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Convex Lenses

Convex Lenses. Thicker in the center than edges. Lens that converges (brings together) light rays. Forms real images and virtual images depending on position of the object. The Magnifier. Concave Lenses. Lenses that are thicker at the edges and thinner in the center.

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Convex Lenses

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  1. Convex Lenses Thicker in the center than edges. • Lens that converges (brings together) light rays. • Forms real images and virtual images depending on position of the object The Magnifier

  2. Concave Lenses • Lenses that are thicker at the edges and thinner in the center. • Diverges light rays • All images areerect and reduced. The De-Magnifier

  3. Let’s get focused… • Just as with mirrors, curved lenses follow same rules as flat interfaces, using local surface normal A lens, with front and back curved surfaces, bends light twice, each diverting incoming ray towards centerline. Follows laws of refraction at each surface. Parallel rays, coming, for instance from a specific direction (like a distant bird) are focused by a convex (positive) lens to a focal point. Placing film at this point would record an image of the distant bird at a very specific spot on the film. Lenses map incoming angles into positions in the focal plane. UCSD: Physics 8; 2006

  4. pinhole object image at film plane object image at film plane lens Cameras, in brief In a pinhole camera, the hole is so small that light hitting any particular point on the film plane must have come from a particular direction outside the camera In a camera with a lens, the same applies: that a point on the film plane more-or-less corresponds to a direction outside the camera. Lenses have the important advantage of collecting more light than the pinhole admits

  5. The Eye • Now for our cameras… • Eye forms image on retina, where light is sensed • Cornea does 80% of the work, with the lens providing slight tweaks (accommodation, or adjusting) Refractive indices: air: 1.0 cornea: 1.376 fluid: 1.336 lens: 1.396 Central field of view (called fovea) densely plastered with receptors for high resolution & acuity. Fovea only a few degrees across.

  6. How You See • Near Sighted – Eyeball is too long and image focuses in front of the retina • Near Sightedness – Concave lenses expand focal length • Far Sighted – Eyeball is too short so image is focused behind the retina. • Far Sightedness – Convex lense shortens the focal length.

  7. Questions • Why can’t we focus our eyes under water? • Why do goggles help?

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