Chapter 27 Properties of Light

Light The two most commonly used models describe light either as a wave or as a stream of particles. Light produces interference patterns like water waves. Light can be modeled as a wave. This model describes light as transverse waves that do not require a medium in which to travel. Light waves are also called electromagnetic waves because they consist of changing electric and magnetic fields.

Electromagnetic Waves Electromagnetic Waves - energy-carrying waves emitted by vibrating electrons HowStuffWorks "How Light Works" Also know as… Light Radiation Photons

These three are the same… Light pure energy Electromagnetic Waves energy-carrying waves emitted by vibrating electrons Photons particles of light

The Electromagnetic Spectrum Radio Waves - communication Microwaves - used to cook Infrared - “heat waves” Visible Light - detected by your eyes Ultraviolet - causes sunburns X-rays - penetrates tissue Gamma Rays - most energetic

Electromagnetic Wave Velocity The speed of light is the same for all forms of light. It is 300,000 kilometers per second or 186,000 miles per second.

Speed of Light Speed of light varies with medium. Light slows down in water Ex. Straw or spoon appears bent in glass of water

The Electromagnetic Spectrum A range of light waves extending in wavelength from radio waves to gamma rays

The Visible Spectrum A range of light waves extending in wavelength from about 400 to 700 namometers.

Transparent Materials Transparent - the term applied to materials through which light can pass in straight lines

Opaque Materials Opaque - the term applied to materials that absorb light When the absorption occurs, the vibrations given to the electrons by the light are often great enough to last for awhile, during which the vibratory energy is shared by collisions with neighboring atoms. The absorbed energy warms the material.

Reflection Light may strike a material and bounces off - this is called reflection. Light shines on metal it sets its free electrons into vibration and the energy is reemitted as visible light.

When light is incident upon a material it can behave in three ways: It can bounce off It can be transmitted through the material It can be absorbed by the material Usually a combination of all three occurs!

Transparent/Opaque Is glass transparent or opaque to visible light? Answer: transparent Is glass transparent or opaque to ultraviolet light? Answer: opaque Atoms give up energy in form of heat as UV light interacts with atoms in the glass. See Figure 27.7 How do UV light bulbs allow UV light transmission? Answer: made of quartz glass

Example Questions Are clouds transparent or opaque to visible light? Answer: opaque Are clouds transparent or opaque to ultraviolet light? Answer: transparent

Shadows Umbra - the darker part of a shadow where all the light is blocked Penumbra - a partial shadow These terms also apply to Solar Eclipses and Lunar Eclipses.

A solar eclipse occurs when the Moon passes in front of the Sun.

A lunar eclipse occurs when the Moon passes into the Earth's shadow.

Questions Which type of eclipse is dangerous to view with the unprotected eye? Why are lunar eclipses more commonly seen than solar eclipses? Lab 70 “Shady Business”

SEEING LIGHT – PARTS OF THE EYE Cornea - does most of the focusing Iris - has the eye color and controls light intensity Pupil - the hole in the eye Lens - does remainder of focusing Retina - location of light sensors, has rods and cones Fovea - center of vision, predominantly cones Blind spot - optic nerve exit, no light sensors

Parts of the Eye Detectors on the Fovea Rods light intensity sensitive Cones color sensitive The blind spot for the eye is cause by the optic nerve.

Color Deficiency

Myopia (Near-Sightedness) People with near-sightedness cannot see clearly at distance.

Hyperopia (Farsightedness) People with far-sightedness cannot see clearly up close.

Doppler Effect and Expanding Universe Doppler effect also occurs for light waves from stars and galaxies. When a light source approaches earth there is an increase in its frequency – called “blue shift” When a light source goes away from earth there is a decrease in its frequency – called “red shift”

Polarized Light and 3-D Polarizing filters transmit parts of incident nonpolarized light that are parallel to the polarization axis, and block components vibrating at right angles to the polarization axis. Emergence of polarized light. Lab 72 “Shades” There are a variety of 3-D viewing techniques, all of which involve each eye seeing a different view – binocular vision.