Chapter 14

LIGHT

The Electromagnetic Spectrum

The Electromagnetic SpectrumRadio Waves: Used to transmit radio and television signalsMicrowaves: Can be tuned to frequencies that are absorbed by water (microwave oven) and cell phonesInfrared: Used to detect heat when no visible light presentVisible Light: What our eyes can detect

Ultraviolet: Frequency of radiation that is blocked by the OzoneX Rays: Have great penetration power used in medical and engineering fieldsGamma Rays: Generated by radioactive elements

Speed of light

Scientists discovered that the speed of light is about 300 million (300,000,000) meters per second. The speed of light is special because nothing in the universe travels faster than light. This idea forms part of Albert Einstein’s theory of relativity.

Polarization

Polarization is a useful property of light waves. Light is a transverse wave of electricity and magnetism. We say a light wave with an up-down electrical pattern is “polarized” in the vertical axis. A light wave with a side-to-side electrical pattern is “polarized” in the horizontal axis.

Color

To understand color we need to look at light as a wave. Like other waves, light has frequency and wavelength. Red light has a frequency of 460 trillion, or 460,000,000,000,000 cycles per second. Because the frequency is so high, the wavelength is tiny.Because of the high frequency and small wavelength, we do not normally see the true wavelike nature of light. Instead, we see reflection, refraction, and color.

Color

Scientists have discoveredcells in the retina of the eyethat contain photoreceptors.That fancy phrase means that they receive lightand release a chemical. When light hits a photoreceptor cell, the cell releases a chemical signal that travels down the optic nerve to the brain. In the brain, the signal is translated into a perception of color.

Color

Two types of Cells

Cones: Respond to Color. One kind responds to Red, one for Green and one for Blue.

Rods: Respond only to a difference in brightness. (Black, white and shades of grey)

Color

Color

All of the colors in the rainbow are really light of different energies. Red light haslow energy compared with blue light. The closer to violet, the higher the energy.Low energy means lower frequency so waves of red light oscillate more slowlythan waves of blue light. We see the different energies of light as different colors.

The human eye allows us to see millions of different colors. When the brain receives a signal only from the red cone cells, it thinks red. If there is a signal from the green cone cells and neither blue nor red, the brain thinks green.

In reality, our brains arereceiving all three color signals just about all of the time. The strength of the signal matters too.

Color

Color

When we see an object, the light that reaches our eyes can come from two different processes.

1 The light can be emitted directly from the object, like a light bulb or glow stick.

2 The light can come from somewhere else, like the sun, and we only see because of the light that is reflected off of them.

The World of Color

Color

Colored fabric gets color from a subtractive process. The dyes subtract out colors by absorption and reflect the colors you actually see. It works because white light is a mixture of red, orange, yellow, green, blue, indigo, and violet. But actually,you need just three primary colors—red, green, and blue—to make white light.

Color

Colored fabric gets color from a subtractive process. The dyes subtract out colors by absorption and reflect the colors you actually see.

Color Absorbs Reflects

Cyan Red Blue and Green

Magenta Green Red and Blue

Yellow Blue Red and Green

Subtractive Primary Colors

Color

The subtractive color process is often called CMYK for the four pigments it uses.

CMYK stands for cyan, magenta, yellow, and black.

ColorThe green pigment, chlorophyll a, is the most important light-absorbing pigment. You can see on the graph that chlorophyll a absorbs light at each end of the spectrum. In other words, it reflects most of the green light and uses blue and red light. Plants are green because they reflect green light. In fact, plants will not growwell if they are placed under pure green light!

Notice that chlorophyll b and carotenoids (orange pigments) absorb light where chlorophyll a does not. These extra pigments help plants catch more light. Leaves change color in the fall when chlorophyll a breaks down and these pigments become visible. They are the cause of the beautiful bright reds and oranges that you see when leaves change color in the fall.

Color

A lens is a polished, transparent disc, usually made of glass.

Converging / Convex: Light is bent together

Diverging / Concave: Light is bent apart

Lenses