Chapter 14: Mirrors and Lenses

Section 1: Mirrors

How do you use light to see? * Your eyes see by detecting light, so anytime you see something, it is because light has come from that object to your eyes. * Light can reflect more than once. * When no light is available to reflect off of objects and into your eye, your eyes cannot see anything. * Light sources send out light waves that travel in all directions. * Even though light rays can change direction when they are reflected or refracted, your brain interprets images as if light rays travel in a single direction.
Seeing Reflections with Plane Mirrors * Plane Mirror: a flat, smooth mirror * Seeing an image of yourself in a mirror involves two sets of reflections. * Every point that is struck by the light rays reflects these rays so they travel outward in all directions. * Your brain thinks that the light rays that reflect off of the mirror come from a point behind the mirror. * Virtual Image: An image which your brain perceives even though no light rays pass through it * The virtual image formed by a plane mirror is always upright and appears to be as far behind the mirror as the object is in front of it.
Concave Mirror: the surface of a mirror is curved inward * Concave mirrors, like plane mirrors, reflect light waves to form images. * Optical Axis: an imaginary straight line drawn perpendicular to the surface of the mirror at its center. * Focal Point: a point on the optical axis * Every light ray traveling parallel to the optical axis as it approaches the mirror is reflected through a point on the optical axis called the focal point. * Using the focal point and the optical axis, you can diagram how some of the light rays that travel to a concave mirror are reflected * A concave mirror has an optical axis and a focal point. When light rays travel toward the mirror parallel to the optical axis, they reflect through the focal point. * Focal Length: The distance from the center of the mirror to the focal point * The image that is formed by a concave mirror changes depending on where the object is located relative to the focal point of the mirror. * The point where the two rays meet is on the reflected image. * The image that is formed by the concave mirror is not virtual. Rays of light pass through the location of the image. * Real Image: formed when light rays converge to form the image. * When an object is farther from a concave mirror than twice the focal length, the image that is formed is real, smaller, and upside down, or inverted. * A flashlight uses a concave mirror to create a beam of light. * A light placed at the focal point is reflected in a beam. * The image formed by a concave mirror changes again when you place an object between it and its focal point. * The location of the reflected image again can be found by drawing two rays from each point.
Convex Mirror: A mirror that curves outward like the back of a spoon * Light rays that hit a convex mirror diverge, or spread apart, after they are reflected. * The reflected rays diverge and never meet, so the image formed by a convex mirror is a virtual image. * The image also is always upright and smaller than the actual object is. * A convex mirror is said to have a wide field of view. * Objects look farther away than they truly are in a convex mirror. * The different shapes of plane, concave, and convex mirrors cause them to reflect light in distinct ways.

Section 2: Lenses

What is a lens? * A lens is a transparent material with at least one curved surface that causes light rays to bend, or refract, as they pass through. * The image that a lens forms depends on the shape of the lens. * Like curved mirrors, a lens can be convex or concave.
Convex Lenses: thicker in the middle than at the edges. * When light rays approach a convex lens traveling parallel to its optical axis, the rays are refracted toward the center of the lens * The focal length of the lens depends on the shape of the lens. * A light ray that passes straight through the center of the lens is not refracted. * The type of image a convex lens forms depends on where the object is relative to the focal point of the lens. * When you use a magnifying glass, you move a convex lens so that it is less than one focal length from an object. This causes the image of the object to be magnified.
Concave Lenses: thinner in the middle and thicker at the edges. * The rays spread out and never meet at a focal point, so they never form a real image. * Concave lenses are used in some types of eyeglasses and some telescopes.
Lenses and Eyesight * If you don’t need eyeglasses, the structure of your eye gives you the ability to focus on these words and other objects around you. * Cornea: causes light rays to bend so that they converge. * Light enters your eye through a transparent covering on your eyeball called the cornea. * The light then passes through an opening called the pupil. Behind the pupil is a flexible convex lens. The lens helps focus light rays so that a sharp image is formed on your retina. * Retina: the inner lining of your eye. * For you to see an object clearly, its image must be focused sharply on your retina. * As an object moves farther from a convex lens, the position of the image moves closer to the lens. * For an image to be formed on the retina, the focal length of the lens needs to be able to change as the distance of the object changes. * As an object gets farther from your eye, the focal length of the lens has to increase. * The lens in your eye changes shape so you can focus on objects at different distances.
Visual Problems * People that have good vision can see objects clearly that are about 25 cm or farther away from their eyes. * A person who is farsighted can see distant objects clearly, but can’t bring nearby objects into focus. * Light rays from nearby objects do not converge enough after passing through the cornea and the lens to form a sharp image on the retina. * As many people age, their eyes develop a condition that makes them unable to focus on close objects. * Another vision problem, called astigmatism occurs when the surface of the cornea is curved unevenly. * When people have astigmatism, their corneas are more oval than round in shape. * Astigmatism causes blurry vision at all distances. Corrective lenses also have an uneven curvature, canceling out the effect of an uneven cornea. * A person who is nearsighted can see objects clearly only when they are nearby. * Objects that are far away appear blurred. * In a nearsighted eye, the cornea and the lens bring light rays from distant objects to a focus in front of the retina

Section 3: Optical Instruments

Telescopes * A telescope uses a lens or a concave mirror that is much larger than your eye to gather more of the light from distant objects. * One common type of telescope is the refracting telescope. * Refracting Telescope: uses two convex lenses to gather and focus light from distant objects. * Incoming light from distant objects passes through the first lens, called the objective lens. * The second convex lens, called the eyepiece lens, acts like a magnifying glass and magnifies this real image. * Several problems are associated with refracting telescopes. * A large lens is heavy and can be supported in the telescope tube only around its edge. * The lens can sag or flex due to its own weight, distorting the image it forms. * Also, these heavy glass lenses are costly and difficult to make. * Reflecting Telescope: uses a concave mirror, a plane mirror, and a convex lens to collect and focus light from distant objects. * Just as in a refracting telescope, a convex lens in the eyepiece then magnifies the image. * Reflecting telescopes use two mirrors to create a real image, which then is magnified by a convex lens. * Earth’s atmosphere blurs the view of objects in space. * The view from telescopes on Earth is different from the view from telescopes in space. * The Hubble telescope is a type of reflecting telescope that uses two mirrors to collect and focus light to form an image.
Microscope: uses two convex lenses with relatively short focal lengths to magnify small, close objects. * A microscope, like a telescope, has an objective lens and an eyepiece lens. * A microscope uses two convex lenses to magnify small objects. * The final image can be hundreds of times larger than the actual object, depending on the focal lengths of the two lenses.
Cameras * A camera works by gathering and bending light with a lens. This lens then projects an image onto light-sensitive film to record a scene. * When you take a picture with a camera, a shutter opens to allow light to enter the camera for a specific length of time. * Some lenses have short focal lengths that produce a relatively small image of the object but have a wide field of view. These lenses are called wide-angle lenses, and they must be placed close to the film to form a sharp image with their short focal length. * Telephoto lenses have longer focal lengths. * A telephoto lens creates a larger image of an object than a wide-angle lens does.