Chapter 27- Light

1. Chapter 27- Light Light is the only thing we see!

2. Early Concepts of Light Light consists of particles! Socrates & Plato believed that vision resulted from streamers emitted by the eye making contact with an object. Light seems to move in a straight line! Light consists of waves! Empedocles & Christian Huygens argued that light travels in waves. Sometimes light does spreads out! 1905- Einstein’s theory is that light consists of particles of electromagnetic energy. Scientists now agree that light is part particle and part wave!

3. The Speed of Light The Speed of Light = 300,000 km/s Roemer found that light coming from Jupiter’s moon Io takes a longer time to reach earth at position D than at position A. The extra distance that the light travels divided by the extra time it takes gives the speed of light.

4. The Speed of Light Michelson used a mirror arrangement to measure the speed of light. The speed of the rotation of the mirror was adjusted so that the round trip of the light was the same as the rotation of the mirror and would reflect the light into the eyepiece. He divided the 70-m distance by the time and found the speed of light. The Speed of Light = 300,000 km/s

5. Electromagnetic Waves Light is energy that travels in electromagnetic waves within a certain range of frequencies. Electromagnetic Spectrum - the range of electromagnetic waves. The lowest frequency of visible light is red and highest is violet. Infrared - electromagnetic waves of frequencies lower than the red light. Ultraviolet - electromagnetic waves of frequencies higher than violet.

6. Light and Transparent Materials • Light waves can force charged particles in materials into vibration. • How the material responds to light depends on the frequency of light & the frequency of electrons in the material. • Light passes through materials whose atoms absorb the energy and immediately reemit it as light. Transparent materials = glass & water. • Ultraviolet & Infrared light cannot pass through glass. Visible light can pass through glass. • Light travels at different speeds through different materials! Vacuum = 300,000 km/s or c, water = 0.75c, glass = 0.67c, diamond = 0.40c. When light emerges from materials into air it travels at its original speed, c.

7. Light doesn’t pass through a material when the energy is changed to random kinetic energy of the atoms. Opaque materials = Wood, Stone, People, etc. Metals are also opaque. When light shines on metal, free electrons vibrate and reemit the light as a reflection. Our atmosphere is transparent to visible light but opaque to ultraviolet waves. Ultraviolet light causes sunburns. Clouds are semitransparent to uv and sand & water reflect uv waves. Opaque Materials

8. Shadows Ray: thin beam of light. Shadow: shaded region that results where light rays can’t reach. Umbra: total shadow. Penumbra: partial shadow due to some light being blocked and other light filling in. Solar and lunar eclipses! Shadows occur in water. Light travels at different speeds in warm and cool water. The difference bends light causing shadows.

9. Light waves are transverse waves! A polarized wave is one that travels back in forth in one direction. (Vertically & Horizontally polarized waves) The sun, a candle flame or an incandescent lamp emits light that is not polarized. This light vibrates in random directions. Polarization: The aligning of vibrations in a transverse wave, usually by filtering out waves of other directions. Polarizing filters transmit components of non-polarized light that are parallel to the polarization axis, and block components vibrating at right angles to the polarization axis. Polarization

10. Vision in 3 dimensions is due to both eyes viewing a scene from a different angle. 3-D movies & stereo grams uses this technique! Polarized Light & 3-D Viewing