Light
White light moving through a prism.
On Earth, humans do not see pure light. Rather we see light that has been filtered through the Earth's atmosphere and that bounces off of objects. Photons (light particles) are created in the photosphere in the Sun and travels towards Earth through space at at about 300 000km per second (or to be exact 299 792.458km per second). One Astronomical Unit in space is approximately 1 500 000km and since the Sun is approximately 5 AU from Earth, it takes about 8 minutes for the light from the Sun to reach the Earth.
The first people to create theories of light were the Greeks. They first described light as a ray, moving in a straight line from point A to point B. There have also been other theories that were later discarded such as a theory proposed by Pythagoras that the eye creates light which bounces off of the object. There are now two main theories associated with light. The particle theory and the wave theory. The particle theory proposes that light acts like particles, or small light bullets, that move outwards from the light source. The wave theory says that light acts like waves or ripples in space, explaining how rainbows operate (the water acts as the refractive medium). In fact, both these theories are correct or at least partially and provide assistance when describing behaviours of light. Along with these theories are also three established laws of light:
a) Light travels in a straight line.
b) The further you are from a light source, the dimmer the light.
c) The angle that a light beam makes when it hits a mirror (the angle of incidence) is the same size as the angle the beam makes when it bounces off the mirror (the angle of refraction).
Isaac Newton, the famous English scientist, was also known for his investigations into the inner-workings of light. He discovered that white light is actually created from all the colours shown at once. He also found out that a piece of glass can be used to separate the colours in the white light. From there it was found that glass prisms were particularly effective with creating this phenomenon. In glass prisms, white light changes them from being clear, to being filled with a colourful rainbow because of a process called optical refraction. This is where the wavelengths in light are caused to bend at slightly different angles. This array of colours is called a spectrum. Spectographs are used to view and separate white light and can be used with viewing the spectrum of stars. The spectrum appears to be a rainbow of bright colours sometimes appearing with dark lines throughout. In the 1800's Gustav Kirchoff discovered what these dark lines meant and created a set of rules to describe them. These rules are:
1. A luminous solid or liquid emits a continuous spectrum of all wavelengths. It has no lines in it.
2. A rarefied, luminous gas emits light whose spectrum shows bright lights. These lines are called emission lines.
3. If the light from a luminous source passes through a gas, the gas may extract certain specific energies from the continuous spectrum. We then see dark lines where the energy has been removed. These dark lines are called absorption lines.
Visible light is only a small part of the electromagnetic spectrum (EM spectrum). This spectrum is the full range of frequencies that are known to exist. The basic principles of this spectrum is that the hotter or more energetic the object the higher the energy radiation. Only the particles that move at extremely high velocities can create high-energy radiation such as X-rays and gamma-rays. Wavelength in the spectrum is measured in nanometres (one billionth of a metre/one millionth of a millimetre). The electromagnetic waves in the spectrum from slowest to fastest (longest to shortest) are:
Visible light is the middle radiation in the electromagnetic spectrum and has a wavelength between approximately 400-700 nanometres. It is the only electromagnetic wave visible to the human eye. This spectrum is made up of seven basic colours (eight if cyan is included). The colours are: red, orange, yellow, green,(cyan) blue and violet. Red has the longest wavelength of about 700nm and so is the slowest form of light. Violet is the shortest wavelength of about 400nm and so is the fastest form of light.
When looking up at a night sky, different stars appear to be bright or faint. This is not an accurate observation due to the fact that a star's visibility in our night sky can depend on how close it is to Earth, its size, brightness and colour. Visible light can tell the temperature and chemical compositions of stars. The Hertzsprung-Russell Diagram (see below) is one way of categorising different aspects of stars, including colour. This diagram shows the average surface temperature, the absolute magnitude (brightness), the class and colour of a star. There are currently five known colours of stars that also help reveal their surface temperature. Red, red-orange, yellow, white and blue or blue-white. Stars that are red are up to 3 500°C, red-orange are up to 5 000°C, yellow comes up to 6 000°C, white comes up to 7 500°C and blue is up to 40 000°C.
