1. LO: understand how to draw ray diagrams for lenses
Converging lens
A converging lens is always convex. It makes rays that are coming in that are parallel converge onto a point. The point where the rays converge is called the principal focus, or focal point.
Converging lenses are used in magnifying glasses and in cameras.

2. LO: understand how to draw ray diagrams for lenses
Diverging lens
A diverging lens is always concave. It makes rays that are coming in that are parallel diverge. The point where the rays seem to diverge from is called the principal focus, or focal point.
Diverging lenses are used to correct short sight

3. LO: understand how to draw ray diagrams for lenses
Focal length
For both lenses, the distance between the centre of the lens and the focal point is called the focal length.

4. Magnification = image height
LO: understand how to draw ray diagrams for lenses
Magnification
The magnification produced by a lens can be worked out using the following formula
Magnification = image height
object height

5. New Skool uses of converging lenses
LO: understand how to draw ray diagrams for lenses
New Skool uses of converging lenses
Newer digital cameras work in the same way. However, instead of photographic film at the back of the camera, they have a CCD imager. When the shutter is opened, the CCD is exposed to light and it forms the image.

6. New Skool uses of converging lenses
LO: understand how to draw ray diagrams for lenses
New Skool uses of converging lenses
When an object is placed too close to a camera, it will appear fuzzy. This is because the object is closer to the lens than the focal length and the lens is not able to focus the image properly onto the CCD.

7. Focal point, virtual, real
The Eye
Understand the structure of the eye
ALL – label the different parts of the eye
MOST – Describe what different parts of the eye do
SOME – Explain how converging and diverging lenses can be used to correct for vision problems
KEYWORDS: converging, diverging, lenses,
Focal point, virtual, real

8. Task LO: understand the structure of the eye Suspensory ligament
Cornea
Optic nerve
Pupil
Blind spot
Lens
Retina
Iris
Ciliary muscle

9. LO: understand the structure of the eye
How the eye works
Regardless of if you are looking at an object very close to you or very far away, your eye is able to focus and you are able to see the object clearly. How is your eye able to refocus based on where the object is?

10. What happens if too much light suddenly enters the eye?
LO: understand the structure of the eye
How the eye works
When light enters the eye, the ciliary muscles change the thickness of the lens
The light is focused by your lens onto the retina
The light sensitive cells in the retina send electrical impulses through the optic nerve to your brain
Your brain processes these impulses and shows you what the object looks like
What happens if too much light suddenly enters the eye?

11. Task Rearrange these sentences into the correct order:
LO: understand the structure of the eye
Task
Rearrange these sentences into the correct order:
The brain processes the impulses and reforms the image
The ciliary muscles change the thickness of the lens
The lens focuses the light onto the retina and forms a real image
The eye senses light and alters the size of the iris to limit the amount of light coming into the eye
The electrical impulses are sent along the optic nerve to the brain
The light sensitive cells in the retina change the image into electrical impulses

12. LO: understand the structure of the eye
Correcting vision
By using our understanding of how the eye works and how lenses work, we can design glasses to correct for sight problems

13. LO: understand the structure of the eye
Short sight
In a normal eye, the lens focuses the image exactly on the retina.
However, in the eye of a person with Myopia (short sighted), the image is formed before the retina. This leads to a blurred image.

14. Correcting short sight
LO: understand the structure of the eye
Correcting short sight
Short sight can be corrected by glasses that have a concave (diverging) lens. This causes the light rays to diffract outwards slightly as they pass the lens so that they are focused exactly on the retina by the lens in the eye.

15. LO: understand the structure of the eye
Long sight
In a person with ‘hyperopia’ (long sight), the image is not correctly focused onto the retina by the eye lens. The image is focused behind the retina, leading to a blurry image.
How can we correct this?

16. LO: understand the structure of the eye
Correcting long sight
Long sight can be corrected by using a convex (converging) lens. This causes the light rays to converge slightly before they hit the lens so that they are refracted perfectly onto the retina.

17. Power of a lens Power = Dioptre (D) Focal length = m
LO: understand the structure of the eye
Power of a lens
The power of a lens can be worked out using the following equation:
Power = 1
Focal length
Power = Dioptre (D)
Focal length = m

18. LO: understand the structure of the eye
Example 1
A converging lens has a focal length of 2cm. Calculate the power of the lens

19. LO: understand the structure of the eye
Example 2
A diverging lens has a focal length of 0.3m. Calculate the power of the lens

20. Example 3 A lens has a power of -3.5D.
LO: understand the structure of the eye
Example 3
A lens has a power of -3.5D.
Is this a converging or diverging lens?
Calculate the focal length of the lens

21. LO: understand the structure of the eye
Task
A lens has a power of +3.5D. How do you know that this is a converging lens?
Work out the focal length of the lens in the question above
A short sighted person is given glasses with lenses of power 6.5D (the sign accompanying the power is missing). Calculate the focal length of this lens and state whether it is a converging or diverging lens.
A diverging lens has a focal length of 0.4m. Calculate the power of the lens.
Lens A has a focal length of 33cm. Lens B has a focal length of 0.46m. What is the difference between the power of the two lenses?