Focimeter & Newtons equation

What is x and x’?

• x : distance from O to F

• x’ : distance from F’ to O’

What are the properties of similar triangles?

From the triangles on the diagram, what three fractions can we use to express the equivilence by M?

How can we rearrange this equation to get Newtons equation?

xx’ = ff’

For a thin lens how do we retire Newtons equation?

What is a focimeter?

A machine that helps you measure the power if an unknown lens ir a contact lens

Draw a focimeter diagram with no lens in the positioning stop

What is a positioning screen

Opaque hole with a hole in the middle at the second focal point

Draw the diagram with an unknown lens placed in the positioning stop

What di we require at the sit for the focimeter to work ?

A collimated beam at the exit (the light rays are parallel to each other or going to infinity)

How do we achieve a verge new coming out of the system to be 0?

• must change the vergence leaving the target

• Simplest way is to move the target

How do we know when we have parallel rays?

When target is sharply in focus

Draw a diagram using x, x’, fs, and fs’

What is the focal length if the test lens to the image (ft) equal to?

x’

Why is the fact that x’ = ft important?

So that we can find the power if the test lens, as it is reciprocal

How can we rewrite Newtons equation to get the focimeter equation?

For a negative test lens, what can we say about x and Ft?

Both less than 0

What if the test lens is positive?

• Must move the target past the first focal point to the right of F

• x and Ft are positive

What kind of relationship does the focimeter equation represent?

Linear, as X changes, the power of the test lens changes in a directionally proportional way