What I learned
❖ Light allows us to perceive the world around us with its shapes, colours, and sizes.
❖ Light sources are all bodies that emit light energy into the environment. We distinguish between natural and artificial light sources. Light propagates linearly in the form of electromagnetic waves until it encounters an obstacle or a change in the environment. It passes through everything transparent, including air, water, glass and the vacuum of space. The speed of propagation of light in a vacuum is the greatest in the universe – 300,000 km/s.
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.
❖ When light rays pass from one medium to another, for example from air to water, they change direction. This phenomenon is known as the refraction of light. It occurs when light rays change direction and speed of propagation as they pass from one medium to another of different densities. The 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 moves in a substance than in a vacuum. It is calculated by dividing the speed of propagation of light in a vacuum by the speed of propagation of light in a given medium. A refractive index is a number that shows the ratio of the two speeds – the speed of light in a vacuum to the speed of light in a substance, therefore it has no unit.
❖ White light is made up of all the colours of the visible spectrum. The bundle of coloured rays that make up white light is called the spectrum of light, consisting of 7 spectral colours. However, it can also be obtained by mixing only three colours of light rays – red, green and blue. These colours are called primary colours.
❖ When light hits a body, it reflects some wavelengths and absorbs others. The colour of the body depends on which wavelengths are reflected. The unabsorbed wavelengths of light are reflected and determine the colour of the body.
❖ 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.
❖ Colour filters only let light waves in a particular colour through, absorbing and stopping other colours. They are widely used in photography and film.
