Thin Lenses

 When light passes through a lens, it refracts twice ◦ Once upon entering the lens and once upon leaving  Exiting ray is parallel to the entering light ray  Lens – a transparent object that refracts light rays causing them to converge or diverge to create an image  Two types of lenses ◦ Converging – thicker in the middle than on the edges  Light rays converge to a single focal point ◦ Diverging – thicker on the edges than in the middle  Light rays scatter

 Both converging and diverging lenses have two focal points, one on each side of the lens ◦ Both focal points have the same focal length ◦ Focal length is measured by the distance from the focal point to the center of the lens  To draw ray diagrams, draw three rays ◦ Parallel ray, central ray, and focal ray  Different for converging and diverging lenses

 Converging lenses can produce real or virtual lenses  Objects at infinity produce a point image at the focal point  Objects beyond 2F produce a small real image between F and 2F beyond the lens  Objects at 2F produce a real image of equal size at the 2F beyond the lens  Objects between F and 2F produce a larger real image beyond 2F on the other side of the lens  Objects at F do not produce an image  Objects inside F produce a larger virtual image on the same side of the lens

 Angular size – the apparent size of an object based on the size of the angle needed to view the entire object ◦ Increases as you get closer to the object ◦ As magnification increases, visible detail increases  Diverging lenses produce only virtual images on the same side of the lens ◦ Smaller than object ◦ Placement of the object does not matter

 The equations to find image distance and height for thin lenses are the same as those for mirrors  1/p + 1/q = 1/f  M = h’/h = -q/p  Magnitude of magnification is greater than one when the image is larger than object  Magnitude of magnification is less than one when the image is smaller than object  Positive magnification is virtual image and upright  Negative magnification is real image and inverted

 Cornea – front of eyeball – acts as a converging lens directing light to the retina  Far-sighted – hyperopia – cornea focuses light behind the retina ◦ Corrected with a converging lens ◦ Cannot see near objects clearly  Near-sighted – myopia – cornea focuses light in front of the retina ◦ Corrected with a diverging lens ◦ Cannot see far objects clearly

 When using combinations of lenses, use one lens at a time ◦ The image formed by the first object is treated as the object for the second lens and so on ◦ To get overall magnification, multiply the magnifications of all lenses  Compound microscopes and refracting telescopes are combinations of converging lenses ◦ Cannot use light microscopes to view atoms because the wavelengths of atoms are smaller than the wavelengths of visible light