Light
Chapter 27
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
Photoelectric Effect
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)
Michelson-Morley Experiment
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
Electromagnetic Spectrum
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
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
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
Eclipses
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
Polarization
Java Applet
Assignment
• Read Chapter 27 (pg. 404-418)• Do Ch. 27 Assessment #24-36
