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Fig. 2. Reflection: a is reflection from a white surface; b is reflection from a gray
surface; c is reflection from a black surface
Based on the geometric concepts of a ray and a luminous point, we
can assume that at any point on a rough surface, an infinitely small beam of
rays is reflected in all possible directions.
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Light refraction
When light passes through the interface between two transparent
media, the light changes the direction of propagation. This phenomenon is
called light refraction. It is explained by the fact that a stick dipped into water
seems to be broken: the light reflected by a part of the stick in the water,
when passing through the water-air interface, is refracted, i.e. light rays are
bent. The degree of light refraction depends on the difference in the speed
of light in both media: the greater the difference is, the stronger the
refraction is. The ratio of the speed of light in vacuum to the speed of light
in a given medium is called the refractive index of the given medium.
Passing from one transparent medium to another one, with different
physical properties, not perpendicular to the interface between these two
media, the rays of light change their original direction. This phenomenon
obeys certain laws.
The refractive index is called absolute if the beam goes from the
“vacuum” to the medium. If the light goes from one medium to another, the
refractive index is called relative. The refractive index for all transparent
bodies is greater than one, but in most cases it does not reach two.
The refractive index is different for various substances:
• air is 1.00029;
• water is 1.334;
• Canadian balsam is 1.54;
• various glass is 1.51–1.92.
LECTURE QUESTIONS
1. How does light propagate in a uniform medium: rectilinearly or
curvilinearly?
2. What is called a ray in geometric optics?
3. What is a luminous point?
4. What determines the intensity of reflected light?
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5. Under what condition can a certain monochromatic radiation be
illuminated surface
освещенная поверхность
light beam
луч света
light propagation
распространение света
light reflection
отражение света
light scattering
рассеяние света
longwave radiation
длинноволновое излучение
optical instrument
оптический прибор
perfectly black surface
идеально черная поверхность
radiation intensity
интенсивность излучения
ray optics
лучевая оптика
refractive index
показатель преломления
selective spectral reflection
селективное спектральное
отражение
shortwave radiation
коротковолновое излучение
spectral absorption
спектральное поглощение
spectral transmission
спектральная передача
transparent medium
прозрачная среда
water-air interface
граница раздела вода-воздух
wave nature
волновая природа, природа волны
reflected from a particle?
6. What phenomenon explains the fact that a stick dipped into water
seems to be broken?
7. What is the refractive index?
8. What is the absolute refractive index?
9. In what interval does the refractive index of transparent bodies vary?
LECTURE VOCABULARY LIST
4. THEORY OF COLOR SENSATION
Sensitivity of the eye centers to waves of various lengths.
Color perception
Color vision distinguishes humans from most animals. Vision allows
people to perceive the surrounding world. Colors play the most important
role in interpreting information received through the eyes.
The human eye is an extremely precise instrument, sensitive to both
color and light. It is capable of distinguishing up to 150 color hues, more
than 10 saturation and 25 brightness levels. Therefore, a person can perceive
a huge number of color combinations. Cultivating a sense of color is quite
an accessible task for every person with normal vision.
Our eyes allow us to perceive the size, shape, texture, brilliance,
transparency, shimmer, and color of objects. One of the developers of the
color density meter said that the human eye is too perfect a system to try to
create an inexpensive device to be comparable to it.
The human eye contains two types of photoreceptors: rods and
cones. The rods provide black and white vision. They are very sensitive.
The cones allow a person to distinguish colors, but their sensitivity is much
lower.
In the dark, only rods work. People say: “All cats are gray at night”.
Rods can determine the brightness of the radiation with different
wavelengths. Therefore, in low light conditions we can determine that
a green apple is lighter than red one but we cannot distinguish the colors of
the apples.
There are three groups of cones that are sensitive to light of different
wavelengths. The first group perceives the light waves of the “blue”
component of the color, the second group–the “green” component, and the
third group–the “red” component. Depending on the wavelength and
intensity of light waves in the light spectrum, certain groups of cones are
excited more or less. Receptors transmit signals to the brain, and the brain
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interprets these signals into a certain color. Therefore, this feature of
the human eye allows us to see various colors based on the three-dimensional
color sensation system.
The eye sensitivity to incident radiation is characterized by a number
of parameters. First of all, you can evaluate the brightness sensitivity of
the eye. The radiation power of different colors, causing the same light
sensation, varies over a wide range since both rods and cones contribute to
the brightness (lightness) sensation.
Figure 3 shows the luminous efficacy of radiation of the average
human eye. It is also called the relative luminous efficiency. The eye is most
sensitive to green rays and the least sensitive to blue ones. In other words,
this is the efficiency of the human eye. Based on this function it is easy to
determine what part of the light is used to create a light sensation. If a blue
color is as bright as yellow or green colors, its energy is to be several times
higher. The experiments of radiations of equal power show that
monochromatic yellow-green radiation with a wavelength of 555 nm causes
the greatest light sensation. The relative spectral luminous efficiency
(denoted by the letter V) of this radiation is taken equal to one. According
to figure the spectral sensitivity depends on the external illumination. At
dusk, the maximum spectral light efficiency shifts towards blue radiation due
to the different spectral sensitivity of rods and cones. Approximate V values
for the red and blue squares are shown in figure 4.
