1. 12.1 Characteristics of Lenses
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2. 12.1 Characteristics of Lenses
Lens: A transparent object with at least one curved side that causes light to refract
Reading stones (made of quartz) were the first lenses developed

3. 12.1
Describing Lenses
Lenses have two sides and either side can be plane, concave or convex
Diverging Lens: cause parallel light rays to spread away from a common point
Converging Lens: cause parallel light rays to come together toward a common point

4. 12.1
Describing Lenses
When light rays pass through a window, they first bend towards the normal (air-to-glass)
When light rays exit the glass, they bend away from the normal (glass-to-air)
The light rays are laterally displaced – the glass is not a lens since no change of direction has occurred

5. 12.1 Converging Lenses Bring parallel light rays toward a common point
Biconvex: A lens that is convex on both sides
Light rays entering from the left (air-to-glass; fast to slow medium) they bend towards the normal
Light rays leave (glass-to-air; slow to fast medium) they bend away from the normal
Because of the direction of the normals, the rays converge

6. 12.1
Diverging Lenses
Spreads parallel light rays away from a common point
Biconcave: A lens that is concave on both sides
Light rays entering from the left (air-to-glass; fast to slow medium) they bend towards the normal
Light rays leave (glass-to-air; slow to fast medium) they bend away from the normal
Because of the direction of the normals, the rays diverge

7. 12.1 Focal Point & Focal Length
Principal Axis: A line that passes through the center of the lens normal to the lens surfaces
Converging Lens: Light rays parallel to principal axis intersect at the focal point(F)
Since light rays can pass from both sides, lenses have two focal points

8. 12.1 Focal Point & Focal Length
Diverging Lens: Since rays diverge, you need to trace the rays backwards to find F
This F is a virtual focus
The distance from the center of the lens to F is the focal length

9. 12.1 Focal Point & Focal Length
The position of F depends on the index of refraction of the lens material and the lens curvature
Materials with a high index of refraction cause light rays to bend more which shortens F
The greater the curvature, the more light rays bend, the shorter the F

10. 12.1
Focal Point & Focal Length

11. 12.1 Focal Point & Focal Length
Large lens curvature (A fish-eye lens) cause severe image distortion due to spherical aberration

12. 12.1 Lenses & Spherical Aberration
Spherical aberration is not noticeable in thin lenses
For thick lenses, spherical aberration is a problem
For thick lenses, only light rays that pass through the lens near the principal axis meet at the focal point to give a sharp image

13. 12.1 Lenses & Chromatic Aberration
Chromatic Aberration: The dispersion of light through a lens
The edges of the lens act as prisms and disperse light into different colors

14. 12.1
Lenses & Chromatic Aberration

15. 12.1 Lenses & Chromatic Aberration
Spherical and chromatic aberration can be corrected by combining lenses with different indices of refraction
Two thin lenses (instead of one thick one) can reduce spherical aberration
Combinations of lenses made of different materials can reduce chromatic aberration

16. 12.1 Lenses & Chromatic Aberration
High quality camera lenses are quite expensive due to these (and other) innovations

17. Do Questions # 1-8 on page 493