1. Lesson 3
To understand the effect that convex and concave lenses have on light.
2. To appreciate real world applications of concave and convex lenses.

2. Experiment
Aim
To look at how convex and concave lenses refract light.

3. Method Draw around the convex lens. 2. Switch on the ray box.
with
triple
slit
Sheet of Paper
Convex Lens
Draw around the convex lens.
2. Switch on the ray box.
3. Sketch over the rays of light.
4. Repeat using a concave lens.

4. Results – Convex (Converging) Lens Copy!
Focal Length
Focus Point
A thicker lens will cause the focal length (measured in metres) to decrease.
Focal Length
Focus Point

5. Results – Concave (Diverging) Lens Copy!
Focal Length
Virtual
Focus
There is no natural focal point. Therefore, the rays are extended backwards to show the virtual focus.
The focal length of a CONCAVE lens is always a NEGATIVE value.

6. Applications of lenses
Lenses are used in a range of important devices used by humans.
Lens inside a PS3.
Lens from inside a photocopier.
Magnifying Glass

7. The Eye The structure of the eye is as shown below: Iris Retina Pupil
Optic
Nerve
Cornea
Lens

8. The Eye
The Cornea
The cornea refracts light as it enters the eye. It is made of hardened tissue that protects the inner parts of the eye.

9. The Eye
The Iris/Pupil
The pupil is merely an opening in the iris, through which light enters the eye.
The iris controls the size of the pupil, allowing different amounts of light to enter the eye.

10. The Eye
The Lens
The lens is used to fine tune the focussing process, further refracting light as it passes through the eye.
It becomes thicker and shorter to focus on nearby objects and thinner and longer to focus on far objects.

11. The Eye
The Retina
The retina has special light sensitive cells called rods and cones, which change the incoming picture into electrical impulses.

12. The Eye
The Optic Nerve
The optic nerve takes electrical signals from the retina and transfers them to the brain.
The optic nerve creates a “blindspot” at the back of the eye.

13. Applications of lenses – The Eye
Copy!
Both the cornea and the lens inside the human eye act as convex lenses, causing light to refract and come to a focus.
Retina
Cornea
Light from
faraway objects.
Lens
In the perfect eye (emmetropia), the light will focus on the retina.

14. Applications of lenses – The Eye
Copy!
Light from nearby objects does not travel in parallel rays. Therefore, the lens would not normally be powerful enough to focus it on the retina.
Cornea
Cornea
Retina
Light from
near objects.
Lens
Lens
In order to compensate for this, the lens will change shape.

15. Short Sightedness Copy!
Myopia, or short sightedness, affects millions of people worldwide. Sufferers can see nearby objects clearly but those far away appear blurry.
Retina
Cornea
Lens
Short sightedness is usually caused by the eye being too long, so the light focuses short of the retina.

16. Short Sightedness – The Cure
Copy!
In order to alter the eyesight of someone who is short sighted, a CONCAVE lens is used.
The concave lens diverges the rays, meaning they now focus on the retina.

17. Long Sightedness
Copy!
Sufferers can see faraway objects clearly but those nearby appear blurry.
Retina
Cornea
Lens
Long sightedness is usually caused by the eye being too short, so the light focuses long of the retina.

18. Long Sightedness – The Cure
Copy!
In order to alter the eyesight of someone who is long sighted, a CONVEX lens is used (like in “reading glasses”).
The convex lens converges the rays, meaning they now focus on the retina.

19. 2000

20. 2007 (Int. 1)