Lenses Chapter 30

Converging and Diverging Lenses Lens – a piece of glass which bends parallel rays so that they cross and form an image Converging Lens – a lens which is thicker in the middle, makes parallel light rays converge Diverging Lens – a lens which is thinner in the middle, makes parallel light rays diverge Principle Axis – line joining the centers of curvature of a lens’ surface Focal Point – the point at which a beam of light converges Incident parallel beams that are not parallel to the principle axis focus at points above or below the focal point Focal Plane – all possible points from incident beams A lens has two focal points and two focal planes

Converging and Diverging Lenses

Image Formation by a Lens Magnification occurs when an image is observed through a larger angle with the use of a lens than with out the lens, and allows more detail to be seen A converging lens will only magnify when the object is between the focal point and the lens Virtual Image – an image formed through reflection or refraction that can be seen by an observer but cannot be projected on a screen because light from the object does not actually come to a focus Real Image – an image that is formed by converging light rays and that can be displayed on a screen When a diverging lens is used alone, the image is always virtual, right-side up, and smaller than the object

Image Formation by a Lens

Constructing Images Through Ray Diagrams Ray Diagrams – show the principle rays that can be used to determine that size and location of an image One end of the object is always placed right on the principle axis 3 useful rays in ray diagrams Ray parallel to the principle axis that passes through the focal point after refraction by the lens A ray through the center of the lens that does not change direction A ray through the focal point in front of the lens that emerges parallel to the principle axis after refraction by the lens

Constructing Images Through Ray Diagrams

Image Formation Summarized If an object is one focal length away from a converging lens, the image is virtual, magnified, and right-side up If an object is beyond one focal length, a converging lens produces a real, inverted image If the object is close to the focal point, the image appears far away; if it is far from the focal point, the image appears nearer When an object is viewed through a diverging lens, the image is virtual, reduced, and right-side up

Some Common Optical Instruments The Camera – consists of a lens and sensitive film mounted in a lighttight box, the amount of light which gets to the film is regulated by a shutter and a diaphragm (which varies the size of the opening) The Telescope – a simple telescope uses a lens to form a real image of a distant object The Compound Microscope – uses two converging lenses of short focal length, forming a real and a virtual image (so we see the object right-side up) The Projector – An arrangement of converging lenses is used

Some Common Optical Instruments

The Eye The amount of light that enters is regulated by the iris, the colored part of the eye that surrounds the opening called the pupil Light enters through the transparent covering called the cornea, passes through the pupil and lens, and is focused on a layer of tissue at the back of the eye—the retina—extremely sensitive to light In both the camera and the eye, the image is upside down, our brain flips the image right-side up for us

The Eye

Some Defects in Vision Farsighted – person’s eyes form images behind the retina, the eyeball is too short Nearsighted – person’s eyes form images in front of the retina, the eyeball is too long Astigmatism – the cornea is curved more in one direction than the other, the eye does not form sharp images

Some Defects in Vision

Some Defects in Lenses Aberrations – the distortions in an image By combining lenses in certain ways, aberrations can be minimized Spherical aberration results 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 Chromatic aberration is the result of the different speeds of light of various colors and hence the different refractions they undergo

Some Defects in Lenses

Assignment Read Chapter 30 (pg. 463-476) Do #15-34 (pg. 478-479)