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the light

It is an electromagnetic radiation that exists within a certain part of the electromagnetic spectrum, and the light is clear to the human eye , and its wavelengths range from 380 to 730 nanometers. That is, between infrared and ultraviolet rays, and the main source of light on the surface of the earth is sunlight , where the energy emitted from the sun arrives as light; To contribute to the creation of sugars in green plants, as part of a process known as photosynthesis .

In the past, fire was one of the important sources of light, then man relied on modern lamps and electric lighting. There are some types of animals that emit light from their bodies, such as fireflies , and some types of squid. Thermal radiation is one of the forms of light sources, and the heat is emitted in it in all directions, and reaches the absorption surface at the speed of light, and it does not need a conductive medium, and one of the most prominent examples is the sun's heating of the Earth's surface.

brief history

The ancient Greeks were the first to come up with a few theories related to light, and these theories remained prevalent without any scientific proof of their validity. Muslim scholars during the Middle Ages , the role of the great verification of these theories, and emerged as Ibn al - Haitham , who proved explanations of the nature of light, the main function for him, and gave explanations about the mirrors , and the case of the eclipse and the eclipse , and explain At science Albesriaat- vision process.

The nature of light

For hundreds of years, scientists have been arguing about whether light is a type of wave or not. In the seventeenth century, the theory of the English scientist Isaac Newton (who was the first to study matter and its properties in detail) that light was a continuous stream of particles, but it was opposed by the Dutch scientist Christian Huygens, whose theory considered light to be waves.

The difference over the nature of light remains until now, due to the fact that the light carries the characteristics of a wave such as reflection . Moreover, light also bears the characteristics of a continuous stream of particles, and in the twentieth century, physicists came to the conclusion that light can be considered both particles and waves at the same time, and this property acquired the name of wave-particle duality.

Light properties

Reflection and refraction

When light falls on a surface between two materials with different refractive indices, two things will happen. Reflection of incident light at an angle between the line perpendicular to the surface and the line of light falling on it; That is, the angle of incidence is equal to the angle of reflection, and the refraction of light at an angle depends on the angle of incidence and the type of material, and therefore the light will pass through the surface with different refractive index. If the material allows light to pass through it (transparent).

Dispersion

The difference in the refractive index for each wavelength of light produces an effect called light scattering , and the most prominent example of this property is the passage of white light in a triple prism. The light inside the prism will be refracted at different angles from each other according to the wavelengths, and when the light leaves the prism, the wavelengths of different colors resulting from the refraction of the white light will appear. The longest wavelength of the resulting radiation is the red wavelength. Whereas, its refractive index is less than that of other refracted rays, such as violet.

Absorption

When light passes through a transparent material, some of its energy will be dispersed in the form of thermal energy, so the light will lose some of its intensity, and when this absorption of the thermal energy of the different wavelengths of light occurs; Light passing through this transparent material will only show wavelengths that have not been absorbed. So; Then this passing light will appear as a color called the material absorption color.

The speed of light

The speed of light in a vacuum reaches 1.07 billion kilometers per hour, and the first experiment to measure this speed was by the Danish physicist Romer in 1676 AD using the telescope , and he concluded that the calculation of the speed of light takes 22 minutes to traverse the diameter of the Earth, despite that; Because he could not find out the speed, and then; Another, more accurate test was carried out by the two French scientists (Hippolyte Vizo) and (Leon Fusselt) in the nineteenth century; Where they reached more accurate results than previous experiences; The results showed that the speed of light is 315 million meters per second, and after that many experiments were conducted by scientists such as (Wilhelm Edward), (Weber), (Huygens), Isaac Newton and Albert Einstein.

How a person sees light

When light falls on an object, this object absorbs some of this light and reflects some of the other (since absorption and reflection are properties of light ), and the absorbed and reflected wavelengths depend on the characteristics of the object itself. When looking at the body, light waves are reflected from the surface of the body and enter the retina that is sensitive to light, so the cones in the retina are activated differently depending on their sensitivity to a specific color (64% of human cones are sensitive to red, 34% of them are sensitive to green 2% sensitive to blue), so the resulting signal travels through the optic nerve to the visual cortex in the brain.Who in turn analyzes it. Humans excel at distinguishing colors over most mammals due to the nature of their cones. However, some animals (such as birds and fish) have more cones, which makes them able to see wavelengths other than those seen by humans (such as ultraviolet rays).