QUESTIONS: COLORS
1. You look at white sunlight through a stained glass window which is colored yellow, and then
through another stained glass window which is colored red. The two windows are placed one in front
of the other. What color is the light that shines through?
2. You are in a dark room and can’t see a thing. You can’t see that in the room is a white sheet of
paper, a red cherry, a yellow lemon, and a green plant. You turn on a lamp, but the lamp only shines
red light. What colors do these objects appear to be?
3. Scientists often say that orange light has a wavelength of 600 nm. Why is this inaccurate?
4. A healthy person stares at a green object. Then she quickly shifts her attention to a similar white
object. Assuming negative afterimage what color may she perceive?
5. A small object radiates light of low intensity in the region 500-600 nm. After 25 min of adaptation
in a dark room a healthy person can see the object. What color does the person perceive??
6. If chlorophyll absorbed only blue wavelengths, what color would leaves be? What if it absorbed
only red wavelengths?
7. (a) What's the difference between light and radio waves?
(b) What's the difference between green light and yellow light?
8. Red light has a wavelength of 700 nm. What is its frequency?
9. The graph shows the response of the three types of cone cells
(z, y, and x are blue, green, and red respectively) to different
wavelengths of light. (a) Where on the graph represents the
wavelength of green light? (b) What wavelength of monochromatic
light looks just like a mixture of equal parts red and green light?
(c) If a person’s z (blue) cells were not working, what are two
wavelengths that of light that would look identical? (d) What
wavelength of monochromatic light stimulates only the z cells?
ANSWERS
1. When you shine white light through the yellow window, only yellow light comes out the other side.
That means that all the non-yellow parts were absorbed by the window. Since yellow light is a mix of
wavelengths that stimulate the green and red cone cells in your retina, the window must be
absorbing all the blue light (that is, the wavelengths that stimulate the blue cells). Similarly, the red
window lets red light through, and blocks the rest (both green and blue). So what colors get through
both windows? Only red light can pass through both, and looking at white light through them will
make the light appear red.
2. The white paper reflects all wavelengths. So it reflects the red light and looks red. The red cherry
reflects only red light, so it reflects that lamp light and still looks red. The yellow lemon reflects both
red and green light (that’s why it looks yellow), so it will reflect the red lamp light and appear red. The
green plant reflects only green light (that’s why it looks green), so it will not reflect the red lamp light,
and it will appear black.
3. While monochromatic orange light has a wavelength of 600 nm, you can also make orange light
by mixing a combination of other wavelengths that stimulate the cones in your eyes in exactly the
same way that monochromatic orange light does. For example, a combination of red and yellow light
will produce orange light, even though it does not include any 600 nm wavelengths.
4. Staring at the green object depletes her green-detecting cone cells. When she looks at a white
object, the white light includes all the wavelengths, including those that stimulate the red cone cells,
those that stimulate the blue cone cells, and those that stimulate the green cone cells. But because
the green cells are depleted, mostly the red and blue ones will be firing, so the person will see
magenta – which is the complementary color to green.
5. When adjusting to a dark room, the cone cells increase their sensitivity as much as possible.
However, after about 10 minutes, if they still can’t see anything, then the more sensitive rod cells
start to increase their sensitivity. Rod cells can’t detect color. So if the person can’t see the object
until 25 minutes of adjustment, then it must be the rod cells that are picking up the light, and it will be
seen as white light regardless of the actual wavelength.
6. Chlorophyll makes leaves green. That means chlorophyll absorbs all the red and blue light, and
reflects the green. If chlorophyll absorbed only blue wavelengths, then it would reflect red and green
light, so that leaves would appear yellow. If chlorophyll absorbed only red wavelengths, then it would
reflect blue and green light, so that leaves would appear cyan. In the fall, when leaves lose their
chlorophyll, you can see the red and yellow colors that were always there, but hidden by the
chlorophyll that was absorbing all that red light.
7. (a) They are both electromagnetic waves. The only difference is their frequency. Visible light is
one of those frequencies our eyes can detect. Radio waves are longer, lower frequency waves.
While visible light waves are hundreds of nanometers long, radio waves can be a kilometer long.
(b) Just as in part (a), the only difference is their frequency. Green light has higher frequency than
yellow light, and our eyes interpret the different frequency as a different color.
8. The equation relating the frequency and wavelength of a wave is given by v = fλ. In the case of
electromagnetic waves like light, the speed of the wave in empty space is c = 3.0 × 108 m/s. So, f =
c/λ = (3.0 × 108 m/s)/(700 × 10-9 m) = 4.3 × 1014 Hz.
9. (a) Green light has wavelength about 510 nm, where only the green cells are being stimulated and
virtually no red or blue cells are stimulated—this is not the wavelength that most stimulates the
green cells, because that wavelength also stimulates red cells. (b) There are two wavelengths of
monochromatic light which stimulate the x and y cells equally, where the graph for x and y cells
cross each other: At about 565 nm (yellow light), and at about 460 nm (although this wavelength
stimulates the z cells as well – and it stimulates the z cells far more the than red or green cells). (c) If
someone’s blue (z) cells weren’t working, 565 nm and 460 nm light would look identical, since they
both equally stimulate the x and y cells. (d) There is no wavelength of monochromatic light that
stimulates only the blue (z) cells. Anything that stimulates z also stimulates x or y cells, at least a
little.