Lenses

Lenses You have two lenses on your bench, one that is thick in the middle (converging) and one that is thin in the middle (diverging). Try looking through them.

Converging and diverging lenses WHY?

Principal axis The straight line that goes through the centre of the lens at right angles to the lens surface

Principal focus Rays parallel to the principal axis, after being refracted by the lens, will all pass through a point on the principal axis called the principal focus.

Focal length The distance from the principal focus to the centre of the lens (denoted by the symbol f)

Finding the focal length - a quick practical Use the “distant light” method to show the students how to find the focal length. Give them a number of unmarked lenses that they have to find the focal length to.

Real and virtual images A real image is where the rays actually pass through the image and it can be projected and seen on a screen

Real and virtual images A virtual image is where no rays of light pass through, only their mathematical extensions. It cannot be displayed on a screen

Let’s try it! – Investigating images practical

Let’s construct some ray diagrams to find out! Why? http://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html Let’s construct some ray diagrams to find out!

Drawing ray diagrams

Drawing ray diagrams

Drawing ray diagrams

Drawing ray diagrams

Drawing ray diagrams You can check by drawing a ray backwards from the object parallel to the axis which has passed through the first principal focus.

Drawing ray diagrams

Now construct the following 5 diagrams An object at greater than 2F an object at 2F an object less than 2F (between F and 2F) an object at F an object at less than F

Virtual image

Let’s try some questions