Problem # 49Light [Solved]

Question: One daily phenomenon that we live with is Light. Give a scientific explanation of that phenomenon and the origin of colors.

Answer:

There are two combined origins or causes of the colors of objects we see: (i) the light, and (ii) the nature of the object.

(1) Nature of Light: light is a wave phenomenon as shown below:

                                          (University of Hawai at Manoa)

A wave phenomenon is characterized by: (i) the wavelength, represented by the Greek letter lambda, λ, is the distance between two crests or two troughs; (b) the frequency or the number of wavelengths per second, represented by the Greek letter gamma, γ; (c) the amplitude or the height of the crest of the wave, and (d) its speed

Light, a wave phenomenon, is characterized by a speed, C:

C = 300,000 Km/s = 300,000,000 m/s: constant

C = λγ

λ =   C/γ ;  γ = C/λ

Since C is constant, the relation C = λγ, a constant, shows that longer λ corresponds to lower γ and vice-versa; shorter lambda corresponds to corresponds to higher gamma and vice-versa.

Light bears energy: E = hγ, where h = Planck constant = 6.626 x 10^-34 Js

As you can notice, since E is proportional to γ, short wavelength light bears higher energy than long wavelength light and vice-versa

White light such as Sunlight or light from an electrical bulb, is also a wavelength phenomenon; but in fact, it is a composite light made of many different lights of different wavelengths and frequencies. Those different lights are characterized, in the visible spectrum, by different colors that can be separated by a glass prism, light diffraction, as shown in the Figures below.

                    Diffraction spectra of white light through a glass prism

(clearinghouse.starnetlibraries.org/physics/145-let-s-see-light-in-a-new-way-diffraction-spectra.html)

Light outside of the visible spectrum region, infrared and ultraviolet, cannot be detected by human eyes (but instruments that can detect light in the invisible region of spectrum exist).

According to their wavelength and frequency, the red light (λ = 740 - 625 nm) is the least energetic light, whereas violet light (λ = 435 - 380 nm) is the most energetic light of the visible spectrum.

(1)   The rainbow

The rainbow is the result of diffraction of the sunlight by droplets of water that play the role of a glass prism as shown in the figure representing the diffraction of light previously. You have certainly noticed that the rainbow appears when there is sunlight on a rainy day.

                                                              Rainbow

(2)    Colors of different objects

The color of an object is the result of its interaction with the light.

- If an object reflects the whole sunlight, it will appear white because the whole white sunlight is reflected back; this is popularly known as white color, but in reality it is all colors together.

- If an object absorbs the whole white sunlight, it appears black; this is popularly known as black color, but in reality, black is the absence of colors or absence of light.

- If an object absorbs a color 1, it will appears as white light from which color 1 has been removed (white sunlight – color 1),  in this case the color of the object is called the complementary color of the absorbed color1.

- If an object is transparent to the sunlight and does not absorb any color, it appears transparent and colorless, because the entire sunlight is transmitted.

Here, we are not going to go into the explanation why some materials absorb some color others not.

Hereafter is a ring of colors representing different colors and their respective complementary colors. Colors in diagonal positions are complementary one another: ex: Yellow and Violet are complementary one another.

                                        Ring of Complementary Colors

                                                         (EasyEdit.pro)

If you are wearing a yellow shirt, according to the ring of colors above, this indicates that your shirt absorbs the violet and reflects the yellow color, its complementary color.

On the other side, the green color of the leaves of the trees, indicates that the leaves absorb the red and reflect the green color.

The violet solution of phenolphthalein in a basic solution indicates that the solution absorbs the yellow and lets the violet color pass.

The table below shows the respective complementary colors: