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