Chapter 11Light Waves

Electromagnetic Waves

•The vibrating electric and magnetic fields in space create the em wave.

•Travel in transverse motion•Range of waves extends from radio to

gamma▫(radio, micro, infrared, visible, uv, xray,

gamma)•Speed of all em waves is 3.00 x 108 m/s

Examples of EM Waves

The Color Spectrum (Visible Light)

•Spread of colors = spectrum ▫Red, orange, yellow, green, blue, indigo,

and violet (ROY G BIV)•What about white & black?

▫White is a combination of all colors of light and black is the absence of light

Transparent & Opaque Materials

•The speed of light can vary when transmitted through opaque & transparent material

•Usually results in a slight decrease in speed.

Light & Colors•When a light wave with a single frequency

strikes an object, the following can occur: ▫The light wave could be absorbed by

the object, in which case its energy is converted to heat;

▫the light wave could be reflected by the object;

▫the light wave could be transmitted by the object.

Color by Selective Reflection• Color comes from frequencies that are reflected

▫ The color is not in the object itself, but in the light which strikes the object.

▫ Reflected light (color) is not at the natural (resonant) frequency of the material

• Most materials absorb light of some frequencies and reflect the rest▫ A red rose

Petals appear red because it absorbs all frequencies except red—reflects the red

• Apparent color depends on the objects natural frequency AND what is illuminating it

Selective Reflection• An object can only reflect light of frequencies

present in its illumination▫ Incandescent (non fluorescent light) emits all

visible frequencies but is more rich in lower frequencies enhancing reds

▫ Fluorescent light is richer in higher frequencies enhancing blues

• Perceived color depends on the light source

Color by Selective Transmission• The color of a

transparent object depends on the color of light it transmits

• A blue piece of glass appears blue because it transmits primarily blue and absorbs all the colors illuminating it

The glass warms due to the absorbed frequencies

Scattering• Why is the sky blue?

▫ light striking atoms and molecules in a gas (far apart from each other) gets re-emitted in many directions

• The tinier the particle, the more higher-frequency light it will re-emit▫ N2 and O2 in atmosphere

• As light moves through atmosphere, most goes in straight lines, but when it hits a molecule, the higher frequencies (violet, blue) especially get scattered in all directions.

Refraction

•Deflection of a wave on passing from one transparent medium into a second medium in which its speed is different

•The bending of rays of light▫Magnification (refraction using a lens to form an image

of an object)▫Spectrum (prism uses refraction to form a spectrum of

colors from an incident beam of light)▫Mirage and Optical

Diffraction• the slight bending of light

as it passes around the edge of an object

• Larger opening than light frequency = less diffraction

• Smaller opening = more pronounced