There's a site here (https://bit.ly/1pKWNNQ) that provides an explanation of how the thin lens (both Gaussian and Newtonian) equations are derived. I thought the previous slides were pretty helpful, but this site includes pictures where the variables are clearly labeled and their purposes with respect to the overall equation are discussed. There's also a snippet on cartesian sign convention that describes some of the assumptions being made about lens diagram drawings that could be useful for understanding what the previous slides are communicating.

There's a site here (https://bit.ly/1pKWNNQ) that provides an explanation of how the thin lens (both Gaussian and Newtonian) equations are derived. I thought the previous slides were pretty helpful, but this site includes pictures where the variables are clearly labeled and their purposes with respect to the overall equation are discussed. There's also a snippet on cartesian sign convention that describes some of the assumptions being made about lens diagram drawings that could be useful for understanding what the previous slides are communicating.