# Excursions in Physics

## Color and Vision

(this is Chapter 16 of Adventures in Physics, available only online)

Ch 17, Color and Vision; 2, 3, 4, 5, 6, 7, 9, 11, 12, 17, 19, 20, 25, 26

28, 31, 32, 33, 38, 40, 41, 47

Calculational 5,6

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17.2 What color is light whose wavelength is 500 nm?

Violet light has a wavelength of 400 nm. This is longer than that; it is blue or blue-green.

17.3 What color is light whose wavelength is 600 nm?

This is longer yet. This is yellow light.

17.4 How would you describe EM radiation whose wavelength is 300 nm?

This is shorter than violet. It is beyond the violet end of the visible spectrum. We call this ultraviolet (UV) radiation.

17.5 What would you call EM radiation whose wavelength is 900 nm?

This is longer than red light. It is beyond the red end of the visible spectrum. We call this infrared (IR) radiation.

17.6 Why are sunburns caused by UV radiation but not by IR radiation?

Biological or cellular damage occurs when all the energy of one "photon" or "lump" of light -- or other EM radiation -- is deposited into a cell. UV radiation, with its shorter wavelength and higher frequency -- carries more energy in each "photon" than does IR radiation.

17.7 "Monochromatic" is a big word that means "single-wavelength". We sometimes talk of monochromatic violet light or monochromatic yellow light or monochromatic red light. Can we reasonably talk about monochromatic white light?

White light is a collection of many different wavelengths so it can not be monochromatic.

17.9 Why might someone say that X-rays are very high frequency light waves?

X-rays have shorter wavelengths and higher frequencies than light waves. Both are EM waves.

17.11 Could someone correctly say that X-rays are very high frequency sound waves?

No, sound is quite different from any EM waves, including X-rays.

17.12 Why is no spectrum produced if white light passes through a rectangular solid made of glass? That is, why are the non-parallel sides of a prism so important in producing a spectrum?

The refractions at the two parallel sides of a rectangular solid simply do and un-do the bending of the light. They cancel each others effect.

17.17 Why is there more "fire" or sparkle in a diamond than in ordinary glass?

The index of refraction of a diamond is quite high. And its dispersion -- the variation of index of refraction with wavelength -- is greater than for ordinary glass.

17.19 Why is the region inside a rainbow brighter than the region outside the rainbow?

Light coming into a spherical raindrop at different distances from the axis of symmetry is scattered at various angles that are all inside the rainbow, none is scattered at angles that would be seen outside the rainbow.

17.20 Why is the region between a double rainbow dark?

After reflecting twice inside a spherical raindrop, the exiting light comes out at angles that are outside the second rainbow but no light comes out at angles that would be seen inside the second rainbow.

17.25 What does the cornea of the eye do?

Most of the bending of the light occurs at the cornea as the light comes from air into your eye.

17.26 What does the crystalline lens of the eye do?

The crystalline lens bends the light too, making small corrections to adjust the focus of the light so a clear and sharp image is formed on the retina at the back of your eye.

17.28 What is the retina of the eye?

The retina is the light-sensitive part of your eye, somewhat similar to the film in a camera.

17.31 Red and blue spotlights shining onto a white screen produce what color?

Magenta (sort of "rose" or "hot-pink" or "fuschia")

17.32 When the blue and green phosphors on a color TV screen are illuminated, what color will be seen?

Cyan (sort of torquoise or aquamarine).

17.33 When the red and green phosphors on a color monitor are glowing, what color will be seen?

Yellow

17.38 If a singer comes on stage wearing a yellow outfit under white lights, what will her costume look like if the lights are suddenly change to blue? What if the lights are suddenly changed to red? What if they are changed to green?

The yellow outfit absorbs blue light and reflects green and red which is then seen as yellow.

Changing the light to blue will mean no light is reflected; the outfit will look black (or dingy grey).

Changing the light to red will mean the outfit still reflects red light and, so, will look red.

Changing the light to red will mean the outfit still reflects green light and, so, will look green.

17.40 What color is produced when yellow paint is mixed with cyan (blue-green) paint?

Yellow paint absorbs blue light. Cyan absorbs red light. Now we have both blue and red light being absorbed. Only the green light will be reflected.

17.41 What color is produced when magenta (or fuschia) paint is mixed with cyan (blue-green) paint?

Magenta paint absorbs green light. Cyan absorbs red light. Now we have both green and red light being absorbed. Only the blue light will be reflected.

17.47 If one spotlight shines white light through a red filter and another spotlight shines white light through a green filter, what will be seen where these two lights overlap on a white screen? Instead, if a single spotlight shines white light through overlaping red and green filters, what will be seen?

Red and green spotlights will produce yellow light (as you have seen in class).

A red filter absorbs green and blue light. A green filter absorbs red and blue light. No light will pass through both filters.

5. What is the speed of light in Heavy Flint glass for red light whose wavelength is 770 nm?

From the table, we find the index of refraction is n = 1.628
v = c/n

v = (3.00 x 108 m/s) / 1.628

v = 1.843 x 108 m/s

6. As light goes from air to water, its speed decreases but its frequency remains the same. Yellow light has a wavelength of 550 nm in air. What is its wavelength in water where the index of refraction is 1.33?

vwater = c/n

vwater = (3.00 x 108 m/s) / 1.33

vwater = 2.256 x 108 m/s

v = (frequency) (wavelength)

frequency = v / wavelength

frequency does not change

frequency in water = frequency in air

fwater = fair

fwater = vwater / wavelengthwater = vair / wavelengthair = fair

vwater / wavelengthwater = vair / wavelengthair

[ c / n ] / wavelengthwater = c / 550 nm

[ wavelengthwater] / [ c / n ] = 550 nm / c

[ wavelengthwater] / [ 1 / n ] = 550 nm

wavelengthwater = 550 nm / n

wavelengthwater = 550 nm / 1.33

wavelengthwater = 414 nm

Since the frequency does not change, going slower in the water means the wavelength must be smaller since v = (frequency) (wavelength).

