Download 7.1 - Signals from Space

Space Physics
7.1 – Signals from Space
The Universe
The universe consists of many galaxies separated by empty space.
Here is what makes up the universe in order of size.
Term
Meaning
moon
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 & heat
galaxy
large ball of matter emitting light
universe
whole of space that can be detected
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.
v  3  10 8 ms -1
t  1 year
 60  60  24  365
 31.536 10 6 s
d?
d  vt

 
 3  108  31.536 106
d  9.5  1015 m
(5,878,625,373,183.61 miles)

It is difficult to comprehend such a large distance, so here are some
times for light reaching Earth from various sources.
Light Source
Time Taken for Light
to Reach Earth
Moon
1.2 s
Sun
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
Astronomical Telescope
long focal length
short focal length
eye
distant
planet
objective
lens
light-tight tube
eyepiece
lens
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.
Ray Diagrams
A ray diagram shows what happens when light passes through a
convex lens.
The convex lens is represented by a vertical line.
Symbol
Meaning
M
O
F
I
OM
IM
optical centre of lens
object
focal length of lens
image
object distance
image distance
Rules
1. 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.
Image
Three facts should be stated about an image.
Is it:
1. real or virtual
2. upright or inverted
3. magnified or diminished or same size
Example 1 (Camera)
M
F
O
2F
F
object distance OM > 2F
I
principal axis
The image is real, inverted and diminished.
Example 2 (Magnifying Lens)
M
I
2F
F
O
F
principal axis
object distance OM < F
The image is virtual, upright and magnified.
Magnifying An Image
The object is closer to the lens than the focus.
This produces a virtual image which is upright and magnified.
M
I
2F
F
O
F
principal axis
M
I
2F
F
O
F
principal axis
The Visible Spectrum
White 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)
Red
Orange
Yellow
Green
Blue
Indigo
Violet
700
decreasing
wavelength
400
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.
v  3  10 8 ms 1
Each radiation in the electromagnetic spectrum has its own
wavelength and frequency.
vfλ
Radiation Detectors
Radiation
low frequency
long wavelength
high frequency
short wavelength
TV and radio
Detector
aerials
microwaves
curved reflector & receiver
infrared
blackened thermometer
visible light
eye
ultraviolet
fluorescent paint
x-rays
photographic film
gamma rays
GM tube
Detecting Signals
Energy 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 Telescope
Large 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.
Microwaves
Astronomers 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 Radiation
Infrared 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 Spectra
White 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