Lenses and vision
A lens is a curved piece of transparent material such as plastic or glass.
Light refracts as it passes through the lens causing a ray to bend.
There are two types of lenses:
1)Double concave lens
- Light passing through the thicker curved areas bend more, causing rays of light to spread out.
2)Double convex lens
- Thicker in the middle
- Refracting rays come together
History note:
-glass originated about 2500BC. It was used as a decoration (in bead form only)
-lenses were not used until the invention of spectacles in the 14th C.
-1st combination of lenses were used as a telescope in 1608.
-1609 – Galileo used the telescope for astronomy and witnessed such phenomena as the moons of Jupiter, the mountains/craters of the moon, and sunspots. This contributed to the Earth-centered model of the universe.
Read page 208 – 209
The lens in the human eye is a convex lens, which takes light rays from distant objects and by refraction, focuses them on a light sensitive surface, the retina.
Some people have eyes that are too long. As a result the image forms in front of the retina. These people are near-sighted, and they have trouble seeing distant objects. Concave lenses help correct this problem.
Other people have eyes that are too short. Thus the image has not formed by the time the light reaches the retina. These people are far-sighted, and they have trouble seeing objects close to them. Convex lenses help to correct this problem.
comparing the camera to the Eye
Eye structures and functions / Camera parts and functionsIris - determines the amount of / Diaphragm - controls aperature of
light entering the eye / the camera
Pupil - opening to the eye / Aperature - opens and closes
to let more or less light in
Eyelid - opens and closes to let / Shutter - opens and closes to let
in and keep out light / in and keep out light
Combination of the cornea and / Lens - focus the image
lens - focus the image
Retina - light sensitive, internal / Film - light sensitive screen that
screen where the eye focuses / retains the image
the image
Flexible eyeball - filled with fluid / Camera body - rigid
Flexible lens - controlled by / Lens - rigid and inflexible
ciliary muscle
Camera lab on page 212
Hand in #1-5.
Focus Please!
Camera – if an object moves closer to the camera, the lens must move further away to keep the image in focus.
Human eye – you can’t move the lens further away from the retina.
ciliary muscles change the shape of the lens to adjust for different object distances – called accommodation.
if the object moves toward you, the ciliary muscles make the lens bulge in the middle.
older people – the lens stiffens and loses the ability to change shape; thus they might need convex lenses to read.
Near point of eye – the shortest distance at which an object is in focus.
Far point of eye – the longest distance at which an object is in focus.
Read page 215
Bringing in the Light
Camera – a diaphragm is a device that controlsaperture (the opening) of a lens or optical system.
On a sunny day, the diaphragm makes the aperture small, allowing less light in.
On a cloudy day, the diaphragm makes the aperture large, allowing more light into the camera.
The iris (coloured part) is like the diaphragm in a camera. The pupil (black part) is the hole in the centre of the iris. The size of the pupil is controlled by the iris. In a dark place, the iris increases the size of the eyes opening to let more light in.
When we go from a dark place to a bright one, the pupil which is open, becomes smaller as the iris adjusts to let more light in.
This natural adjustment in the size of the pupil is called iris reflex.
Read page 216-217.
Seeing the Image
Read page 217 – look at figure 3.37.
In the human eye, the retina senses light. When the cells in the retina detect light, they produce small electrical impulses that travel from the retina to the brain through the optic nerve. It is the brain that interprets the electrical impulses sent along the optic nerve, and deciphers or decodes them!
The point where the optic nerve enters the retina, does not have any light sensing cells – This point is the blind spot.
(NOTE: each eye sees what the other misses, because the blind spots are not in the same place).
To keep the eye from collapsing, the eyeball is filled with fluid – called humours.
Eye Internet Assignment – page 219
Read “DID YOU KNOW” on page 218.
Read Career-Connect on page 220.
On page 2-20
#1-5