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Chapter 5

chapter 5

Astronomy & Astrophysics
Course Code
Stefan Mochnacki

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5.1 light in everyday life
Telescopes collect light of distant objects and astronomers extract info from the light
Sunlight transfers energy to the molecules in your skin
Energy in light is radiative energy
Rate of energy flow = power; measured in units of watts
1 watt = 1 joules
prism split light into rainbow of light = spectrum
basic colors are red, orange, yellow, green, blue, violet
white = basic colors mixed in abt equal proportion (all colors of rainbow)
black = we perceive when there is no light, no color
primary colors of vision = red, green, blue
diffraction grating, piece of plastic or glass etched with many closely spaced line
4 ways light interact
emission: eg. Light bulb emits visible light, energy of light from electrical potential
energy supplied to light bulb
absorption: your hand absorbs light when placed on light bulb, light bulb then
warms your hand
materials that absorb light are opaque
transmission: some forms of matter, like glass or air, transmit light which means
allows it to pass through
materials that transmit light are transparent
reflection /scattering: reflection bouncing in same general direction or scatter
(bouncing is more random)
red glasses transmits red light but absorbs all other color
green lawn reflects green light but absorbs all other color
5.2 Properties of light
particle is a thing
can sit still or it can move from one place to another
waves pattern revealed by its interaction with particles
waves carry energy outward from one place to another
consists of wavelengths (distance from one peak to the next), frequency (number of
peaks passing by any point each second, measured cycles per second aka hertz), speed
(how fast the energy travels from one place to another)
wavelength X frequency = speed
Energy = h X frequency of photon
h planck’s constant, 6.626 X 10-34 joules X s)
light waves are vibrations of both electric and magnetic fields caused by the motions
of charged particles
light is a electromagnetic wave
all light travel at the same speed, c= 300,000 km/s

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longer the wavelength, lower the frequency, lower energy
light has characteristics of both particles and waves, its photons
only visible light splits into a rainbow (400 nm to 700 nm)
complete spectrum = electromagnetic spectrum, light is sometimes called
electromagnetic radiation
light with wavelengths longer than red light = infrared
longest wavelengths of light = radio waves
radio waves are a form of light not sound
wavelengths range from micrometer to millimeter is microwaves
wavelengths somewhat shorter than blue light = ultraviolet
lighter even short are x rays
shortest wavelength light is gamma rays
5.3 properties of matter
Democritus, greek philosopher
matter can be broken down into smallest piece called atom
all materials were composed of 4 elements: fire, water, earth, air
he believed different properties of elements can be explained by physical
characteristics of their atom
atoms make up elements & are made of protons, neutrons and electrons
protons and neutrons are found in nucleus at the center of atom
nucleus is really small but contains most of the mass
properties of atom depend on electrical charge in nucleus
oppositely charged particles attract and same charge repel
most atoms have same number or protons & electrons
atomic number number of protons
atomic mass number number of protons and neutrons
isotopes same number of protons but different number of neutrons
same type of atoms form molecules (eg. O2)
different types of atoms form compounds (NaCl)
phases of matter:
solid eg ice (strong bonds)
liquid eg water
gas eg water vapor (much weaker bonds)
plasma hot gas which atoms have become ionized
chemical bond interactions between electrons that hold the atoms in a molecule
changes in pressure or temperature or both can change phases
temperature: substance is heated, average kinetic energy increases, enabling particles
to break bonds holding them together
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