1. Lens Applications

2. Reflecting Telescope (Newtonian)
The incoming rays are reflected from the mirror and converge toward point A
At A, an image would be formed
Instead, a small flat mirror, M, reflects the light toward an opening in the side which passes the light into an eyepiece
The image formed after reflecting off of the flat mirror is then magnified by the eyepiece

3. Refracting Telescopes
Two lenses are arranged at separation equal to the sum of their focal points
Light from distant objects enters the objective lens and forms a real inverted image between the two lenses
• The first image is near the focal point of the eyepiece lens

4. Refracting Telescopes
Light from the first image then passes through the eyepiece lens and forms a virtual image that appears to come from beyond the objective lens
The final image is inverted and larger than the image produced by the objective lens
The distance between the two lenses, ƒo + ƒe, corresponds to the length of the tube of the telescope

5. Cameras
A camera gathers light from large, distant objects and forms smaller, real images on film or a sensor as the light passes through a converging lens
When the object is beyond twice the focal length (2F) the camera lens produces a real, inverted, and smaller image of the object on a film or digital sensor where it is recorded
To get a sharp image you move the lens in and out to focus.

6. Magnifying Glass The simplest optical device
Converging lens in which the object is located between F’ and the lens
No real image is produced by the object at this location
Refracted rays diverge to our eyes and we project these rays backwards, forming and enlarged, virtual image located on the same side of the lens as the object

7. The Human Eye

8. Parts of the Eye
Cornea
Fluid
Ciliary Muscles
Lens
Iris
Pupil
Retina

9. The Anatomy of the Eye
-Light entering the eye first meets the cornea, a thin protective membrane (refractive index n= 1.38) that refracts light before entering the eye
-Light passes through a fluid to the crystalline lens (n=1.44), the shape of which can be adjusted by the ciliary muscles

10. The Anatomy of the Eye
-The ciliary muscles adjust the length and thickness of the lens allowing the lens to focus light from both nearby and distant objects onto the retina
-The back of the eye is the retina, the “screen” on which images are formed. Its detects the intensity and frequency of light and sends the information to the brain via the optic nerve.

11. The Anatomy of the Eye
-The amount of light entering the eye can be controlled by the iris (the coloured part of the eye) which can dilate to admit more light
-The black part of the eye, or the pupil, is not a proper anatomical part of the eye. It is just the opening for the light and appears black because all incident light is absorbed and not reflected

12. Rods and Cones
-Rods & Cones are the light sensitive cells, or, photoreceptors that make up the retina
-Rods are more sensitive to light and dark changes, shape, and movement but contain only one type of light-sensitive pigment and are therefore not good for color vision. Rods are more numerous than cones in the periphery of the retina.

13. Rods and Cones
Cones are most sensitive to one of three different colours (green, red or blue). Signals from the cones are sent to the brain which then translates these messages into the perception of colour. Cones, however, are not as sensitive to light as rods. That's why you cannot see colour very well in dark places.

14. Colour Blindness
Someone who is colour blind does not have a particular type of cone in the retina or one type of cone may be weak. In the general population, about 8% of all males are color blind and about 0.5% of all females are color blind.

15. Focusing problem – Hyperopia
Hyperopia (Far-sightedness)
Can’t see objects that are up close
Eye doesn’t refract light well enough to form an image
Usually occurs because the distance between the lens and the retina is too small or because the cornea-lens combination is too weak
Light ends up focused behind the retina

16. Hyperopia

17. Correction for Hyperopia – Converging lens
The eye needs help in refracting light
A converging lens called a positive meniscus lens can be used to correct this problem

18. Focusing problem - Myopia
Myopia (Near-sightedness)
Can’t see objects that are far away
Usually occurs because the distance between the lens and the retina is too large or because the cornea-lens combination converges light too strongly
Light ends up focused in front of the retina

19. Myopia

20. Solution for Myopia – Diverging lens
The eye needs help in refracting light
A diverging lens, called a negative meniscus, helps to diverge the incoming rays so they end up focused on the retina

21. Focusing problem - Presbyopia
lens loses its elasticity over time
typically affects people who are over 40
results in a form of far-sightedness

22. Laser Eye surgery
The cornea is re-shaped to ensure proper refraction of light
Cornea needs to be flattened to decrease refractive power
Increase the slopes of the cornea (make it fatter) to increase refractive power