1.6 Apollo 11 astronauts left an array of "corner mirrors" on the Moon, each one consisting of three mirrors arranged at mutual
right angles like the corner of a box. A corner mirror has the property that light shined on it from any angle is reflected back
exactly in the direction from which it came. Scientists subsequently directed a brief pulse of light from a powerful laser at the
mirror assembly on the Moon, and measured the time required for it to travel to the array and return back to Earth: 2.56 s.
How far away was the array of mirrors at that time?
m
1.7 The human eye contains receptors for just
three colors of light: red, green, and blue. All
the colors we perceive in the world are mixed
in our brains from different proportions of
these three primary colors.
Computer and video display screens, both
CRTs and flat panels, take advantage of this
fact by using color-generating elements of the
same three colors to create the appearance of
any imaginable color. An image on the screen
is built of tiny dots of color called picture
elements, or "pixels". (By default, the period at the end of this sentence consists of just one black pixel.) Each pixel has three
different color levels associated with it, one level for each of red, green, and blue, with integer values from 0 (representing no
color), to 255 (representing the brightest level of a color). For example, (R, G, B) = (255, 0, 0) represents the brightest pure
red, (170, 170, 0) represents an equal mixture of red and green, without any blue, which comes out a sort of dark yellow, and
(127, 127, 127) represents an equal mixture of all three primary colors, each one at half its maximum intensity: a color
sometimes called "50% gray".
(a) If black is the "color" corresponding to a lack of visible light, what are the RGB values of a black pixel? (b) If white is the
"color" corresponding to the brightest possible equal mixture of all colors, what are the RGB values of a white pixel? (c) How
many different colors can a pixel be? (The answer is large, but enter it as a whole number.)
(a) ( , , )
(b) ( , , )
(c) colors
1.8 Thebrightness of a computer screen color is
a percentage value calculated by dividing the
brightest of its three color levels by 255, the
maximum possible brightness. For example,
the brightness of the magenta hue (R, G, B) =
(127, 0, 135) is 135/255 or 53%. What is the
brightness of each of the following colors: (a)
black (0, 0, 0); (b) brightest white
(255, 255, 255); (c) chartreuse (132, 251, 67);
(d) brown (114, 73, 32).
(a) %
(b) %
(c) %
(d) %
1.9 Equally balanced computer screen colors like
(R, G, B) = (0, 0, 0), (255, 255, 255), or
(120, 120, 120) have no predominant hue, and
they look black, white, or some shade of gray
in between: The grays with greater brightness
are closer to white, and the grays with less
brightness are closer to black. A color like
(104, 100, 100) will appear slightly red, but it is
mostly the gray color (100, 100, 100) with only
a small additional amount of red. Color
specialists say that this kind of color is
"unsaturated".
Colors that are "saturated" have a rich predominant color with little gray mixed in. For example the RGB color (246, 15, 18) is
a very saturated red. It only contains a slight constituent of gray, corresponding to its lowest-level component: (15, 15, 15).
Thesaturation of an RGB color equals the percentage of its brightest component color that does not form part of its gray
constituent. If the brightest component of an RGB color has value N, and the dimmest component has value n, then its gray(^572) Copyright 2000-2007 Kinetic Books Co. Chapter 30 Problems