Physics 11

Name: __________________________________

Light (Electromagnetic Waves) EM Radiation - ________________ waves of energy - The speed of light depends on the ______________ (denser  __________) - The speed of light in a vacuum is:

1) The Sun is 1.50x108 km from Earth. How long does it take for the light from the Sun to reach us?

- White light is actually a mixture of many different colours (_ _ _ _ _ _ _ _)

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EM radiation is emitted from ______________ _________.

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After energy is absorbed by matter is emitted as EM radiation.

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Each element emits very specific frequencies of light known as _____________ _______________.

- Different colours of light have different _____________ and ______________. Red has the ____________ λ and violet the ___________ λ

2) Alpha Centauri is the nearest solar system to ours, at 4.07x1013 km away. How long does it take for light to travel from Alpha Centauri to us?

Physics 11

Name: __________________________________

Electromagnetic Waves We know that light transfers energy; for example, light from the sun provides us with the energy necessary to sustain life on earth. Humans are familiar with two basic methods of transferring energy:  

Particles – for example, a baseball travelling through the air has kinetic energy which can be transferred to another object in a collision. Waves – for example, water waves transfer energy to the shore and cause erosion.

When scientists began developing a model of light, they attempted to use these theories (particles and waves) to develop two different models – the particle model and the wave model. In the latter part of the seventeenth century, a group of scientists proposed a particle model of light. The most prominent of these scientists was Isaac Newton. This model proposed that light was made up of extremely small particles that travelled extremely fast. It was reasoned that the particles must be extremely small, since two beams of light could be observed to pass through one another without any interference; the particles must be moving very fast, since beams of light appear to travel in straight lines (just as the curvature of a projectile's path is reduced as the particle's speed is increased). This model could be used to explain reflection of light, as well as some other properties. This model gained acceptance because it could be used to explain various properties of light, and it had the backing of the famous Newton. Around the same time as Newton and others were proposing the particle model of light, another group of scientists led by Christian Huygens was putting forward a wave model of light. They proposed that light actually consists of waves; since all waves at this time required a medium, these scientists also proposed that all of space was filled with an ether that provided the medium for these light waves. Huygen's wave model was not as well accepted as Newton's particle model, mainly due to Newton's reputation; however, by the early to mid 1800's it began to gain more acceptance for the following reasons. 

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Around the beginning of the nineteenth century, Young performed his double slit experiment to show that light passing through two slits demonstrated the same interference pattern as two sources of water waves; a wave theory of light began to make more sense now as this alone could explain the interference pattern. This will be looked at later in this module. Also, in 1850, the speed of light was shown to be lower in water than in air; this supported Huygen's theory of refraction and contradicted Newton's theory of refraction.

By the middle of the nineteenth century, the wave model of light became the more widely accepted model of light. This model was not, however, without its problems. For example, there was no evidence of the ether that was supposedly required for the transmission of waves. Electromagnetic Theory In the latter part of the nineteenth century, James Maxwell improved upon Huygen's wave model. Maxwell predicted that an accelerating electric charge will emit interacting electric and magnetic waves (electromagnetic waves) that require no medium (just as electric and magnetic fields require no medium). He further calculated that in order for these waves to continue to travel and interact together, they must be travelling at a speed of 3.0 x 108 m/s –the same speed as the speed of light!! The logical conclusion was that light is a type of electromagnetic wave.