Lenses

History of Lenses October 18, 2010 Greeks thought that eyes send out invisible rays that allow a person to perceive objects. They thought vision takes place in space since objects appear to be outside of a person’s body. This was until 1625 when a priest demonstrated it is within the eye. By scraping the sclera off the back of an eye he created a small inverted and reversed image on a screen.

How Eyes Operate Eyes collect light from the environment and focus that light on a light-sensitive structure inside the eye.

In order for any object to be visible, it must either produce light or reflect it. Radiant light Sun, candle flame, and a lightbulb all produce Reflected light Moon, candle stick, and people reflect Transmitted light When light shines THROUGH an object

Refraction Light travels through transparent media (air, water, glass) in a straight line as long as it is of the same consistency When it changes composition, it bends at the point of contact called Refraction.

Every medium has an index of refraction that tells the degree that light will bend

Lens A lens is a clear, curved object that has a different index of refraction than its surrounding medium The curvature of the surface of the lens bends light rays differentially.

Bill Nye http://www.youtube.com/watch?v=cFVbLnXWn6A&feature=related

Focus When light passes through it bends so all of the rays of light come together at one point. This point is called the focus point

Focal Length A lens refracts light rays coming in parallel to one another and they converge at a point known as the principal focus or focal length for that lens. When the rays come together to focus, they form an image

Real Image Converging (convex) lens is thicker in the middle, and as the light rays pass through, they converge (meet) at one point. Below shows an image of the optical bench applet with beams (many parallel light rays) passing through the convex lens.

Virtual Images Diverging (concave) lens is thinner in the middle, the lens curves in. As the light rays pass through, they diverge (spread out), the rays do not meet like the convex lens. Below shows an image of the optical bench applet with beams (many parallel light rays) passing through the concave lens.

Projected Image If light passing through a convex lens is bent or refracted so that a real image forms on a surface, it is a projected image. These images are always inverted and reversed because the light rays cross. The distance between the lens and the screen on which a projected image is focused is the image focal distance. The only ray that is not refracted is the one that enters the center of the lens absolutely perpendicular to the plane of the lens.

Inside of the Eye The image that is projected on the screen is smaller than the actual object. This size reduction allows the image to fit inside the eyeball. After the image focuses on the retina, it travels to the brain where it can be interpreted as an image that is in its correct orientation.

Two Lenses The eye has two lenses The lens in the eye itself is the most obvious but not as important as the cornea. The lens in the eye make slight adjustments in focus as you look at something far away and then close up.

The curved surface of the cornea and the fluid behind it act as the main lens for focusing and image for eyes that function in the air. The cornea bends light more than the lens because the difference between the index of refraction of the cornea and that of the air is greater than the difference between the index of refraction of the lens and that of the fluid that surrounds it in the eye. Ex.) opening eyes under water with no mask, then with a diver’s mask.

Muscles Adjustments in focus is called accommodation This is controlled by delicate muscles called ciliary muscles. As we age, the ciliary muscles continue to pull on the lens, but no longer change shape to create a focused image of an object that is close to the eye.

Glasses and Contacts Focus can be corrected with glasses or contacts The lenses in glasses and contacts work with the lens in the eye to change the overall focal length of the lens system.

Capturing Images Using what they knew about how the eye works, scientists were able to capture images using a dark box and light-sensitive paper. The brighter the light, the faster the reaction on the paper.

Cameras and the Eye Cameras are similar to the eye in many ways. Both have a lens that allows light to enter a dark container and then focuses the image on the back. If you replace light sensitive paper with light sensitive living tissue, you have an eye. The screen in a camera is like the retina in the eye.