The phenomenon of refraction of light is ... The Law of Refraction of Light

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 falls on a section between two media at an angle of 90 °, then nothing happens, it continues its movement in the same direction at a right angle to the interface. If the angle of incidence of the beam differs from 90 °, a phenomenon of light refraction occurs. This, for example, produces such strange effects as an apparent fracture of an object partially submerged in water or mirages observed in a hot sand desert.

History of the discovery

In the first century AD. E. Ancient Greek geographer and astronomer Ptolemy attempted to mathematically explain the magnitude of refraction, but the law he proposed later proved to be unreliable. In the XVII century. The Dutch mathematician Willebrord Snell developed a law that determined the magnitude associated with the ratio of the incident and refracted angles, which was subsequently called the refractive index of the substance. In fact, the more a substance is capable of refracting light, the greater this index. The pencil in the water is "broken" because the rays coming from it change their path at the air-water interface before reaching the eye. To the dismay of Snell, he never managed to find the cause of this effect.

In 1678 another Dutch scientist Christian Huygens developed a mathematical dependence explaining Snellius's observations and suggested that the phenomenon of refraction of light is a result of the different speed with which the beam passes through two media. Huygens determined that the ratio of the angles of light passing 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 greater refractive index, the light moves more slowly. In other words, the speed of light through the material is inversely proportional to its refractive index. Although later the law was experimentally confirmed, for many researchers of the time this was not obvious, since there were no reliable means of measuring the speed of light. Scientists thought that its speed does not depend on the material. Only 150 years after Huygens' death, the speed of light was measured with sufficient accuracy, proving his rightness.

Absolute refractive index

The absolute refractive index n of a transparent substance or material is defined as the relative velocity at which light passes through it relative to the speed in a vacuum: n = c / v, where c is the speed of light in a vacuum, and v is in the material.

Obviously, the refraction of light in a vacuum devoid of any substance is absent, and the absolute value in it is 1. For other transparent materials, this value is greater than 1. Refraction of light in air (1,0003) can be used to calculate the indices of unknown materials.

Snell's Laws

We introduce some definitions:

• The incident beam is a ray that is approaching the separation of media;
• The point of incidence is the point of separation into which it falls;
• The refracted beam leaves the separation of media;
• Normal - a line drawn perpendicular to the separation at the point of incidence;
• The angle of incidence is the angle between the normal and the incident ray;
• The angle of refraction of light can be determined as the angle between the refracted ray and the normal.

According to the laws of refraction:

1. The incident, refracted ray and the normal are in the same plane.
2. 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 the first medium: sin i / sin r = n _r / n _i .

The law of refraction of light (Snellius) describes the relationship between the angles of two waves and the refractive index of two media. When the wave passes from a less refractive medium (eg air) to a more refractive (for example, water), its velocity decreases. Conversely, when light passes from water to air, the speed increases. The angle of incidence in the first medium with respect to the normal and the angle of refraction in the second will differ in proportion to the difference in the refractive indices between these two substances. If the wave passes from a medium with a low coefficient to a medium with a higher one, then it bends in the direction to the normal. And if on the contrary, it is removed.

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 the constant, which is the ratio of the velocities of light in both media.

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 are the result of refraction are often observed in everyday life. The effect of a "broken" pencil is one of the most common. The eyes and the 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 exerts a great influence on the refraction of light . In other words, the substance has many refractive indices, which can differ when the color or wavelength changes.

Such a change occurs 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 the wavelength. As the wavelength increases, the phenomenon of light refraction becomes less pronounced. This is confirmed by the fact that violet refracts more red, since its wavelength is shorter. Due to the dispersion in ordinary glass, a certain splitting of light into its components occurs.

Decomposition of light

At the end of the 17th 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 purple through blue, green, yellow, orange and ending in red. Working in a darkened room, Newton placed a glass prism in a narrow beam that penetrated through the hole in the window shutters. When passing through the prism, light was refracted - the 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 "scatter" the white light, refracting each color from a different angle. Newton could not separate the colors, passing them through the second prism. But when he put the second prism very close to the first in such a way that all the dispersed colors entered the second prism, the scientist established that the colors recombine, again forming a white light. This discovery convincingly proved the spectral composition of light, which can be easily separated and connected.

The phenomenon of dispersion plays a key role in a large number of diverse phenomena. The rainbow occurs as a result of the refraction of light in rain drops, producing an impressive spectacle of spectral decomposition, similar to the one that occurs in the prism.

Critical angle and total internal reflection

When passing through a medium with a higher refractive index in a medium with a lower wave path, the angle of incidence is determined by the separation of the two materials. If the angle of incidence exceeds a certain value (depending on the refractive index of the two materials), it reaches a point where light is not refracted 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 the critical angle, then the light is reflected back. This phenomenon is called the total internal reflection. Examples of its use are diamonds and optical fibers. Diamond cuttings contribute to complete internal reflection. Most of the rays entering through the top of the diamond will be reflected until they reach the upper surface. This is what gives brilliance to their brilliance. The optical fiber is a glass "hair", so thin that when the light enters one end, it can not come out. And only when the beam reaches the other end, he can 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 mug of beer seems more complete, and the sun sets a few minutes later than it really is. Millions of people use refractive power to correct vision defects with glasses and contact lenses. By understanding these properties of light and controlling them, we can see details invisible to the naked eye, regardless of whether they are on a microscope slide or in a distant galaxy.