# Chapter 5

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Unit 5 ā Light and Matter
How do we Experience Light:
īEnergy that light carries is called radiative energy.
Energy and Power:
īIn astronomy we are concerned more with the rate of energy transfer not the amount of
energy transfer
ī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)
īMaterials that transmit light are transparent, and opaque if they absorb light.
īMany materials are never perfectly transparent or opaque.
ī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:
īA particle of mater can sit still or it can move from one place to another.
īIn essence, a particle is a thing, while a wave is a pattern revealed by its interaction with
particles.
īWavelength: distance from one peak to the next (one trough to the next)
īFrequency: number of peaks passing by any point each second.
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īCycles per second often are called hertz (Hz)
īSpeed of the waves tell you how fast the peaks travel
īFormula defined as: wavelength x frequency = speed
Light as an Electromagnetic Wave:
īLight is a wave but not your conventional wave where it moves through a medium. In a
sense light is its own medium
īWe can describe the forces that charged particles exert on one another in terms of electric
and magnetic fields.
ī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)
īAll light travels through empty space at the same speed ā the speed of light
īVery important relationship between wavelength and frequency for light: the longer the
wavelength, the lower the frequency, vice versa.
Photons: āParticles of Lightā
īLight behaves as both a wave and a particle. So, we say that light come in āpiecesā called
photons.
ī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?
ī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 lightās wave lengths.
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## Document Summary

energy that light carries is called radiative energy. 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) 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.

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