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Light Year A light year is a distance. It is the distance travelled by light in one year. Example Calculate the length of one light year. (5,878,625,373, miles)
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Space Physics
7.1 – Signals from Space
The universe consists of many galaxies separated by empty space.Here is what makes up the universe in order of size.
The Universe
Term Meaningmoon an object orbiting a planet (reflects
light)planet an object orbiting a star
(reflects light and produces no energy)star large ball of gases producing light &
heatgalaxy large ball of matter emitting lightuniverse whole of space that can be detected
Light YearA light year is a distance. It is the distance travelled by light in one year.
ExampleCalculate the length of one light year.
?d
-18 ms 103v year1t tvd
68 1031.536103 365246060 s 1031.536 6 m 109.5d 15
(5,878,625,373,183.61 miles)
Light SourceTime Taken for Light to Reach
EarthMoon 1.2 sSun 8 minutes
Nearest start after the Sun
(Proxima Centauri)
4.3 years
Other side of our galaxy 100,000 years
Our nearest galaxy
(Andromeda)2,200,000 years
It is difficult to comprehend such a large distance, so here are some times for light reaching Earth from various sources.
Astronomical Telescope
objective lens
eyepiece lens
eyedistant planet
light-tight tube
The objective lens collects light and forms an inverted image of the distant planet which is magnified by the eyepiece lens.A larger diameter objective lens collects more light, so brighter image is formed.
long focal length
short focal length
Ray DiagramsA ray diagram shows what happens when light passes through a convex lens.The convex lens is represented by a vertical line.
Symbol MeaningMOFI
OMIM
optical centre of lensobject
focal length of lensimage
object distanceimage distance
Rules1. A ray parallel to the principal axis passes through the focus
after refraction.2. A ray passing through the centre of the lens continues
straight on undeviated.
ImageThree facts should be stated about an image.Is it:
1. real or virtual2. upright or inverted3. magnified or diminished or same size
Example 1 (Camera)
Example 2 (Magnifying Lens)
FF
2FOIM principal axis
F2F OM F principal axis
I
The image is real, inverted and diminished.
The image is virtual, upright and magnified.
object distance OM < F
object distance OM > 2F
Magnifying An ImageThe object is closer to the lens than the focus.This produces a virtual image which is upright and magnified.
F2F OM F principal axis
I
F2F OM F principal axis
I
The Visible SpectrumWhite light is made up from a range of colours.A prism will split white light into its component colours, by refraction of the white light.
Colour Wavelength (nm)
RedOrangeYellowGreenBlueIndigoViolet
700
400
decreasing wavelength
The Electromagnetic Spectrum
Visible light is just a small part of a whole range of radiation known as the electromagnetic spectrum.Each type of radiation in the electromagnetic spectrum travels at the speed of light.
18 ms 103v
Each radiation in the electromagnetic spectrum has its own wavelength and frequency.
λ fv
Radiation DetectorsRadiation Detector
low frequencylong wavelength aerials
microwaves curved reflector & receiverinfrared blackened thermometer
visible light eyeultraviolet fluorescent paint
x-rays photographic filmgamma rays GM tube
TV and radio
high frequencyshort wavelength
Detecting SignalsEnergy is omitted from objects in space at many different wavelengths.To get a full picture of our universe information of all wavelengths is collected.This is done using many types of telescope.
Radio TelescopeLarge unpolished metal dishes collect and direct weak radio waves to an aerial at the focus.To see fine detail, the opening of the radio telescope should be as large as possible. This is hard to achieve with one telescope, so several are usually used, and computer software combines the information.
The 305-metre Arecibo dish in Puerto Rico is the world's most sensitive radio telescope.
MicrowavesAstronomers can detect radiation from space that has a wavelength of several millimetres.This can give information on the temperature of stars and hence their age. As the star reactions change, so will its temperature.Microwave ovens at home use microwave radiation which has a wavelength of about 12cm.
Infrared RadiationInfrared radiation arrives on Earth from objects in space.Objects at different temperatures give off different colours of light. The hotter an object is, the smaller the wavelength of emitted light.
Line SpectraWhite light sources produce a continuous spectrum emitting all colours of the rainbow.
Other sources of light, for instance street lights, emit only particular wavelengths of light.
Line spectra tell us about a stars chemical composition, as each element has its own line spectrum pattern.
Example 1