Lenses. Not in your text book

Transcription

1 Lenses Not in your text book Objectives: 1. Students will be able to draw a ray diagram for a lens 2. Students will be able to explain the difference between a real and a virtual image

2 Different Lenses, Different Purposes Lenses work under the principle of refraction. The light bends as it passes through the lens. focus principle axis Each curve for a lens will have a focal point, or a focus. The focus is the singular point at which all the light rays that travel parallel to the principle axis will cross. The plural of focus is foci. (pronounced pho-sigh).

3 Different Lenses, Different Purposes A concave shape curves inward (like a cave) A convex shape has an outward bulge (not like a cave) Lenses have two shapes: convex and concave.

4 Different Lenses, Different Purposes The convex shapes cause all the light rays to converge after passing through the lens. principle axis The concave shapes cause all the light rays to diverge after passing through the lens.

5 Different Lenses, Different Purposes With your partner, identify which type of lens goes with the examples you gave for the warm up question today. How do we know what something will look like when it is seen through a lens?

6 Ray Diagrams Similar to what we did with mirrors, we draw ray diagrams to help us see how an image will be seen through a lens. The light will always come from an object and passing through to the other side of the lens. The result is an image of the original object. Typically, we draw three different rays of light to find the image.

7 Ray Diagrams Ray 1: Starts at the top of your object, runs parallel to the centerline until it gets to the middle of the lens. It then passes through the focus on the other side of the lens. Hint: when you are drawing these rays, draw them through the focus f f

8 Ray Diagrams Ray 2: Starts at the top of your object and passes through the middle of the lens. This ray is a straight line. f f

9 Ray Diagrams Ray 3: Starts at the top of your object, runs through the focus toward the bottom of the lens, and then bends to be parallel to the centerline after leaving the lens. f f

10 Ray Diagrams Try this one in your notes. We ll check our work in a couple minutes. The arrow on the right side of the lens is the image. Discuss with your partner what differences you see between the image and the actual object.

11 Ray Diagrams Real images image created by light coming from the image or passing through it. Real images can be projected on a screen, will be on the opposite side of the lens from the object, and will be the reciprocal of the original object. Lens demonstrations

12 Eye Wish Eye Could See Better Your eye has a lens that focuses the light that enters it onto the retina at the back of the eye. This is one example of how we all use refraction. Depending on the shape of the eye, the light may not focus in the right place, causing us to have poor vision.

13 Eye Wish Eye Could See Better For you folks that are nearsighted, the light focuses in front of the retina. This is because the eye is elongated, which means the back of the eye is farther from the front of the eye than it should be.

14 Eye Wish Eye Could See Better For you folks that are farsighted, the light focuses behind the retina. This is because the eye is compressed, which means the back of the eye is closer to the front of the eye than it should be.

15 Eye Wish Eye Could See Better For you folks that have astigmatisms, the shape of the eye is ok but the shape of the cornea is not. This causes light to focus in different places on the retina so the image you see isn t clear.

16 Eye Wish Eye Could See Better The oblong shape causes the light to refract differently before it gets to the lens. This causes the lens to focus the light in different places.

17 Eye Wish Eye Could See Better Each condition has a different correction. Look at the diagram; the top picture shows what each eye sees without correction and the bottom shows how to correct it. Talk to your partner and figure out the correction differences.