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An observer O , facing a mirror, observes a light source S . Where does O perceive the mirror image of S to be located? 1. 1 2. 2 3. 3 4. 4 5. Some other location. 6. The image of S cannot be seen by O when O and S are located as shown.
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An observer O, facing a mirror, observes a light source S. Where does Operceive the mirror image of Sto be located? 1. 1 2. 2 3. 3 4. 4 5. Some other location. 6. The image of Scannot be seen by Owhen Oand Sare located as shown.
Gwen seems a bit perplexed as she views her image in the plane (flat) mirror. She is perplexed because she cannot figure out exactly how to describe her image in the mirror. When you view your image in a plane mirror, is your image (1) inverted top-to-bottom? (2) inverted left-to-right? (3) inverted front-to-back? (4) not inverted at all? You may vote for as many of the above answers as you wish, as long as you are consistent.
Suppose that you hold the transparency in the photograph below in front of a mirror. How will its reflection appear? Is the image: (1) inverted top-to-bottom? (2) inverted left-to-right? (3) inverted front-to-back? (4) not inverted at all? Write out the word LIGHT as it will appear in the mirror.
Refraction: Refraction is another means of changing the direction of light. Refraction – the bending of light rays as it passes the boundary between two different media. Why does the light ray bend? The speed of light changes from one medium to the next. The speed of light is supposed to be a constant how can it change? The absorption and reemission of light from molecule in the medium cause a delay in the travel time through the medium. This corresponds to a lower average speed for light in different media. Different materials have different rates of absorption and reemission. The ratio of the speed of light in a vacuum to the speed of light in a particular medium is the index of refraction. n > 1 for most materials n = 1 for a vacuum n ~ 1 for air n – index of refraction c – Speed of Light in a vacuum [m/s] v - Speed of Light in a medium [m/s] The index of refraction is actually a complex number, with the real part corresponding to the amount of light transmitted and the imaginary part corresponding to absorption.
Let us look at what would be observed for refraction at different boundaries. q1 Incident Reflected When n2 > n1 the light ray bends towards the surface normal. n1 Air n2 Glass Refracted q2 q2 Incident When n1 > n2 the light ray bends away from the surface normal. Reflected Refracted q1
What causes these tendencies for a light ray to bend towards or away from the normal? Wavefront Consider plane waves incident on the interface between air and glass. (n1 < n2) When one edge of the wavefront enters the new medium, what happens? n1 Air The edge in the new medium travels slower than the rest of the wave that remains in the initial medium, causing the far edge to rotate about the near edge. n2 Glass The direction of rotation is reversed when n1 >n2. The interaction between the electric and magnetic fields of the plane wave and the medium is the reason for the speed change. Wavefront – surface connecting points of equal phase on all waves. The result of this interaction is a simple expression that can be used to relate the incident angle to the refracted angle. Snell’s Law of Refraction
A group of sprinters gather at point Pon a parking lot bordering a beach. They must run across the parking lot to a point Qon the beach as quickly as possible. Which path from Pto Qtakes the least time? You should consider the relative speeds of the sprinters on the hard surface of the parking lot and on loose sand. 1. a 2. b 3. c 4. d 5. e 6. All paths take the same amount of time.
