The phenomenon of refraction of light is a physical phenomenon that occurs every time a wave moves from one material to another, in which its propagation velocity changes. Visually, it manifests itself in the fact that the direction of wave propagation changes.
Physics: Refraction of Light
If the incident beam enters the section between two media at an angle of 90 °, then nothing happens, it continues to move in the same direction at a right angle to the interface. If the angle of incidence of the beam differs from 90 °, the phenomenon of refraction of light occurs. This, for example, produces such strange effects as the seeming kink of an object partially submerged in water or mirages observed in a hot sandy desert.
Discovery story
In the first century AD e. the ancient Greek geographer and astronomer Ptolemy tried to mathematically explain the magnitude of refraction, but the law he proposed later turned out to be unreliable. In the XVII century. Dutch mathematician Willebord Snell developed a law that determined a value related to the ratio of incident and refracted angles, which was later called the refractive index of matter. In fact, the more a substance is able to refract light, the greater this indicator. The pencil in the water is “broken” because the rays coming from it change their path at the air-water interface before they reach the eyes. To Snell's disappointment, he was never able to discover the cause of this effect.
In 1678, another Dutch scientist, Christian Huygens, developed a mathematical relationship explaining Snell's observations and suggested that the phenomenon of refraction of light is the result of different speeds at which a beam passes through two media. Huygens determined that the ratio of the angles of light transmission through two materials with different refractive indices should be equal to the ratio of its velocities in each material. Thus, he postulated that through media having a higher refractive index, light moves more slowly. In other words, the speed of light through a material is inversely proportional to its refractive index. Although the law was subsequently experimentally confirmed, for many researchers of that time this was not obvious, since reliable means of measuring the speed of light were lacking. It seemed to scientists that its speed was independent of material. Only 150 years after Huygens' death, the speed of light was measured with sufficient accuracy, proving its correctness.
Absolute Refractive Index
The absolute refractive index n of a transparent substance or material is defined as the relative speed at which light passes through it relative to the speed in vacuum: n = c / v, where c is the speed of light in vacuum and v is in the material.
Obviously, there is no refraction of light in a vacuum devoid of any substance, and the absolute value in it is equal to 1. For other transparent materials this value is greater than 1. To calculate the performance of unknown materials, refraction of light in air can be used (1,0003).
Snell's Laws
We introduce some definitions:
- incident beam - a beam that approaches the separation of media;
- point of fall - the point of separation at which it falls;
- the refracted beam leaves the separation of media;
- normal - a line drawn perpendicular to the separation at the point of incidence;
- angle of incidence - the angle between the normal and the incident beam;
- the angle of refraction of light can be determined as the angle between the refracted beam and the normal.
According to the laws of refraction:
- The incident, refracted ray and normal are in the same plane.
- The ratio of the sines of the angles of incidence and refraction is equal to the ratio of the refraction coefficients of the second and first medium: sin i / sin r = n r / n i .
The law of light refraction (Snellius) describes the relationship between the angles of two waves and the refractive indices of two media. When a wave passes from a less refractive medium (for example, air) to a more refracting one (for example, water), its speed drops. On the contrary, when light passes from water to air, the speed increases. The angle of incidence in the first medium relative to the normal and the angle of refraction in the second will differ in proportion to the difference in the refractive indices between the two substances. If a wave passes from a medium with a low coefficient to a medium with a higher coefficient, then it bends towards the normal. And if on the contrary, then it is deleted.
Relative refractive index
The law of refraction of light shows that the ratio of the sines of the incident and refracted angles is equal to a constant, which is the ratio of the speeds of light in both environments.
sin i / sin r = n r / n i = (c / v r ) / (c / v i ) = v i / v r
The ratio n r / n i is called the relative refractive index for these substances.
A number of phenomena that result from refraction are often observed in everyday life. The effect of a “broken" pencil is one of the most common. The eyes and brain follow the rays back into the water, as if they are not refracted, but come from the object in a straight line, creating a virtual image that appears at a shallower depth.
Dispersion
Careful measurements show that the wavelength of the radiation or its color has a great influence on the refraction of light. In other words, a substance has many refractive indices, which can vary with a change in color or wavelength.
Such a change takes place in all transparent media and is called dispersion. The degree of dispersion of a particular material depends on how much the refractive index varies with wavelength. With increasing wavelength, the phenomenon of light refraction becomes less pronounced. This is confirmed by the fact that purple refracts more than red because its wavelength is shorter. Due to the dispersion in ordinary glass, a known splitting of light into its components occurs.
Decomposition of light
At the end of the XVII century, Sir Isaac Newton conducted a series of experiments that led to his discovery of the visible spectrum, and showed that white light consists of an ordered array of colors, ranging from violet through blue, green, yellow, orange and ending with red. Working in a darkened room, Newton placed a glass prism in a narrow beam, penetrating through the hole in the window shutters. When passing through a prism, light was refracted - glass projected it onto the screen in the form of an ordered spectrum.
Newton came to the conclusion that white light consists of a mixture of different colors, and also that the prism “scatters” white light, refracting each color from a different angle. Newton could not separate the colors, passing them through the second prism. But when he set the second prism very close to the first so that all dispersed colors entered the second prism, the scientist found that the colors recombine, again forming white light. This discovery convincingly proved the spectral composition of light, which can be easily divided and connected.
The phenomenon of dispersion plays a key role in a large number of diverse phenomena. A rainbow arises from the refraction of light in raindrops, producing an impressive spectacle of spectral decomposition similar to that which occurs in a prism.
Critical angle and total internal reflection
When passing through a medium with a higher refractive index into a medium with a lower wave path, it is determined by the angle of incidence relative to the separation of the two materials. If the angle of incidence exceeds a certain value (depending on the refractive index of two materials), it reaches a point where light does not refract into a medium with a lower index.
The critical (or limiting) angle is defined as the angle of incidence resulting in a refraction angle of 90 °. In other words, while the angle of incidence is less than critical, refraction occurs, and when it is equal to it, the refracted beam passes along the place of separation of the two materials. If the angle of incidence exceeds a critical angle, then the light is reflected back. This phenomenon is called full internal reflection. Examples of its use are diamonds and optical fibers. Diamond cutting contributes to full internal reflection. Most of the rays entering through the top of the diamond will be reflected until they reach the top surface. This is what gives diamonds their bright shine. An optical fiber is glass “hair” so thin that when light enters one end, it cannot come out. And only when the beam reaches the other end can it leave the fiber.
Understand and manage
Optical instruments ranging from microscopes and telescopes to cameras, video projectors, and even the human eye rely on the fact that light can be focused, refracted, and reflected.
Refraction produces a wide range of phenomena, including mirages, rainbows, optical illusions. Because of the refraction, a thick-walled beer mug seems more complete, and the sun sets a few minutes later than it actually is. Millions of people use the power of refraction to correct vision defects with glasses and contact lenses. By understanding and managing these properties of light, we can see details invisible to the naked eye, whether they are on a microscope slide or in a distant galaxy.