Physiological features
People cannot determine the magnitude of any stimulus in absolute
values. People cannot determine that it is 19.853 degrees Celsius outside.
They cannot accurately decompose the color of a red apple into a printing
triad. To record exact values of parameters, different devices were invented.
People, on the other hand, are able to determine relative changes, relying
either on direct comparison of two different quantities, or on a comparison
of a quantity with some value from their memory.
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Fig. 3. Spectral sensitivity curve of a human eye
Fig. 4. Approximate V values for red and blue squares.
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In the field of color perception, people can distinguish only two
colors according to their brightness or hue if the difference between them
exceeds a threshold value. Therefore, the measurement system is based on
the estimation of the difference of a threshold from the standard value. The
number of thresholds for hue, brightness and saturation is limited.
Therefore, the number of colors distinguishable by the eye is also finite. As
a result of research, it was determined that the human eye is able to
distinguish up to 100 thousand colors. At the same time, the number of
distinguishable colors of non-luminous objects is less, which makes it
possible to create a color assessment system based not on measuring
parameters, but on comparison with a sample from a catalog of color
standards. Designers select a color according to Pantone Guides.
There are no two people who perceive the same color in the same
way. This is due to the fact that the number of receptors responsible for the
perception of certain wavelengths is individual for each person. The color
perception changes with age. It depends on visual acuity, on the nationality
of people, and even on the color of their hair and what they ate. For example,
after dinner the sensitivity of the human eye to the short-wave (blue part of
the spectrum) increases. However, these differences relate mainly to subtle
shades of color. Therefore, we can assume that most people perceive
primary colors in the same way.
Psychological features
Human vision is a unique mechanism. One of the features of
the human eye is its sensitivity which is constantly changing depending on
different parameters. The eye is constantly adapting to its environment that
leads to very interesting results. Let us consider some of them.
First, let us consider the eye adaptation to brightness. At dusk,
the human eye sensitivity is automatically adjusted to perceive the maximum
dynamic range. In other words, the black and white spots of the eye are
adjusted and the halftone transfer curve changes.
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Secondly, let us consider the eye adaptation to colors. Color
brightness sensitivity
чувствительность к яркости
color combination
сочетание цветов
color density meter
измеритель плотности цвета
color perception
цветовосприятие
color sensation
ощущение цвета
eye adaptation
адаптация глаза
perception of the human eye may change under the influence of previous
lighting conditions. If a person is in a room with bright red light for some
time and then comes into a room with normal lighting, his eye will perceive
the surrounding objects in a greenish tint, which will be especially noticeable
in white areas. This phenomenon is explained by the fact that the lightsensitive pigment regenerates. But this regeneration does not happen
instantly. Firstly, one group of receptors works intensively, and then other
cones start working predominantly in this place of the retina to examine
a white field. This is the ultimate example of color adaptation. There are also
much less noticeable, but much more important results of this process.
LECTURE QUESTIONS
1. How many color tones can the human eye distinguish?
2. What light-sensitive receptors does the human eye contain?
3. Which receptors are responsible for black and white vision?
4. Which receptors allow a person to distinguish colors?
5. What is light sensation?
6. What is the name of the catalog of color standards?
7. How does color perception change the human vision after eating?
8. What is the essence of brightness adaptation?
9. What is the mechanism of color adaptation?
LECTURE VOCABULARY LIST
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hue
оттенок
light sensation
ощущение света
lighting conditions
условия освещения
light-sensitive pigment
светочувствительный пигмент
luminous efficiency
интенсивность излучения
precise instrument
точный инструмент
retina
сетчатка глаза
surrounding world
окружающий мир
5. PRINCIPLES OF COLOR HARMONY
Color circle and cone
Color harmony has been of interest to many scientists and artists for
a long time. The English physicist Isaac Newton was the first who tried to
create a color system and systematize colors. He arranged all the colors of
the solar spectrum in a circle, adding to them the missing purples. Newton's
color wheel began to be used for scientific and artistic purposes. He used
three primary colors - blue, yellow and red.
After that, various color systems were created. One of the most
convenient systems is the color wheel developed by V.M. Shugaev. Unlike
Newton, Shugaev used four primary colors, including green in the triad of
primaries.
Although the green color is derived from a mixture of yellow and
blue, it is perceived as absolutely neutral in relation to its “parents”. Shugaev
placed four colors in a circle at the ends of mutually perpendicular
diameters. Intermediate colors were placed between the main colors in four
quarters, which made up four groups:
• yellow-red;
• blue-red;
• blue-green;
• yellow-green.
Each color is characterized by three parameters: hue, brightness, and
saturation.
Two colors that have the same parameters are equal to each other.
If at least one parameter is different, then the colors differ. For example, if
you take red, then purple will be close in color tone on one side, and redyellow (orange) on the other. Yellow is adjacent to orange, then yellowgreen is adjacent to green, and then blue-green is adjacent to blue and
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