Homework 3- Answers to help you study

COLOR

1. If a patch of cloth only reflects back all wavelengths greater than 670 nm, what color will we perceive when the cloth is shown under a spotlight of normal white light?

The patch of cloth is absorbing all wavelengths that are shorter than 670 (e.g. all the blues, greeens, etc.) White light contains all wavelengths in the visual spectrum. If it is shone onto this cloth, the cloth will only reflect back the wavelengths longer than 670. (i.e. only the wavelengths longer than 670 will reach our eye). If we look at the light spectrum diagram it shows that the wavelengths above 670 are all reddish, so the answer of what color we perceive is red.

If that same patch of cloth is shown under a spotlight that contains the following wavelengths (560, 610, and 680), what color will we perceive and why?

Since the cloth only reflects back wavelengths larger than 670, only the 680 nm wavelength will be reflected back at our eye. So we will only perceive red.

If that same patch of cloth is shown under a spotlight that contains the following wavelengths (480, 520, and 630), what color will we perceive and why?

Since these wavelengths are all below the 670 cutoff, they are all absorbed by the cloth. This means no wavelengths will be reflected back at the eye. So we will perceive the color of the cloth to be black in this situation.

 

2. A spotlight of 420 nm is shone onto the same white surface as a spotlight of 680 nm. What color of light will we perceive shining on that white surface and why?

This is additive color mixing. A 420 nm light is a bluish color. A 680 nm light is a reddish color. These two put together would yield the perception of a purple (red and blue == stimulation of short cones and stimulation of long cones leads to purple (see relative proportion of cone responding diagram)). Note in the additive color diagram that red and blue together make a purple color.

(The only time combining wavelengths will achieve gray is when the wavelengths are exact complementary colors. e.g. a specific blue and yellow or a specific green and magenta).

3. A beam of light made up of 400, 550 and 650 nm wavelengths passes through a light filter that shows all wavelengths between 450 and 600 nm. What color light will be perceived and why?

Subtractive color mixing using a filter. The filter will only allow waves between 450 and 600, so the only wavelengths to be let through to reach the eye is the 550 nm wavelength. This is a greenish color, so what we'll perceive is a green.

If that same filtered light (coming out of the filter) then contacts a patch of paint that reflects wavelengths between 400-500 nm and 600-700 nm, what color will be perceived and why?

That filter light is only letting through the 550 nm wavelength. If that contacts a patch that only reflects either 400-500 and 600-700, then that 550 nm wave will be absorbed by the paint and will not be reflected back at the eye. So, in this situation only black will be perceived.