Unit 3 Light and Optical Systems Topic 4 Lenses and Vision Remember to name and date your notes!

A lens is a curved piece of transparent material (glass/plastic). When light rays pass through it, the light is refracted, causing the rays to bend.

A double concave lens is thinner and flatter in the middle than the edges.

Light passing through the thicker more curved areas of the lens will bend more than light passing through the thinner areas, causing the light to spread out or diverge.

A double concave lens

Concave mirror

A double convex lens is thicker in the middle than around the edges. This causes the light to come together at a focal point, or converge.

A convex lens refracts the light rays from an object so they can be focused.

Different size lenses can converge the light rays at different distances, enabling corrections to be made to focal points. Here is the focal point….where light comes together after passing through the lens.

The lens in the human eye is a convex lens, which focuses the light rays entering your eye to a point on your retina (a light sensitive area at the back of the eye).

The image you see is formed on the retina.

Some people however have eyes that are too long or too short. If their eye is too long, the image forms in front of the retina - this is a condition called near-sightedness

If their eye is too short, the image forms behind the retina, making object that are close to them difficult to see. This condition is called far-sightedness.

Notice anything odd about this?

Make sure you are relaxed Make sure you are relaxed. Keep your eyes on the lighter areas where the black lines intersect. You should probably notice circles coming from the center of the design. The trick is that a light color appears brighter when it contrasts against a darker color. So, people see the areas adjacent to the intersections to be slightly brighter, causing what appears to be circles.

Can you find all 9 people in this picture?

HOW MANY BLACK DOTS DO YOU SEE?

Stare at the center of the spiral for about 60 seconds Stare at the center of the spiral for about 60 seconds. Now look at your hand... it will appear to look like bugs are crawling UNDER your skin!

Try some optical illusions online. http://www.optillusions.com/ http://www.grand-illusions.com/ http://www.liquidgeneration.com/sabotage/optical_sabotage.asp

http://www.ewart.org.uk/science/Waves/wav7.htm