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Chapter 36

Chapter 36. Image Formation 2 Thin Lens Multi lens/mirror system. Thin lenses. Lenses are commonly used to form images by refraction. We discuss spherical lenses only. They are part of two spheres. There are two types of lenses Converging Diverging

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Chapter 36

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  1. Chapter 36 Image Formation 2 Thin Lens Multi lens/mirror system

  2. Thin lenses • Lenses are commonly used to form images by refraction. • We discuss spherical lenses only. They are part of two spheres. • There are two types of lenses • Converging • Diverging • When the thickness of the lens is negligible, the lens is called thin lens. • Principal axis: the line that goes through the two centers of the two spheres. • Paraxial rays are those close to the principal axis. • Lenses are used in optical instruments • Cameras • Telescopes • Microscopes Principal axis converging Principal axis diverging

  3. Converging and diverging Lens Shapes • Converging: • positive focal lengths • thickest in the middle • Diverging: • negative focal lengths • thickest at the edges

  4. Focal Length of a Converging Lens • The parallel rays pass through the lens and converge at the focal point • The parallel rays can come from the left or right of the lens

  5. Focal Length of a Diverging Lens • The parallel rays diverge after passing through the diverging lens • The focal point is the point where the rays appear to have originated

  6. Notes on Focal Length and Focal Point of a Thin Lens • Because light can travel in either direction through a lens, each lens has two focal points • One focal point is for light passing in one direction through the lens and the other is for light traveling in the opposite direction • However, there is only one focal length • Each focal point is located the same distance from the lens

  7. Ray Diagrams for Thin Lenses – converging • Ray diagrams are convenient for locating the images formed by thin lenses or systems of lenses • For a converging lens, the following three rays are drawn: • Ray 1 is drawn parallel to the principal axis and then passes through the focal point on the back side of the lens • Ray 2 is drawn through the center of the lens and continues in a straight line • Ray 3 is drawn through the focal point on the front of the lens (or as if coming from the focal point if p < ƒ) and emerges from the lens parallel to the principal axis • The image is real • The image is inverted • The image is on the back side of the lens

  8. Ray 1:Parallel to axis, then passes through far focal point Ray 2:Passes unchanged through center of lens Ray 3:Passes through near focal point, then parallel to axis The 3-ray diagram again

  9. ho F do di f hi Object F Real image, inverted, smaller

  10. Real, inverted, smaller F F f 2f Real, inverted, same size F F f 2f Object distance, 5 cases: 1 -- 2

  11. Real, inverted, larger F F f 2f F F f 2f No image Object distance, 5 cases: 3 -- 4

  12. Object distance, 5 cases: 5 Virtual, upright, larger F F f 2f Like in the converging mirror case, there are 5 possible object locations that produce different images. While in the diverging lens case, like in the diverging mirror case, no matter where the object is placed, you always get a virtual, upright and smaller image.

  13. Ray Diagrams for Thin Lenses – Diverging • For a diverging lens, the following three rays are drawn: • Ray 1 is drawn parallel to the principal axis and emerges directed away from the focal point on the front side of the lens • Ray 2 is drawn through the center of the lens and continues in a straight line • Ray 3 is drawn in the direction toward the focal point on the back side of the lens and emerges from the lens parallel to the principal axis • The image is virtual • The image is upright • The image is smaller • The image is on the front side of the lens

  14. Ray 1Parallel to axis, virtual ray passes through near focal point Ray 2Straight through center of lens Ray 3Virtual ray through far focal point, virtual ray parallel to axis The 3-ray diagram again

  15. ho hi do di Object Virtual image, upright, smaller F f F

  16. The Camera • The photographic camera is a simple optical instrument • Components • Light-tight chamber • Converging lens • Produces a real image • Film behind the lens • Receives the image

  17. The Eye • The normal eye focuses light and produces a sharp image • Essential parts of the eye: • Cornea – light passes through this transparent structure • Aqueous Humor – clear liquid behind the cornea

  18. The Eye – Close-up of the Cornea

  19. Combinations of Thin Lenses • The image formed by the first lens is located as though the second lens were not present • Then a ray diagram is drawn for the second lens • The image of the first lens is treated as the object of the second lens • The image formed by the second lens is the final image of the system

  20. Combination of Thin Lenses, example

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