What creates a water wave? - a vibration in the water or on the surface of the water

What creates a sound wave? - a vibration in the air

So, what action is likely involoved to create a light wave? - a vibration of electrons in certain materials

Make two more drawings like the one at left. The second drawing should show the electron jumping from: n = 1 to n = 3 and back again. and the third drawing should show from: n = 1 to n = 4 and back again. Then check out the "How Light is Created Animation" below.

Luminous - anything that emits light waves

Illuminated - anything that has light waves striking it.

The sun is luminous. It emits light waves.

The moon is illuminated. It reflects light waves.

Visible light is an Electromagnetic wave.

In Ch.15 what happened to the pitch of the sound we heard as something vibrated more frequently?

The frequency of a light wave determines:   the color of light we see.

Red has the lowest frequency and Violet has the highest frequency.

This means that Violet has the shortest wavelength and Red has the longest.

Doppler Shift:

The Doppler shift happens to all types of waves (water, sound, light, etc.) This means that if there is an object that is luminous, and it is also moving, the frequency observed (seen by an observer) will be different than the frequency emitted by the object.

Relationship between velocity, frequency and wavelength:

Just like Ch.14 & 15, the speed of any wave is constant in a given medium. So light in air has a constant velocity.

Remember our race ?

long steps      =        long wavelength of light
few steps       =        low frequency of light

short steps      =       short wavelength of light
several steps   =       high frequency of light

Luminous flux - the rate at which light is emitted from a light source.

For luminous flux we use the symbol: L_f.
The units for luminous flux are called lumens, with symbol: lm.
[ It is a measure of how much light is given off ]

Luminous Intensity - how much light falls on a given surface.

It can also be called the illumination of a surface.
For luminous intensity we use the symbol: L_i.
Its units are lumens per square meter.      lm/m²
To simplify, the units lm/m² are called lux,   with symbol: lx      [ So 1 lm/m² = 1 lx ]

L_i =   L_f           (Draw a line under L_f to show division by 4p d²)
        4p d²

where:

L_f = the luminous flux
L_i = the luminous intensity
p = 3.1415926535
d = the distance the light is from the surface

Transparent materials - materials that transmit most of the incident light and it can be seen clearly.

Examples - air, water, glass, some plastics, plexiglass

Translucent materials - materials that transmit most of the incident light but not so that it
                            can be seen clearly.

Examples - frosted glass, ice, some plastics

Opaque materials - materials that reflect or absorb all of the incident light. None is transmitted.

Examples - soil, brick, wood, steel, silver, gold

The Spectrum

Sunlight is made up of seven colors, the same as those seen in a rainbow. They are Red, Orange, Yellow, Green, Blue, Indigo, and Violet. Together, they are called the spectrum. The first letter of each color in this series is: R O Y G B I V. The spectrum is quite commonly refered to using this abbreviation. When all seven of these colors of light are mixed together in the correct amounts, we see... the color white.

The 3 primary colors of light

Even though there are seven colors that make up sunlight, the same effect can be created by mixing just three of those colors. Because these three can be combined to give the same effect as the complete spectrum, they are called the primary colors of light. (Primary meaning: most basic elements.) When these three are mixed in the proper proportions... we get white light.

complimentary colors:      primary color of light    +    secondary color of light    =    white light

The color of a transparent material is the color of the light that it transmits or reflects.

Selective transmission - The ability of a transparent material to transmit (and reflect)
                              one color of light and absorb all others.

Red light      ----- >      Green glass      ----->      NO LIGHT !!

Green glass absorbs all colors except green. That means that all the red light gets absorbed. So therefore no light passes through.

Now lets look at the color of opaque objects.

Red objects are called red because they reflect red light and absorb all others.

Blue objects are called blue because they reflect blue light and absorb all others.

White objects are called white because they reflect all colors of light and don't absorb any.

The color of an opaque material is the color of light that it reflects .

Selective reflection - The ability of an opaque material to reflect one color of light
                          and absorb all others.

Mostly, the colors of opaque bodies are as a result of selective reflection. A blue book appears blue when illuminated by white light because it reflects the blue part of the white light and absorbs all others. A blue book with blue light on it will appear blue because it reflects the blue light that is shining on it.

Our eyes sense the color black when no light is striking them. When we see an object that doesn't
transmit or reflect any light, we see that object as black.

What color will a blue book illuminated with red light appear?     It will appear black.

The blue book absorbs all colors except blue. Only red light is shining on the book. The red light gets absorbed by the book. Therefore no light reflects off the book.

White objects reflect all colors (frequencies) of light. When a white object is illuminated by different colors of light, it appears to be ... what ever color of light strikes it.

Our eyes sense each of the individual colors when only that specific frequency of light strikes our eye.
When an object reflects only blue to our eyes, we see that object as blue.

Pigments and Dyes absorb certain colors of light and reflect the rest to your eye. (called subtractive process because they absorb (take out) or "subtract" colors of light) Pigments and dyes are commonly used in paints to color all sorts of opaque objects. They are also commonly used to color ink (like in your in pen) and fabrics like clothing, drapes or furniture.

Yellow, Cyan and Magenta are called the primary pigments.

Primary pigments absorb one primary color of light and reflect the other two primary colors of light.

Example #1 :    yellow is a primary pigment because it absorbs only blue and reflects both red and green.

When both red light and green light reflect off an object they combine and we see yellow.
( red light & green light     ---->     yellow light )

Example #2 :    cyan is a primary pigment because it absorbs only red and reflects both blue and green.

When both blue light and green light reflect off an object they combine and we see cyan.
( blue light & green light     ---->     cyan light )

Example #3 :    magenta is a primary pigment because it absorbs only green and reflects both red and blue.

When both red light and blue light reflect off an object they combine and we see magenta.
( red light & blue light     ---->     magenta light )

Red, Blue and Green are called the secondary pigments.

Secondary pigments absorb two primary colors of light and reflect the other one primary color of light.

Example #1 :    red is a secondary pigment because it absorbs both blue and green and reflects only red.

When only red light reflects off an object we see it as red.

Example #2 :    blue is a secondary pigment because it absorbs both red and green and reflects only blue.

When only blue light reflects off an object we see it as blue.

Example #3 :    green is a secondary pigment because it absorbs both red and blue and reflects only green.

When only green light reflects off an object we see it as green.

When combined, the three primary pigments will absorb all primary colors of light and will reflect no light, so you'll see black. (this is how black pigment is made)