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Optical Instruments

Optical Instruments. Chapter 25. The Human Eye: Corrective Lenses. Parts of the eye Iris Adjusts to control the amount of light entering the eye Pupil Retina Contains nerves and sensors known as rods and cones Changes light energy into electrical energy Fovea Cones are closely packed

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Optical Instruments

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  1. Optical Instruments Chapter 25

  2. The Human Eye: Corrective Lenses • Parts of the eye • Iris • Adjusts to control the amount of light entering the eye • Pupil • Retina • Contains nerves and sensors known as rods and cones • Changes light energy into electrical energy • Fovea • Cones are closely packed • Sharpest image and best color discrimination

  3. The Human Eye: Corrective Lenses • Cornea • Location where most of refraction is done • n = 1.376 • Ciliary muscles change the curvature, and focal point, of the lens • Distant objects • Ciliary muscles are relaxed • Lens is thin • Close objects • Ciliary muscles are tightened • Lens is thick • Shortens the focal length

  4. The Human Eye: Corrective Lenses • Near point • Closest distance at which the eye can focus • Average = 25 cm • Far point • Farthest distance at which the eye can focus • Average = infinity • Nearsightedness, or myopia • Can only focus on near objects; distant objects are not clear • Eye is too long or curvature is too great • Diverging lens fixes problem • Farsightedness, or hyperopia • Can only focus on distant objects;near objects are not clear • Eye is too short or curvature is too small • Converging lens fixes the problem

  5. Example • Sue is farsighted with a near point of 100 cm. Reading glasses must have what lens power so that she can read a newspaper at a distance of 25 cm? Assume the lens is very close to the eye.

  6. Example • A nearsighted eye has near and far points of 12 cm and 17 cm, respectively. Assume that the lens is 2 cm from the eye, which is typical for eyeglasses. • What lens power is needed for this person to see distant objects clearly? • What then will be the near point?

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