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In this link: http://www.rocketmime.com/astronomy/Telescope/Magnification.html

In magnification equation part there is an image showing two angles that light rays make and gives the formula that makes sence, but why the light rays arent a line? They change direction at focal point I guess those are to seperate light rays so how can it be proved while useing two different light rays?

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  • $\begingroup$ If I understand which image are you talking about, there isn't any change in the direction of the rays at the focal point. I think that the fact that a ray is highlighted in light blue before the focal point and another one, after that point, is highlighted in red is misleading you. They are not the same ray. $\endgroup$ – JackI Jul 5 '17 at 20:44
  • $\begingroup$ Yes it uses highlited rays ( two seperate) to prove magnification formula. How can he compare two seperate rays shouldnt he use a single ray? $\endgroup$ – Nemexia Jul 6 '17 at 7:04
  • $\begingroup$ No, the formula for the angular magnification just uses the angle between the optical axis and one ray (since before the first lens and after the second one all rays are parallel). The choice to use the highlighted rays is due to convenience: it's easy for them to write, in paraxial approximation, an expression of the angles that cancel out the height of the focal point, leaving with a ratio between the focal lengths. $\endgroup$ – JackI Jul 6 '17 at 7:10

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