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

Transparent Objects

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

Opaque Materials

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

Shadows

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