Main topic information
Vocabulary/new concepts:
A light source is a body that emits light, e.g. the Sun, a light bulb…
The refraction of light is the change in the direction of rays as they pass from one medium to another of different densities. Rays bend towards the perpendicular if they are decelerating (going from a less dense medium to a higher-density medium), and refract away from the perpendicular if they are accelerating (going from a denser to a less dense medium).
The refractive index of a substance expresses how many times slower light travels in a substance than in a vacuum.
The light spectrum is the bundle of coloured rays that make up white light.
The colour of a body corresponds to the component of white light that the body reflects from its surface.
Light sources
Light allows us to perceive the world around us with its shapes, colours, and sizes. Without light, there is no life!
We distinguish different luminous objects, which are called light sources. They can be:
- natural, created by nature – Sun, Moon, stars, fire, lightning, aurora borealis, and some living organisms (fireflies, fish, jellyfish and sponges; this phenomenon is called bioluminescence);
- man-made – light bulbs, candles, torches, lanterns, computer and phone screens, lasers, etc.
Light sources emit light energy into the environment. Most of them emit light when heated and their heat energy is converted into light energy. But some have no heat, even though they emit bright light – for example, fireflies, lasers, etc. The most important source of light for us is the Sun.
Propagation of light
Light propagates in a straight line as electromagnetic waves until it encounters an obstacle or environmental change. It passes through everything transparent, including air, water, glass and the vacuum of space. This allows us to see the objects around us. However, a shadow is formed when it encounters an obstacle, e.g., opaque substances.
Source: https://static6.depositphotos.com/1002818/607/i/600/depositphotos_6077170-stock-photo-colorful-sunset.jpg
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The speed of propagation of light in a vacuum is the greatest in the universe – about 300,000 kilometres per second. In 8 minutes and 19 seconds, light from the Sun travels a distance of 150 million kilometres to reach Earth. That is why we perceive light almost instantaneously. The speed of light in a medium other than a vacuum is lower and depends on the properties and characteristics of the given medium.
Light and vision
- Reflection of light
Most bodies do not emit light, but we see them because the light that falls on the surfaces is reflected in all directions. Some of the reflected rays reach our eye, which contains light-sensitive cells that provide the sense of sight. The brain interprets the signals from the eye and creates an image of the object. In the absence of light, these processes do not take place and we cannot see in the dark.
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Mirroring. Source: https://depositphotos.com
Transparent materials, such as glass, transmit light without changing its direction of propagation. However, they also reflect some light, which is why we can see them.
Opaque (matte) materials do not transmit light and therefore cast a strong shadow.
We perceive objects in different colours because light waves have different lengths. We see short waves in blue and long waves in red.
An interesting fact is that the vision of animals differs from that of humans and depends on the specifics of the environment they inhabit and their lifestyle. Many of them see only in certain colours and shades. For example, for cows, the world is orange-red, for hamsters – blue-green, for sharks – black and white. Others see better at night than during the day, as they pick up fluorescent and colour tones invisible to the human eye. Owls, cats and frogs are typical examples of excellent night vision.
Check out the video:
The World through the eyes of animals
- Refraction of light
In some media, light does not propagate in a straight line, but changes direction – refraction of light occurs. This is due to a change in the speed of light in a different medium. When light rays pass from one medium to another, for example from air into water or through glass, they change direction. This phenomenon is known as refraction.
Semi-transparent materials, such as plexiglass or frosted glass, transmit light but refract it – changing its direction.
If we put a straw in a glass of water, it will look bent or broken. This is because the light from the straw bends as it passes between the water, the glass and the air in our eye. Our brain assumes that the light has come in a straight line, so the image of the straw in the water is distorted.
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Refraction is the change in the direction of propagation of electromagnetic waves when they slow down or speed up. Light waves refract when they pass from one medium to another, and water waves refract when they pass between deep and shallow water. Refraction occurs when the speed of wave propagation changes. After refraction, the frequency of the waves does not change, but the wavelength changes. If the wave slows down, it refracts in the direction of the normal (towards the perpendicular). If a wave is accelerated, it is refracted away from the normal (from the perpendicular).
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The lenses in glasses, cameras and telescopes use refraction to bend and focus light rays, thus creating an image. Almost all optical instruments work on this principle.
Refraction of light can create interesting optical illusions – distortion of images, mirages (e.g. of water in the desert), Fata Morgana (an optical phenomenon that creates the illusion of water mirrors or distorted images above the horizon).
- Spectrum of light
We’ve all observed an impressively beautiful celestial phenomenon after a rain shower – the rainbow. It forms when sunlight refracts and decomposes into water droplets in the atmosphere. The colours of the rainbow are always arranged in the same order – red, orange, yellow, green, blue, indigo (dark blue) and violet. We can only see the rainbow from a certain standing position, as the rays are refracted at different angles – the violets are the strongest and the reds the weakest. But if we approach the rainbow, it disappears, even though it doesn’t have a beginning and an end, because the countless water droplets that form it are scattered in space. The rainbow is a circle, but we see only part of it.
The white light emitted by the Sun is made up of these rainbow colours, called spectral colours. The bundle of coloured rays that make up white light is called the spectrum of light. Isaac Newton first proved in 1666 that the colour white is complex and contains these seven colours and that all the colours of the visible spectrum make up white light.
Each spectral colour has a different refractive index and propagates through bodies and substances at different rates.
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In conclusion, we can say that the colour we see of different objects depends on the spectrum of the light falling on them and the way they reflect and refract different colours. Therefore, we perceive the same colours of bodies differently depending on their illumination and whether we see them during the day or in the evening.
Colours and filters
White light is made up of all the colours of the visible spectrum. However, it can also be obtained by mixing only three colours of light rays – red, green and blue. These colours are called primary colours. By mixing these three primary colours in different proportions an unlimited number of colours can be obtained. This is used in modern displays of electronic devices such as computers, televisions and smartphones, in which millions of colours, including white, are created by emitting red, green or blue light of different intensities.
When light hits a particular body, it reflects some wavelengths and absorbs others. The colour of the body depends on which wavelengths are reflected. When an object absorbs all the colours of the visible light spectrum without reflecting any, we see it as black. Black is the absence of colour.
It is also possible to get one particular colour if the white light is separated from the others. This is done using colour filters – transparent materials made in a particular colour. They only let light waves in that colour through, absorbing and stopping the other colours. They are widely used in various areas of industry, such as photography and film.
So:
- White light contains all the colours of the visible spectrum. It can also result from the overlap of only red, green and blue light beams.
- There are three main colours – red, green and blue.
- The colours of a body depend on the wavelengths in the spectrum it reflects. Mixing two beams of light with primary colours produces a different colour – magenta, cyan or yellow. Each can also be obtained by separating one of the primary colours from the white light.
- Light filters absorb most colours, but some can pass through and be reflected.
