Lesson 25 Lenses Eleanor Roosevelt High School Chin-Sung Lin

Lenses Lenses – Is a piece of glass that had just the right shape, as it bends parallel rays of light so that they cross and form an image

Lenses When arranged in certain positions, prisms bend incoming parallel rays so they converge to or diverge from a single point

Lenses Real lenses are made up of solid pieces of glass with surfaces that are usually ground to a spherical shape

Converging Lenses Converging lenses occur where it’s the thickest in the middle, causing parallel rays of light to converge to a focus

Diverging Lenses Diverging lenses are the thinnest in the middle and causes parallel rays of light to diverge

Thin Lenses

Principal axis – a line joins the centers of curvature of the surfaces of a lens Focal points – For a converging lens, the point at which a beam of parallel light, parallel to the principal axis converges. For a diverging lens, an incident beam of light parallel to the principal axis is not converged to a point, but is diverged so that light appears to come from a point Focal planes – Incident parallel beams that are not parallel to the principal axis focus at points above or below the focal point. All such possible points make up a focal plane Focal lengths – the distance between center of the lens and its focal point, whether it’s converging or diverging

Image Formation by Lens An object can be seen through a small angle of view, when it’s far away. That same object can be seen through a larger angle, when it’s closer

Image Formation by Lens Magnification occurs when an image is observed through a wider angle with the usage of lens than without lens which allows more detail to be seen

Image Formation by Lens With a magnifying glass, it’s held closer to an object. Converging lens will magnify only when the object is between the focal point and lens

Image Formation by Lens A virtual image is formed through refraction that can be seen by an observer but cannot be projected on a screen since light from the object doesn’t actually come to a focus

Image Formation by Lens When the object is far away from the focal point of a converging lens, light from the object does converge and can be focused on a screen, and a real image is formed

Image Formation by Lens Converging lenses are used for projecting slides, motion pictures on a screen

Image Formation by Lens When a diverging lens is used alone, image is always virtual and smaller than the actual object. It does not matter how far or how close the object is

Construction of Images: Ray Diagrams Ray diagrams show principal rays that can be used to determine the size and location of an image Size and location of the object, distance from the center of lens must be known Paths of two rays from a point on the object must be known Path of one refracted ray is known from the definition of the focal point

Construction of Images: Ray Diagrams

A ray parallel to principal axis will be refracted by the lens to the focal point A ray of light will pass through the center with no appreciable change in direction A ray of light that passes through the focal point in front of the lens emerges from the lens and proceeds parallel to the principal axis Any two of the rays is enough to locate the relative size and location of the image

Construction of Images: Ray Diagrams For converging lenses: when the distance from the lens to the object is more than the focal length

Construction of Images: Ray Diagrams For converging lenses: when the distance from the lens to the object is less than the focal length

Construction of Images: Ray Diagrams For diverging lenses

Construction of Images: Ray Diagrams

Optical Instruments Eyes

Optical Instruments Telescope

Optical Instruments Compound Microscope

Optical Instruments Camera

Defects in Vision Normal Vision

Defects in Vision Nearsighted & Farsighted

Defects in Vision Nearsighted vision correction

Defects in Vision Farsighted vision correction

Defects in Lens Spherical aberrations occurs when light passes through the edges of a lens and focuses at a slightly different place from light passing through the center of the lens

Defects in Lens Chromatic aberration is a result of the different speeds of light of various colors, the different refractions they undergo, and hence they don’t come to focus in the same place

The End