WHAT DO WE KNOW ABOUT

LIGHT?

What is Light?• Light is a wave that we can see.

– Light can carry heat and warmth.– Light has color.– Light can be bright or dim.– Light travels 300,000,000 meters per

second (5,880,000,000,000 miles per year!).

– Light can be bent by lens or reflected by mirrors.

What makes light?• The movement of electrons in atoms produce light.

– Electrons are arranged in levels. Electrons in each level have different amounts of energy.

– As the electrons move from a lower level to a higher level, they gain energy.

– As the electrons move to a lower level, they release energy.

Examples of making light• Glow in the dark material

– When light hits Phosphorus atoms, the electrons absorb light and rise to a higher level. When they fall back down they give off their stored light.

• Glow sticks & Fire– A chemical reaction causes atoms to move around. When electrons

move from a higher level to a lower level, light is produced.

• Lightning and Sun– When energy moves through the sun or air, the atoms in the sun or

the air rearrange their electrons producing light.

Electromagnetic Spectrum (p. 229)

• The light we see is only a small part of all light.

• The whole range is called the electromagnetic spectrum and visible light is in the middle of it.

• On the low energy end of the spectrum are radio waves. On the high energy end are gamma rays.

Polarization• What kind of wave is light?

– Light is a transverse wave.

• A polarizer is a partially transparent material that only lets one angle of light through.

• If two polarizers are used, the amount of light can be controlled.

• If the two polarizers are not in the same direction, some or all of the light can be blocked.

• Glare screens and most sunglasses are made of polarizing film. They only let light from one direction pass through.

Frequency and Wavelength

• Light has frequency and wavelength.• Red light has a frequency of 460,000,000,000,000

waves/sec and a wavelength of 0.00000065 m.• Because light waves are so small, we do not see the

waves. Instead, we see how the light interacts with objects.

• The wavelength is so small that we use nanometers (nm) as the unit of measurement.

1 nm = 1x10-9 meter• The frequency is so large we use terahertz (THz).

1 THz = 1 Trillion Hertz

• How does the wavelength of Red light compare with the wavelength of Violet light?• How does the frequency of Red light compare with the frequency of Violet light?

Energy of light• All of the colors in a rainbow are really

different energies.• Red light has low energy compared with blue

light.• The closer to violet, the higher the energy.• Low energy means low frequency, so waves of

red light oscillate more slowly than waves of blue light.

Eyesight

• The photoreceptors of the retina of the eye respond to the different energies.

• Our eyes have two different types of photoreceptors, rod cells and cone cells.

– Cone cells respond to color. Different cone cells respond to different frequencies of light.

– Rod cells respond only to differences in brightness. Rod cells are more sensitive and work at very low light levels.

How can we see other colors?

• When the brain received a signal from only the red cone cells, it thinks “red”.

• When the brain receives a signal from both the red and green, the energies add together and the sensation is different from either red or green.

Additive Process• The additive process: New

colors are formed by the addition of more than one color signal.

• The additive primary colors are RED, GREEN, and BLUE.

• If the signal is equal parts red, green, and blue we perceive this as white light.

Color of things• The light that reaches our eyes comes from two

different processes:

– The light can be emitted directly from the object, like a light bulb.

– The light can come from somewhere else, and we only see objects by their reflected light.

• If you remove the source of light, there isn’t any light to reflect, so you see nothing.

What gives objects their color?

• The reason a blue cloth looks blue is because the pigment in the cloth absorbs all the frequencies of light other than blue.

• Since blue frequencies is all that is reflected, it is the one that reaches our eyes.

Subtractive process• Dyes and pigments subtract out colors by

absorbing some colors and reflecting others.

• The primary pigments are cyan, magenta, and yellow. These colors are the result of the absorption of one of the primary colors of light.

• Black is made when no light is reflected. If you add magenta, cyan, and yellow together, they absorb all light so it looks black.