rainbows appeared in the sky. The rainbow is produced because the refraction of the light by each water droplet in the sky causes red light to “bounce back” through 42 degrees and blue light through 40 degrees from the direction of the sun.

Total Internal Reflection Consider light traveling from a slow medium and striking an interface with a “fast” medium (like from water into air). Normally there is some reflection and some refraction observed. However, if the angle of incidence is gradually increased, eventually an angle, called the “critical angle” is reached beyond which no light escapes from the “slow” medium. Beyond this angle, you get “total internal reflection”. For comparison, a good mirror is normally only about 90% reflective … the other 10% being lost. This picture shows the transition to total internal reflection as the angle of incidence approaches the critical angle. Prisms are used in a variety of optical instruments because of total (100%) internal reflection. Physics Place figure

Lenses Physics Place figure The refractive property of light enables us to make lenses of different shapes to make light rays converge (come together) or diverge (spread apart) to achieve desired effects. A lens is just a carefully ground or molded piece of transparent material which refracts light rays in such a way as to form an image. Lenses can be thought of as a series of tiny refracting lenses, each of which refracts light to produce their own image. When these prisms act together, they produce a bright enough image to be focused at a point. A converging lens is thicker in the middle, and focuses light at a point. A diverging lens is thinner in the middle and diverges the light. Lens terminology …. There are important terms used to describe all lenses. Center of Curvature Center of Curvature Physics Place figure

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Image Formation by a Lens Lenses can be used to concentrate light so that images can be formed or magnified. A single converging lens can be used to project a REAL image outside the focal point of the lens. Upside down REAL image projected on screen outside the focal point. Object outside the lens’ focal point A single diverging lens can be used to create an upright virtual image (one that appears to be located somewhere that it isn’t): For this reason, a single diverging lens is used as a “sighting” lens, or a “viewfinder”. eye