For an explanation, see the main splats page
The principles of light
- Light is a form of electromagnetic radiation, part of the electromagnetic spectrum. It has electrical and magnetic properties, just like radio waves and X-rays.
- Light is a form of energy, and it is usually released when other forms of energy are converted. Hot bodies radiate light, if they are sufficiently hot.
- Light can be converted to other forms of energy such as electricity in photovoltaic cells and it is converted into chemical energy in plants.
- Because light radiates out in all directions, we can treat light as straight-line rays. Rays of light do not exist, but they are a convenient 'fiction'.
- Light and other forms of radiation are reflected. Most surfaces are rough, and reflect light in many directions but a mirror has a comparatively smooth surface.
- Light can be 'piped' through a carefully designed optic fibre, and the light source can be modulated to carry signals from one place to another.
- As you get further from a light source, the apparent intensity drops according to the inverse square law, dropping to a quarter when the distance is doubled.
- As a general rule, light travels in a straight line, but it will bend when it travels through a transparent medium that is not a vacuum, such as water or glass.
- Projection systems and pinhole cameras rely on light travelling in straight lines, which can be assumed in a uniform medium with no massive bodies nearby.
- Light can be bent away from a straight line of travel by the force of gravity when it passes very close to a very large mass such as a star or a black hole.
- The way that light bends when it travels through a transparent medium is called refraction. Curved lenses and prisms work because of refraction.
- In 1666 Isaac Newton demonstrated the composite nature of white light while carrying out studies directed at minimizing chromatic dispersion in lenses.
- Rainbows are seen when sunlight shines on small spherical water droplets, and is reflected and refracted several times inside the drops before exiting again.
- Some colours are produced by dispersion, the effect where a prism bends different wavelengths to different extents, separating white light into components.
- The colours in white light may be separated by filtering light through coloured transparent material that absorbs some wavelengths while transmitting others.
- Some colours are produced by selective absorption and reflection of different wavelengths of white light giving objects the colour of the reflected light.
- A 'blue' object absorbs other colours and reflects blue light. A black object absorbs all colours equally. A white object reflects all colours equally
- The Tyndall effect, where dust causes scattering of light, is behind the blue colour of the sky, and the red colour of sunsets, and the moon in a lunar eclipse.
- White light is what we see when we look at all of the colours that form the visible spectrum in combination. These colours have different wavelengths.
- The different wavelengths (colours) that make up white light are separated into the standard colours of the spectrum by a prism in a process called dispersion.
- When light is refracted in a medium such as a lens, different colours are refracted to different extents, causing coloured fringes like a rainbow.
- Some forms of colour are produced by selective scattering, as in the Tyndall effect, where dust in the atmosphere scatters blue light as it lets red light pass.
- The colours in some animals arise from birefringence, which depends on anisotropy, a measurable difference in optical properties in different directions.
- In 1665 Robert Hooke and Christiaan Huygens pointed out that the colours of an oil film are explained by combining the wave theory of light with interference.
- Light travels through a vacuum at a speed of 300,000 kilometres per second. That means it takes 500 seconds for light from the Sun to reach Earth.
- Light has a constant velocity of 300,000 kilometres a second when it travels in a vacuum, but when light enters a more dense medium, it slows down.
- In the first century AD, Hero of Alexandria said the speed of light must be infinite. He thought light came from the eyes, and we see as soon as we open them.
- In the early 1600s, Galileo Galilei tried measuring the speed of light by flashing lanterns from two hilltops, and decided the speed of light was infinite.
- The distance of the hilltops Galileo Galilei used were only about one forty-thousandth of a light-second apart, making it hard to reach a reliable estimate.
- In 1676, Ole Rømer used variations in the eclipses of Jupiter's moons to estimate the speed of light at around 227 million kilometres a second, about 25% out.
- About 1690, Christiaan Huygens estimated that the speed of light was might be as high as 35 million kilometres a second, which he thought extreme but possible.
- Armand Fizeau developed an earth-bound variation on the method developed by Galileo Galilei to measure the speed of light, using a toothed wheel and reflection.
- Fizeau's speed of light was about 5% higher than the value we accept to day, but this was adjusted the following year, when Jean Foucault refined the method.
- Light is produced when fuels burn because energy is released, and some of this is used to energize some of the electrons in atoms in the flame.
- In 1782, Aimé Argand invented the highly efficient fuel lamp which is still known as the Argand lamp. It had a hollow flame or wick and was much brighter.
- The combination of the Fresnel lens and the Argand lamp allowed much better light-houses to be set up, visible from a much greater distance in bad weather.
- Light of a suitable wavelength, shining on certain metals, can generate a charge by the photoelectric effect by energizing electrons in the metal.
- We cannot see ultraviolet radiation: black light is another name for ultraviolet light, which makes certain objects fluoresce at visible wavelengths.
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