The first people to create theories of light were the Greeks. They first described light as a ray, moving in a straight line from point A to point B. There have also been other theories that were later discarded such as a theory proposed by Pythagoras that the eye creates light which bounces off of the object. There are now two main theories associated with light. The particle theory and the wave theory. The particle theory proposes that light acts like particles, or small light bullets, that move outwards from the light source. The wave theory says that light acts like waves or ripples in space, explaining how rainbows operate (the water acts as the refractive medium). In fact, both these theories are correct or at least partially and provide assistance when describing behaviours of light. Along with these theories are also three established laws of light:
a) Light travels in a straight line.
b) The further you are from a light source, the dimmer the light.
c) The angle that a light beam makes when it hits a mirror (the angle of incidence) is the same size as the angle the beam makes when it bounces off the mirror (the angle of refraction).
Isaac Newton, the famous English scientist, was also known for his investigations into the inner-workings of light. He discovered that white light is actually created from all the colours shown at once. He also found out that a piece of glass can be used to separate the colours in the white light. From there it was found that glass prisms were particularly effective with creating this phenomenon. In glass prisms, white light changes them from being clear, to being filled with a colourful rainbow because of a process called optical refraction. This is where the wavelengths in light are caused to bend at slightly different angles. This array of colours is called a spectrum. Spectographs are used to view and separate white light and can be used with viewing the spectrum of stars. The spectrum appears to be a rainbow of bright colours sometimes appearing with dark lines throughout. In the 1800's Gustav Kirchoff discovered what these dark lines meant and created a set of rules to describe them. These rules are:
1. A luminous solid or liquid emits a continuous spectrum of all wavelengths. It has no lines in it.
2. A rarefied, luminous gas emits light whose spectrum shows bright lights. These lines are called emission lines.
3. If the light from a luminous source passes through a gas, the gas may extract certain specific energies from the continuous spectrum. We then see dark lines where the energy has been removed. These dark lines are called absorption lines.
Visible light is only a small part of the electromagnetic spectrum (EM spectrum). This spectrum is the full range of frequencies that are known to exist. The basic principles of this spectrum is that the hotter or more energetic the object the higher the energy radiation. Only the particles that move at extremely high velocities can create high-energy radiation such as X-rays and gamma-rays. Wavelength in the spectrum is measured in nanometres (one billionth of a metre/one millionth of a millimetre). The electromagnetic waves in the spectrum from slowest to fastest (longest to shortest) are:
- Radio waves- slowest waves in the spectrum, heard through sound systems and the actual radio.
- Micro waves- short but intense waves that astronomers use to learn about structures of our own and nearby galaxies, also used for telephone reception through the large dishes placed for that purpose.
- Infra-red waves- the infra-red light emitted by our skin is the reason why we can be seen in the dark, the waves are used in space to map the dust particles between stars.
- Visible light- radiation emitted by everything from fireflies to the Sun.
- Ultraviolet radiation- emitted by stars, is what causes our skin to burn (UV rays)
- X-rays- hot gasses produce x-rays, used on Earth for doctors to look at bones and dentists to look at teeth. These are very dangerous.
- Gamma-rays- emitted by radioactive materials and can also be emitted in particle accelerators that scientists use to help them understand the makings of matter. These rays are extremely dangerous.
Visible light is the middle radiation in the electromagnetic spectrum and has a wavelength between approximately 400-700 nanometres. It is the only electromagnetic wave visible to the human eye. This spectrum is made up of seven basic colours (eight if cyan is included). The colours are: red, orange, yellow, green,(cyan) blue and violet. Red has the longest wavelength of about 700nm and so is the slowest form of light. Violet is the shortest wavelength of about 400nm and so is the fastest form of light.
When looking up at a night sky, different stars appear to be bright or faint. This is not an accurate observation due to the fact that a star's visibility in our night sky can depend on how close it is to Earth, its size, brightness and colour. Visible light can tell the temperature and chemical compositions of stars. The Hertzsprung-Russell Diagram (see below) is one way of categorising different aspects of stars, including colour. This diagram shows the average surface temperature, the absolute magnitude (brightness), the class and colour of a star. There are currently five known colours of stars that also help reveal their surface temperature. Red, red-orange, yellow, white and blue or blue-white. Stars that are red are up to 3 500°C, red-orange are up to 5 000°C, yellow comes up to 6 000°C, white comes up to 7 500°C and blue is up to 40 000°C.