Facts About Light



  • Light is the fastest thing in the Universe, traveling at 299,792,458 meters per second.
  • Light rays always travel in straight lines.
  • Light rays change direction as they pass from one material to another. This is called refraction.
  • colors are different wavelengths of light.
  • The longest light waves you can see are red, and the shortest are violet.
  • Light is a form of electromagnetic radiation (see magnetism and radiation), and a light ray is a stream of tiny energy particles called photons.
  • Photons of light travel in waves just 380 to 750 nanometers (millionths of a millimeter) long.
  • Faint light from very distant stars is often recorded by sensors called CCDs (see observatories). These count photons from the star as they arrive and build up a picture of the star bit by bit over a long period.
  • The electromagnetic spectrum (range) includes ultraviolet light and X-rays, but light is the only part of the spectrum our eyes can see.
  • All light is given out by atoms, and atoms give out light when ‘excited’ – for example, in a nuclear reaction.
  • Distances in space are so vast that the fastest thing in the Universe – light – is used to measure them.
  • The speed of light is about 300,000 km per second.
  • A light-second is the distance light travels in a second – 299 million meters.
  • A light-year is the distance light travels in one year – 9.46 trillion km. Lightyears are one of the standard distance measurements in astronomy.
  • It takes about 8 minutes for light from the Sun to reach us on Earth.
  • Light takes 5.46 years to reach us from the Sun’s nearest star, Proxima Centauri. This means the star is 5.46 light-years away – more than 51 trillion km.
  • We see Proxima Centauri as it was 5.46 years ago, because its light takes 5.46 years to reach us.
  • The star Deneb is 1800 light-years away, which means we see it as it was when the emperor Septimus Severius was ruling in Rome (AD200).
  • With powerful telescopes, astronomers can see galaxies 2 billion light-years away. This means we see them as they were when the only life forms on Earth were bacteria.
  • Parsecs may also be used to measure distances. They originally came from parallax shift measurements (see distances). A light-year is 0.3066 parsecs.
  • Distances in space are so vast that they are measured in light-years, the distance light travels in a year. Note: not all of these stars can be seen in the northern hemisphere
  • Light is a form of energy. It is one of the forms of energy sent out by atoms when they become excited. Light is just one of the forms of electromagnetic radiation (see electromagnetic spectrum). It is the only form we can see. Although we are surrounded by light during the day, very few things give out light. The Sun and other stars and electric lights are light sources, but we see most things only because they reflect light. If something does not send out or reflect light, we cannot see it.
  • Light beams are made of billions of tiny packets of energy called photons (see moving light). Together, the photons behave like waves on a pond. But the waves are tiny — 2000 would fit on a pinhead
  • Light travels in straight lines. The direction can be changed when light bounces off something or passes through it, but it is always straight. The straight path of light is called a ray.
  • When the path of a light ray is blocked altogether, it forms a shadow. Most shadows have two regions: the umbra and penumbra. The umbra is the dark part where light rays are blocked altogether. The penumbra is the lighter rim where some rays reach.
  • When light rays hit something, they bounce off, are soaked up or pass through Anything that lets light through, such as glass, is transparent. If it mixes the light on the way through, as does frosted glass, it is translucent. If it stops light altogether, it is opaque. 84
  • When light strikes a surface, some or all of it is reflected. Most surfaces scatter light in all directions, and all you see is the surface. But mirrors and other shiny surfaces reflect light in exactly the same pattern in which it arrived, so you see a mirror image.
  • When light passes into transparent things such as water, rays are bent, or refracted. This happens because light travels more slowly in glass or water, and the rays swing round like the wheels of a car running onto sand. What you see is not the object itself but a reduced image Light rays from the object
  • Glass lenses are shaped to refract light rays in particular ways. Concave lenses are dishshaped lenses — thin in the middle and fat at the edges. As light rays pass through a concave lens they are bent outwards, so they spread out. The result is that when you see an object through a concave lens, it looks smaller than it really is.
