1. Light
Chapter 27

2. Early Concepts of Light
Light has been studied for thousands of years
Up until the time of Newton, most scientists thought that light consisted of particles
The Dutch scientists, Christian Huygens, argued that light was also a wave
In Einstein’s photoelectric effect, light consists of particles—massless bundles of concentrated electromagnetic energy—called photons
Now scientists agree to a dual nature for light, part particle and part wave

3. Photoelectric Effect

4. The Speed of Light
The first demonstration that light travels at a finite speed was supplied by the Danish astronomer Olaus Roemer ~1675, using the relationship between Io and Jupiter
The most famous experiment for the speed of light was done by the American physicist Albert Michelson in 1880, using reflected light from a mirror in the distance and an eyepiece (won the Nobel Prize)
We now know that the speed of light in a vacuum is a universal constant (c = 3.0x108 m/s)

5. Michelson-Morley Experiment

6. Electromagnetic Waves
Light is energy that is emitted by accelerating electrons in atoms, it travels in a wave that is partly electric and partly magnetic – electromagnetic wave
Light is a small portion of the large family of electromagnetic waves (radio waves, microwaves, X-rays, etc.)
Electromagnetic Spectrum – the range of electromagnetic waves
Infrared – electromagnetic waves of frequencies lower than the red of visible light
Ultraviolet – electromagnetic waves of frequencies higher than those of violet

7. Electromagnetic Spectrum

8. Light and Transparent Materials
When light is incident upon matter, electrons in the matter are forced to vibrate
How a receiving material responds when light is incident depends on the frequency of the light and the natural frequency of electrons in the material
Transparent – materials that allow light to pass through
When light passes through a transparent material, there will be a slight time delay as it must force the electrons in the material to vibrate, but the light will have the same frequency as before when it reemerges

9. Transparent Objects

10. Opaque Materials
Opaque – materials which absorb light without reemission and thus allow no light through them
Any coordinated vibration given by light to the materials atoms is transformed into random kinetic energy (become warmer)
Metals appear shiny because of a release of free surface electrons, by light vibrations, into the visible spectrum
Our atmosphere is transparent to visible light and infrared, but almost opaque to high-frequency ultraviolet waves

11. Opaque Materials

12. Shadows Ray – a thin beam of light
When light shines on an object, some of the rays may be stopped while others pass on
Shadow – formed where light rays cannot reach
Sharp shadows are produced by a small light source close to the object
There is usually a dark part on the inside and a lighter part around the edges
Umbra – total shadow
Penumbra – partial shadow

13. Shadows

14. Eclipses

15. Polarization Light travels in waves
The waves are transverse, not longitudinal, demonstrated by polarization
When the vibrations of the light wave are back and forth, the wave is polarized
A wave can be polarized in either the horizontal or the vertical
A pair of polarizing sunglasses cuts out the waves in one direction, allowing the other waves to go through

16. Polarization
Java Applet

17. Assignment
Read Chapter 27 (pg )
Do Ch. 27 Assessment #24-36