Uploaded by Mir Shahaab Hussain

pcs chapter 2

The science of astronomy
Science: explanation of what occurs in the real world
Not all knowledge comes from science
The idealized “scientific method”
Based on proposing and then testing a hypothesis
But science rarely proceeds just via this idealized procedure…
Sometimes we’ll “observe” an event or phenomenon as we systematically vary conditions to try to
deduce underlying, more fundamental causes “laws” for what we see;
But sometimes we just follow our intuition rather than fellow a particular line of evidence…
Science is both a creative as well as an Imaginative activity;
Hallmarks of science:
1. Modern science seeks explanations for observed phenomena that rely solely on matural causes.
#2 Science progresses through the creation and testing of models of nature that explain the
observations as simply as possible (simplicity = “Occam’s razor)
#3: A scientific model must make testable predictions about natural phenomena that would force us to
revise or abandon the model if the predictions do not agree with observations.
How can we distinguish science form non-science?
Science” seeks explanations that rely solely on natural causes; progresses through the creation and
testing models of nature.
What is a scientific theory?
The word theory has a different meaning in science compared
How do we experience light?
The warmth of sunlight tells us that light is a form of energy
A standard unit of energy (“work done”) is the Joule (J)
We can measure the flow of energy carried by light in units of watts:
1 watt= 1 joule per second = 1 joule/s
Colours of light
White light is made up of many different colours
How do light and matter interact?
o Transparent objects transmit light
o Opaque objects block (absorb) light
o Light rebounds in every direction (no change in direction)
o Light rebounds in every direction (if light isn’t gone through it rebound back)
Mirrors reflect light in a particular direction
Movie screen scatters light in all directions
Interactions between light and matter determine the appearance of everything around us
Properties of light
What is light
Light may act either like a wave or like a particle
Particles of light are called photons
A wave is a pattern of motion that can carry energy without carrying matter along with it
Properties of waves
Wavelength: is the distance between two wave peaks
Light: Electromagnetic waves
A light wave is a vibration of electric and magnetic “fields”
Light interacts with charged particles through these electric and magnetic particles
Wavelength x frequency = speed of light = constant
Particles of light
Particles of light are called photons
Each photon has a wavelength and a frequency
The energy of a photon depends on its frequency (or wavelength)
Wavelength, frequency and Energy
Lamdax frequency = speed of light (3.0x10^8 m/s)
E= h x f = photon energy
H= 6.626 x10^-34 joule x s = is a constant… tells us how much energy in each photon for light of a given
frequency (colour)
Visible light = 400 nanometers
Colour can be measure of temperature (Wien’s law)
Solids liquids and dense gases emit electromagnetic radiation (light)
The colour of the light emitted tells us the temperature of the material
We call this “thermal radiation”
All colours (in theory) are emitted; but the light is brightest at a particular colour (wavelength)
that is directly proportional to the material’s temperatures
Red star are cooler in temperature
Blue star are hotter in temperature
Thermal radiation
Nearly all dense and/or opaque objects emit thermal radiation, including stars, planets, you…
An object’s thermal radiation
Properties of thermal radiation
1. Hotter objects emit more light at all frequencies per unit area
2. Hotter objects emit photons with a hogher average energy
Wien;s law for thermal radiation