3. Lenses

really great notes on this topic http://www.funsci.com/fun3_en/lens/lens.htm

there are many different uses of lenses and also many everyday object which can act as lenses. Projectors, microscopes, binoculars, telescopes, reading glasses, contact lenses, cameras ....... Other objects that can act as a lens include vases, fishbowls, droplets (water or other) .....

A general definition for a lens would be any transparent material that has at least one curved surface.

Fat lenses have short focal lengths, skinny ones have longer focal lengths

Images formed by single thin lenses.

V +ive for real & -ive for Virtual

f +ive for Converging lens (convex) -ive for diverging (concave)

Ray Diagrams for Lenses Rules

  1. If the ray passes through the center of the lens it carries on without bending
  2. If the ray hits the lens parallel to the optic axis it leaves passing through the focal point
  3. If the ray passes through the focal point it leaves parallel to the optical axis

Draw ray diagrams for an object

Outside 2f, @2f, between 2f & f, @ f, btw f & p

So what happens if the object is inside the focal length of the of the lens ?

The Converging lens

The image making muliplier lens.

The eye: ;

optical structure

how does the eye work ??

check out physics.info here


Power of lens: P

Two lenses in contact: P = P1 + P2

The lenses in contact can be taken to be one single lens.

The Optometrist Formula

Pnormal vision = Plens + Ppatient.

Convert the power of your glasses to the power of contact lenses

Q6 2006

The power of an eye when looking at a distant object should be 60 m–1. A person with defective vision has a minimum power of 64 m–1.

Calculate the focal length of the lens required to correct this defect. (12)

Pdesired vision = Plens + Ppatient

60 m-1 = Plens + 64 m-1.

60 m-1 - 64 m-1 = Plens = - 4 m-1.

P = 1/f → f = 0.25 m

What type of lens is used? Name the defect. (6)

the minus reading means a diverging lens is required


for combining powers of glasses lens to contact lenses .... what differences can you imagine there might be ?

short sight, long sight, and corrections.

Long sighted can see things far away, hyperopia

short eyeball / under curved cornea

remedy - diverging lens


Short sighted can see things close by, myopia.

long eyeball / over curved cornea

remedy - converging lens


When we get older our sight deteriorates, this is called Presbyopia

Simple exercises on lenses by ray tracing.

in parallel out through the focal point

in through the focal point out parallel

no deviation

Simple exercises on lenses by use of formula.

Uses of lenses.


Use of cameras & focusing

Conjugate foci

Because of the lens makerformula 1/f = 1/u + 1/v has 2 possible solutions everytime, that is the image position and object position are interchangable, these positions are called conjugate foci.

I also thought in complex optical instruments, telescopes, where there are more than 1 lens that the conjugate foci was the shared focus

diagram needed

4. Measurement of the focal length of a converging lens

Title Measurement of the focal length of a converging lens.



  1. Ensure your hands are clean
  2. obtain the lens, be careful not to touch the optical surface, hold by the edge
  3. Find an approximate focal length by focussing an image from inifinity onto a screen

There should be more distance between the light box and the cross hair object. Take note of this when doing this experiment

1 Carry out this experiment with different measured locations of an object, measure the corresponding values that the image is found at.

2 Repeat at least 6 times.

There are two ways to obtain a value for f the focal length of the lens.

Find different values of f from the lens maker formula, find an average of these values of f


plot the values 1/u vs 1/v,

It can be hard to measure u & v very accurately, due to the true values including some small distance within the lens.

It can also be more difficult to find the value of v as you must get the image to its sharpest (a judgement call)

for a little background on the simple converging lens


Telescope look at the