  • ( nnvex lenses bulge outwards. They are fatter in the middle and thin around the edges. As INIn rays pass through a convex lens they are bent inwards, so they come together, or converge. When you see an object through a convex lens, it looks magnified.
  • All light comes from atoms. They give out light when they gain energy – by absorbing light or other electromagnetic waves when hit by other particles.
  • Atoms are normally in a `ground’ state. Their electrons circle close to the nucleus where their energy is at its lowest ebb.
  • An atom emits light when `excited’ by taking in energy. Excitement boosts an electron’s energy so it jumps further out from the nucleus.
  • An atom only stays excited a fraction of second before the electron drops back towards the nucleus.
  • As the electron drops back inwards, it lets go the energy it gained as a tiny packet of electromagnetic radiation called a photon.
  • Electrons do not drop in towards the nucleus steadily like a ball rolling down a hill, but in steps, like a ball bouncing down stairs.
  • Since each step the electron drops in has a particular energy level, so the energy of the photon depends precisely on how big the steps are. Big steps send out higher-energy short-wave photons like X-rays. 86
  • The color of light an atom sends out depends on the size of the steps its electrons jump down.
  • Each kind of atom has its own range of electron energy steps, so each sends out particular colors of light. The range of colors each kind of atom sends out is called its emission spectrum (see spectrum). For gases, this acts like a color signature to identify in a process called spectroscopy.
  • Just as an atom only emits certain colors, so it can only absorb certain colors. This is its absorption spectrum.
  • Our main sources of natural light are the Sun and the stars. The hot gases on their surface glow fiercely.
  • The brightness of a light source is measured in candelas (cd); one c andela is about as bright as a small candle.
  • For 0.1 millisecond an atom bomb flashes out 2000 billion candelas for every square meter (m2).
  • The Sun’s surface pumps out 23 billion candela per m2. Laser lights are even brighter, but very small.
  • The light falling on a surface is measured in lux. One lux is how brightly lit something is by a light of one candela 1 m away. You need 500 lux to read by.
  • Electric lightbulbs are incandescent, which means that their light comes from a thin tungsten wire, or filament, that glows when heated by an electric current.
  • Lightbulbs are filled with an inert (unreactive) gas, such as argon, to save the filament from burning out.
  • Electric lights were invented independently in 1878 by Englishman Sir Joseph Swan and Americans Thomas Alva Edison and Hiram Maxim.
  • Fluorescent lights have a glass tube coated on the inside with powders called phosphors. When electricity excites the gases in the tube to send out invisible UV rays, the rays hit the phosphors and make them glow, or fluoresce.
  • In neon lights, a huge electric current makes the gas inside a tube electrically charged and so it glows.
  • Light is the fastest thing in the Universe.
  • The speed of light is 299,792,458 meters per second.
  • Scientists remember light’s speed from the number of letters in each word of this sentence: ‘We guarantee certainty, clearly referring to this light mnemonic.
  • Isaac Newton suggested in 1666 that light is made up of sterams of tiny particles, or corpuscles.
  • The Dutch scientist Christiaan Huygens (1629-1695) said in 1678: no, light is waves or vibrations instead.
  • Maxwell introduced the concept of electromagnetic waves which included light, and predicted radio waves. .
  • On a sunny day one thousand billion photons fall on a pinhead every second.
  • In 1804 Thomas Young showed that light is waves in a famous experiment with two narrow slits in a light beam. Light coming through each slit creates bands of shadow that must be caused by waves interfering with each other.
  • James Clerk Maxwell suggested in the 1860s that light is electromagnetic waves.
  • Albert Einstein showed with the photoelectric effect that light must also be particles called photons.
  • Light sometimes looks like photons, sometimes like waves. Weirdly, in a way scientists can’t explain, a single photon can interfere with itself in Young’s slit experiment.