12.1 Characteristics of Lenses (Page 199)

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

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

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

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

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

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

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

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

12.1 Focal Point & Focal Length

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

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

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

12.1 Lenses & Chromatic Aberration

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

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

Do Questions # 1-8 on page 493