Typical multiple-guess questions:

1. The sensation of color is seen when light falls on the eye's

a) rods

b) cones

c) cornea

d) crystalline lens

2. The cones in the retina of the eye are

a) most densely packed at the center of vision, the fovea.

b) uniformly spread along the visual field.

c) concentrated along the periphery of vision.

d) concentrated at the "blind spot" where the optic nerve attaches to the retina.

3. Color depends on what characteristic of light?

a) wavelength

b) frequency

c) both of these ( frequency x wavelength = speed )

d) neither of these

4. The colored dots that make up the color on a TV screen are

a) red, yellow, green

b) red, yellow, blue

c) red, green, blue

d) cyan, magenta, yellow

5. Magenta light is a mixture of

a) red and yellow

b) cyan and yellow

c) red and blue

d) yellow and blue

6. Colors seen on TV result from color

a) subtraction

c) neither of these

d) either of these

7. Colors seen on the cover of a magazine result from color

a) subtraction

c) neither of these

d) either of these

8. Different colors of light correspond to different light

a) velocities

b) intensities

c) polarities

d) frequencies

9. Look at a full-color picture in a magazine with a magnifying glass and you'll see that the inks used are

a) red, green, cyan, and blue.

b) red, green, blue, and black.

c) magenta, cyan, yellow, and black.

d) magenta, cyan, yellow, and blue.

10. A sheet of red paper will look black when illuminated with

a) red light

b) cyan light

c) yellow light

d) magenta light

11. A blue object will appear black when illuminated with

a) blue light

b) yellow light

c) red light

d) cyan light

12. A mixture of red and green light produces

a) blue

b) magenta

c) yellow

d) cyan

13. A spotlight produces white light which then passes through a filter that absorbs the yellow light. What color is the light that passes through?

a) red

b) green

c) blue

d) yellow

14. A spotlight produces white light which then passes through a filter that absorbs the red light. What color is the light that passes through?

a) red

b) yellow

c) magenta

d) cyan

15. Different colors are dispersed by a prism because different colors in the prism have different

a) speeds

b) directions

c) energies

d) momenta

16. Rainbows are a result of

a) dispersion and reflection.

b) dispersion and polarization.

c) reconstruction and polarization.

d) constructive and destructive interference.

17. A single or primary rainbow is the result of

a) two reflections and one refraction

b) two reflections and one refraction

c) one reflection and two refractions

d) one reflection and one refraction

within or by or from a raindrop.

18. A secondary or double rainbow is the result of

a) two reflections and two refractions

b) two reflections and one refraction

c) one reflection and two refractions

d) one reflection and one refraction

within or by or from a raindrop.

1. The sensation of color is seen when light falls on the eye's

a) rods

b) cones

c) cornea

d) crystalline lens

2. The cones in the retina of the eye are

a) most densely packed at the center of vision, the fovea.

b) uniformly spread along the visual field.

c) concentrated along the periphery of vision.

d) concentrated at the "blind spot" where the optic nerve attaches to the retina.

3. Color depends on what characteristic of light?

a) wavelength

b) frequency

c) both of these ( frequency x wavelength = speed )

d) neither of these

4. The colored dots that make up the color on a TV screen are

a) red, yellow, green

b) red, yellow, blue

c) red, green, blue

d) cyan, magenta, yellow

5. Magenta light is a mixture of

a) red and yellow

b) cyan and yellow

c) red and blue

d) yellow and blue

6. Colors seen on TV result from color

a) subtraction

c) neither of these

d) either of these

7. Colors seen on the cover of a magazine result from color

a) subtraction

c) neither of these

d) either of these

8. Different colors of light correspond to different light

a) velocities

b) intensities

c) polarities

d) frequencies

9. Look at a full-color picture in a magazine with a magnifying glass and you'll see that the inks used are

a) red, green, cyan, and blue.

b) red, green, blue, and black.

c) magenta, cyan, yellow, and black.

d) magenta, cyan, yellow, and blue.

10. A sheet of red paper will look black when illuminated with

a) red light

b) cyan light

c) yellow light

d) magenta light

11. A blue object will appear black when illuminated with

a) blue light

b) yellow light

c) red light

d) cyan light

12. A mixture of red and green light produces

a) blue

b) magenta

c) yellow

d) cyan

13. A spotlight produces white light which then passes through a filter that absorbs the yellow light. What color is the light that passes through?

a) red

b) green

c) blue

d) yellow

14. A spotlight produces white light which then passes through a filter that absorbs the red light. What color is the light that passes through?

a) red

b) yellow

c) magenta

d) cyan

15. Different colors are dispersed by a prism because different colors in the prism have different

a) speeds

b) directions

c) energies

d) momenta

16. Rainbows are a result of

a) dispersion and reflection.

b) dispersion and polarization.

c) reconstruction and polarization.

d) constructive and destructive interference.

17. A single or primary rainbow is the result of

a) two reflections and one refraction

b) two reflections and one refraction

c) one reflection and two refractions

d) one reflection and one refraction

within or by or from a raindrop.

18. A secondary or double rainbow is the result of

a) two reflections and two refractions

b) two reflections and one refraction

c) one reflection and two refractions

d) one reflection and one refraction

within or by or from a raindrop.

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