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Lecture

Chapter 5


Department
Astronomy & Astrophysics
Course Code
AST101H1
Professor
Clifford Orwin

Page:
of 5
Unit 5 – Light and Matter
How do we Experience Light:
Energy that light carries is called radiative energy.
Energy and Power:
In astronomy we are concerned more with the rate of energy transfer not the amount of
energy transfer
Rate of energy flow is called power and is measured in units called watts. (1 watt = 1
joule/s)
How do Light and Matter Interact:
Emission: bulb emits visible light. The energy of the light comes from electrical potential energy
supplied to the light bulb.
Absorption: when you place your hand near a light bulb, your hand absorbs some of the light,
and this absorbed energy warms your hand.
Transmission: Some forms of matter, such as glass or air, transmit light, which means allowing
it to pass through
Reflection/scattering: light is able to bounce off of matter, leading to what we call reflection
(bouncing all in the same general direction) or scattering (when the bouncing is more random)
Materials that transmit light are transparent, and opaque if they absorb light.
Many materials are never perfectly transparent or opaque.
All the information that light brings to earth from the universe was encoded by one of the
four basic interactions between light and, matter common to our everyday experience.
Particles and Waves in Everyday Life:
A particle of mater can sit still or it can move from one place to another.
In essence, a particle is a thing, while a wave is a pattern revealed by its interaction with
particles.
Wavelength: distance from one peak to the next (one trough to the next)
Frequency: number of peaks passing by any point each second.
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Cycles per second often are called hertz (Hz)
Speed of the waves tell you how fast the peaks travel
Formula defined as: wavelength x frequency = speed
Light as an Electromagnetic Wave:
Light is a wave but not your conventional wave where it moves through a medium. In a
sense light is its own medium
We can describe the forces that charged particles exert on one another in terms of electric
and magnetic fields.
Light waves are vibrations of both electric and magnetic fields caused by the motions of
charged particles. (this we say that light is an electromagnetic wave)
All light travels through empty space at the same speed – the speed of light
Very important relationship between wavelength and frequency for light: the longer the
wavelength, the lower the frequency, vice versa.
Photons:Particles of Light
Light behaves as both a wave and a particle. So, we say that light come in “pieces called
photons.
Each photon of light carries a specific amount of radiative energy. The shorter the
wavelength of light, the higher the energy of photons.
What is the Electromagnetic Spectrum?
There is light that the human eye cannot see. Why? Visible light that splits into the
rainbow of colour is only a tiny part of the complete ranges of light’s wave lengths.
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Because light is sometimes described as an electromagnetic wave, the entire spectrum of
light is called electromagnetic spectrum. Light itself is also called electromagnetic
radiation.
Light with wavelengths longer than red light is called infrared
Radio waves are not to be mistaken as a form of sound but a form of light, the longest
wavelength light.
Opposite of that, light with wavelengths shorter than blue light I called ultraviolet. It lies
beyond the blue/violet end of the rainbow
Light with even shorter wavelengths is called x-rays and shorter than that is called
gamma rays.
What is the Structure of Matter?
All ordinary matter is composed of atoms, and the properties of ordinary matter depend
on the physical characteristics of their atoms.
Not invisible because they are made up of even slammer particles.
Atomic Structure:
Atoms are in turn made of particles called: protons, neutrons, and electrons
Protons and neutrons are found in the nucleus where electrons surround the nucleus.
Properties of an atom depend on the electrical charge in its nucleus. Protons have a
positive charge, electrons have a negative charge, and neutrons are electrically neutral (no
charge).
Impossible to pinpoint the exact location of electrons on an atom.
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