Lenses• Mirrors = “Reflection”

• Lenses = “Refraction”

Convex Lens (converging)

Examine the outside of the eye.How many parts of the eye can you identify?

Sclera: the white, tough, outer coveringCornea: the clear covering over the front of the eyeIris: the coloured part of the eyePupil: the dark oval in the middle of the iris

Source: adapted from www. http://www.exploratorium.edu

Cut around the middle of the eye

You’ll end up with two halves

Remove the cornea from the front half and place it on the cutting board

Try cutting through the cornea – why is it so strong?

Make an incision in the sclera in the middle of the eye

Remove the lens

It is a clear lump about the size and shape of a squashed marble

Pull out the iris – it should be between the cornea and the lens, but may have stayed with the back of the eye

The hole in the centre of the iris is the pupil, which lets light into the eye

Put the lens down on a newspaper and look through it at the words – what do you see?

The lens feels soft on the outside and hard in the middle

Hold the lens up and look through it – what do you see?

Concave Lens (diverging)

Normal Eye

Myopia (Short sightedness)

Mirrors• Reflects light• Concave mirror = converging mirror• Convex mirror = diverging mirror

Lenses • Refracts light• Concave lens = diverging lens• Convex lens = converging lens

Ray Diagrams

The two important rules:

1. Parallel to pa → Focal point

2. Focal point → Parallel to pa(pa = principal axis, the horizontal line)

Convex (Converging) Lens

Size Orientation Nature

Behind 2F

At 2F

Between 2F & F

At F

Between F & P

1. 2. 3.

4. 5.

Convex (Converging) Lens

Size Orientation Nature

Behind 2F Diminished Inverted Real

At 2F Same Inverted Real

Between 2F & F Enlarged Inverted Real

At F No image No image No image

Between F & P Enlarged Upright Virtual

Convex (converging) lensesThe image will be real and inverted if the object is located further away from the principal focus.

The image will be virtual, upright and enlarged if the object is located between the lens and the principal focus.

Concave (diverging) lensesConcave lenses always produce virtual images which are always upright and smaller than the object.

Formulae ONE

• Descartes’ Formula:

• and:

• m = magnification factor• h = height• d = distance from the lens

oi ddf

111

o

i

o

i

d

d

h

hm

• Newton’s Formula:

• and:

• S = distance from the focal point• All distances are positive but care must be

taken when calculating Si or So

2fSS oi

f

S

S

f

h

hm i

oo

i

Formulae TWO

• f is + if the lens is convex (converging)

• f is − if the lens is concave (diverging)

• di is + if the image is real and located on the opposite side of the lens

• di is − if the image is virtual and located on the object’s side of the lens

• hi is + if the image is upright/virtual

• hi is − if the image is inverted/real

• m > 1 if the image is enlarged