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.

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