Light intensity notes

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Sam Davyson         PHY//12//        19/09/05

Mr Ford – Chapter 12 – Revision Notes

Light Intensity

In space the light intensity from distant stars can be used to approximate the distances between the stars and us. This idea can be mimicked in the laboratory with a light box, a lux meter and a measuring rule. Taking measurements and plotting them reveals a linear relationship between light intensity and the reciprocal of the distance squared. It turns out that they are in fact proportional to one another.

There are some obvious problems with this technique:

  • Don’t know how bright the stars really are.
  • We must assume that the intensity is not affected by different mediums (including the atmosphere).

Measuring Distance using Parallax

Parallax is a technique that uses trigonometry and the difference between the apparent positions (referring to the “fixed” background stars) of a distant object from two different points along a base line. The longer the base line the further you can see. On earth the best we can do is looking at the object twice with 6 months between each viewing. This gives a baseline of 2Au = 300 Million km. The angle subtended by the distant star and the two viewing points in two lots of the parallax angle. So using the diagram parallax angle = . This method is only really usable over relatively short distances.

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The Parsec

A parsec, or 1 pc, is a unit of distance. It is defined as the distance between the Sun and an object when the parallax angle is equal to one second of arc ( of a degree).

The 1 Au on the diagram is the distance between the Earth and the Sun. Using this definition it is found that 1pc = 3.08 x 1016 m = 3.26 light yrs. If a star is further from Earth then the parallax angle is reduced. Roughly speaking:

Distance (in pc) =  (when the angle is measured in seconds of arc)

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