ColorColor

The Color Spectrum Isaac Newton passed a narrow beam of sunlight through a triangular-shaped glass prism showed that sunlight is composed of a mixture of all the “colors of the rainbow”. This selection of colors is called a spectrum: red, orange, yellow, green, blue, and violet.

Sunlight The light from the sun is a composite of all the visible frequencies but the color frequencies have uneven brightness. Yellow-green light is the brightest part of sunlight, (the most heat).

The human eye is most sensitive to yellow- green, which is why more new fire engines are painted this color; it attracts attention easier. Yellow- green is also easy to see at night

Color by Transmission When something is transparent the light is simply transmitted through. Ordinary window glass is colorless and it transmits all colors and visible frequencies of light. A colored transparent glass contains a pigment in the glass that selectively absorbs some colored light and transmits others. A piece of blue glass transmits blue light and absorbs all other frequencies.

Light transmitting through blue glass

Color by Transmission cont… Electrons in the pigment selectively absorb light of certain frequencies in the illuminating light. Light that is not part of the selective frequencies is reemitted from atom to atom in the glass.

Mixing Colored Light When red, blue, and green light are projected onto a screen, the overlapping areas appear different colors. Where all the three overlap, white is produced. RED + GREEN =YELLOW RED+ BLUE = MAGENTA GREEN + BLUE = CYAN Additive primary colors are red, blue, and green because these colors produce the highest number of different colors.

Mixing Colored Pigments -Mixing red, green, and blue paint is entirely different from the mixing of colored light. -Pigments absorb light of a relatively wide range of frequencies. -Subtractive primary colors are three paint or dye colors that are more useful in color mixing by subtraction (absorbing) are MAGENTA, YELLOW, and CYAN

Color of objects Objects are a certain color because of the light they reflect. (Ex. Blue objects are blue because they reflect blue light and absorb other colors) Different elements have different frequencies for absorbing and emitting radiation (electrons changing energy levels).

Color of objects cont… An object can reflect only light of the frequencies present in the illuminating light. The appearance of a colored object therefore depends on the kind of light used to illuminate it. Colors in the daylight appear different from the way they appear when illuminated with manmade lamps. The color seen from an object is subjective and depends on the source of the light.

Seeing color The color an object appears depends on the colors of light it reflects. For example, a red book only reflects red light: White light Only red light is reflected

Using colored light If we look at a colored object in colored light we see something different. For example: White light Shorts look blue Shirt looks red

In different colors of light this would look different: Red light Shirt looks red Shorts look black Blue light Shirt looks black Shorts look blue

What is Rayleigh scattering? (or why is the sky blue) The shorter the wavelength, the more light is scattered – blue is scattered more than red. – this is why the sky is blue and sunsets are red. Think of white light from sun as a mixture of R, G and B Blue is scattered the most so sky looks blue when we look away from the sun For same reason sun looks yellow (red + green) More atmosphere allows next shortest wavelengths (green) to scatter so sunset looks red

Why blue and not violet? Spectral response of cones in typical human eye relative response Our eye sensitivity to the blue color is much stronger than that to violet!

Earth Atmosphere Color of Day-time sun vs. Sunset Rays from Sun (not scattered) White or yellow At the end of the day the light rays have more atmosphere to travel through.

Larger particles scatter red as well as blue and hence look white. Dust or smoke Clouds Milk Light scattering in milk and smoke?

Why Water is Greenish Blue Water is transparent to almost all the visible frequencies of light. The color is actually the reflected color of the sky Red is absorbed by the molecules in the water

When you keep looking at the colors in the flag, your eye gets tired and the cells in your eye stop responding to the colors. When you look away from the flag, your brain says, "Hey, I'm not getting any signals that this color is yellow (red & green), so it must be blue."