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Light Waves

Light Waves. Chapters 16 and 17. Light as a Ray. Light travels as a ray (could be a particle or a wave) that travels in a straight line unless it runs into an obstruction Comes from a source: Luminous – emits its own light Illuminated – reflects light Opaque – only reflects

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Light Waves

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  1. Light Waves Chapters 16 and 17

  2. Light as a Ray • Light travels as a ray (could be a particle or a wave) that travels in a straight line unless it runs into an obstruction • Comes from a source: • Luminous – emits its own light • Illuminated – reflects light • Opaque – only reflects • Transparent – allows light to pass through, reflects only a little • Translucent – falls in between the two extremes

  3. Light Intensity • Follows the inverse square law • Brightness decreases 4x when the distance from the source is doubled • However, the area covered increases (think about pointing a flashlight at the wall, then backing up) • Measured in candela (cd)

  4. Light Speed • Measured from several experiments • Consensus is about 3 x 108 m/s in a vacuum • Unlike sound waves, any media actually decreases this speed • Means light travels 9.45 x 1012 km in one year (this distance is called a light year)

  5. Diffraction • The bending of light around a barrier or through an opening • Indicates that light is a wave • Why you lose some radio reception around mountains and tall buildings • Occurs more for higher frequency waves (why AM gets further reception than FM)

  6. Electromagnetic Spectrum • Light is a transverse wave created by vibrating electric and magnetic fields • Unlike sound, it can travel in a vacuum • Composed of a variety of waves that have different frequencies and wavelengths (but all travel at c)

  7. Color • Occurs because of different frequencies • Red has least energy, violet has most • All combine to form white light • 2 colors that add together to make white are complements (ex. Magenta and green)

  8. How we see color • Based on what frequency gets to our eyes • A red object absorbs all frequencies except for red. Red is reflected to our eye, so we “see” red. • Only light incident on an object can be reflected.

  9. Polarization • A wave being forced to vibrate in only one direction • Occurs only for transverse waves (can’t polarize sound) • 2 filters at right angles block all waves!!!

  10. Doppler Effect and Light • Occurs for all waves, not just sound • Used often in astronomy to measure the movement of stars, moons and planets • One famous conclusion from Doppler studies is that the universe is expanding: all light from other galaxies is red shifted, meaning they are moving away from us

  11. Law of Reflection • When something hits a barrier, it will reflect at the same angle with the normal as which it hit. • Normal is a line perpendicular to the barrier • Works for light, billiard balls, and any other object • Wavelength and frequency don’t matter, all reflect the same

  12. Diffuse Reflection • Each individual light ray obeys its own LOR • On a bumpy surface, this results in rays reflecting in a variety of directions, based on the angle that they hit the barrier • This results in a spread of light, and no image can be formed

  13. Plane Mirrors • A very flat, poorly absorbing surface makes the best image • Your brain gets confused because it doesn’t think light can reflect. It assumes that the source of the light proceeds straight back into the mirror. • This creates a virtual image in your mind

  14. Plane Mirrors Continued • The reflection of all these rays obeying LOR, from different parts of your body, bouncing to your eye creates a consistent virtual image • The heights of the image and object are the same • The distance from the mirror of the image and object are the same • The image has a back to front reversal based on how the light reflects back to you

  15. Curved Mirrors • When a surface is curved, there are different normals because the perpendicular line to the surface changes • This means that as rays obey the LOR, they create an image that will differ from the object in a variety of ways • Principal axis – the normal at the point that divides the mirror in half • Focal point – the point where reflected light rays come together (F) • Focal length – the distance between the mirror and focal point (f)

  16. Concave Mirrors • Mirror curves away from the object • F occurs in front of the mirror • f is half the distance between the object and the mirror • f is a positive distance • Results in an image that is inverted if the object is further than F • This is a real image (it can be projected on a screen) • Results in a magnified, virtual image if the object is closer than F

  17. Convex Mirrors • A mirror that curves out toward the object • F occurs behind the mirror • f is negative • Results in a smaller than normal, right side up image • This is a virtual image (cannot be projected onto a screen)

  18. Comparing Mirrors

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