Chapter 18: Light and Optics

Objectives Summarize the science of optics. Describe how mirrors control reflection. Describe how mirrors produce images.

Optics is the science of light and vision Optics is the study of visible light and the ways in which visible light interacts with the eye to produce vision. Mirrors, lenses, eyeglasses, cameras, lasers are all optical tools.

Mirrors use Regular Reflection When light waves strike an object, they either pass through it or they bounce off its surface. The law of reflection states that the angle of reflection equals the angle of incidence.

Regular Reflection Surface of the object is smooth, like a mirror, light rays that come from the same direction will bounce off in the same new direction. The reflection of parallel light rays all in the same direction is called regular reflection.

Diffuse Reflection If the surface is not very smooth, light rays strike it from the same direction and bounce off in many new directions. The reflection of parallel light rays in many different directions is called diffuse reflection.

An Image When you look in the mirror, you see an image of yourself. An image is a picture of an object formed by waves of light. The image of yourself is reflected off of you, onto the mirror.

Flat Mirrors Looks Exactly like you But, if you raise your right hand, in the mirror it appears that your raising your left hand.

Concave & Convex Mirrors A convex mirror is curved outward, like the bottom of a spoon. In a convex mirror, parallel light rays move away from each other. A concave mirror is curved inward toward the center, like the inside of a spoon. Parallel light rays reflecting off a concave mirror move toward each other.

Concave/Convex

Focal Point The rays striking a concave mirror cross and then move apart again. The point at which the rays meet is called the focal point of the mirror. P. 597

Review Explain the term optics in your own words? Optics is the study of light and vision. How is diffuse reflection similar to regular reflection? Different? Diffuse reflection and regular reflection both obey the law of reflection sends parallel light rays in many different directions; regular reflection sends parallel rays in the same direction.

Review Describe the path that light rays take when they form an image of your smile when you look into a flat mirror? Light rays reflected from your smile strike the mirror, bounce off, and enter your eye.

18.2 Lenses form images by refracting light Objectives Identify how a material medium can refract light. Describe how lenses control refraction. Recognize how lenses produce images.

A medium can refract light Some light rays reflect off the surface of glass. Refraction occurs when a wave strikes a new medium—such as the window—at an angle other than 90˚ and keeps going forward in a slightly different direction.

Refraction of Light Page 600 Waves moving at an angle into a denser medium turns toward the normal. Waves moving at an angle into a thinner medium turn away from the normal

Refraction and Rainbows Rainbows are caused by Refraction and reflection of Light through spherical water Drops, which acts as prisms. Only one color reaches Your eye from each drop. Red appears at the top of a Rainbow because it is coming From higher drops, while violet Comes from lower drops.

Shape determines how lenses from images Just as a curved mirrors distort images, certain transparent mediums called lenses alter what you see through them. A lens is a clear optical tool that refracts light. Different lenses refract light in different ways and form images useful for a variety of purposes.

Convex & Concave Lenses A convex lens causes parallel light rays to meet at a focal point A concave lens causes parallel light rays to spread out. Page 601 & 602

Images Formed by Lenses Notice the distance between the penguin and the lens in the illustration on page 602. The distance is measured in terms of a focal length, which is the distance from the center of the lens to the lens’s focal point.

To summarize If you look at an object through a concave lens, you’ll see an image of the object that is right side up and smaller than the object normally appears. If you look at an object through a convex lens, the image could be formed upside down and larger if two focal lengths away. Or it will appear right side up and larger if one focal length away.

How does a camera lens form an image A camera lens uses refraction to focus light on the film, or in a digital camera. When a ray of light passes from a less dense to a more dense medium (such as from air to glass) it slows down. If it strikes the glass surface at an angle, it is also bent a little, and this is called refraction. When it passes back into air, it speeds up again, and is again refracted if the surface is at an angle.

Review What quality of a material affects how much it refracts light? The speed of light in the material. How does the curve in a lens cause it to refract light differently from a flat piece of glass? Because the angle at which light strikes a curved surface varies across the surface, the amount of refraction for different light rays varies also.

Review How does a camera lens form an image? A camera lens refracts light waves inward toward a focal point.

18.3 The Eye is a Natural Optical Tool Objectives Recognize how the eye depends on natural lenses. Explain how artificial lenses can be used to correct vision problems.

Eyes gathers and focuses light Eyes transmit light, refract light, and respond to different wavelengths of light. Eyes contain natural lenses that focus images of objects and then sends signals to the brains. The brain interprets these signals as shape, brightness, and color.

How Light Travels through the Human Eye Light enters the eye through the cornea, a transparent membrane that covers the eye. The cornea acts as a convex lens and does most of the refracting in the eye.

How Light Travels through the Human Eye Light continues through the pupil, a circular opening that controls how much light enters the eye. The pupil is surrounded by the iris, which opens and closes to change the size of the pupil.

How Light Travels through the Human Eye Next, the light passes through the lens. It refracts light to make fine adjustments for near or far objects. The lens is connected to tiny muscles that contract and relax to control the amount of refraction that occurs and to move the focal point.

How Light Travels through the Human Eye 4. The light then passes through he clear center of the eye and strikes the retina. The retina contains specialized cells that respond to light. Some of these cells sends signals through the optic nerve to the brain.

How the Eye Forms Images For a complete image to be formed in the eye and communicated to the brain. The Retina plays a very important role.

Retina: Rod Cells Rod cells distinguish between white and black and shades of gray. Help in night vision.

Retina: Cone Cells Cone cells- respond to different wavelength of light so they detect color. There are three types: Red, Blue, and Green. They respond to other colors by using a combination of the three.

Corrective Lenses can Improve Vision How many of you wear contacts or glasses? About 36 million wear contact lenses When the image formed by the lens of the eye does not fall exactly on the retina. A blurry image is formed. Artifical lenses are used to correct this problem.

Corrective Lenses Nearsighted cannot see objects clearly unless they are near. It occurs when the lens of the eye focuses the image in front of the retina. Can be corrected with glasses with concave lenses. They spread out the rays of light before they enter the eye.

Farsighted Objects are clearer to a farsighted person when the objects are farther away. It occurs when the lens of the eye focuses an object’s image behind the retina. This can be caused by aging. Can be corrected by convex lenses.

Surgery Doctors can use surgical procedures to shape the cornea. To correct nearsightedness, surgeons remove tissue from the center of the cornea. To correct farsightedness, surgeons remove tissue from around the edge of the cornea. http://www.youtube.com/watch?v=dZZguJNitnU

Contacts Contact lenses also correct vision by changing the way the cornea refracts light. They fit directly over the cornea and float on a thin layer of tears. This is only a temporary fix.

Review Where are images focused in an eye with perfect vision? On the Retina What causes people with nearsightedness to see blurry images of objects at a distance. The eye focuses the image in front retina

Review What kind of lens is used for correcting farsightedness? Why? Convex lens; it bends light inward, which moves the focal point of the image forward toward the retina.

18.4 Optical Technology makes use of Light Waves Objectives Describe how mirrors and lenses can be combined to make complex optical tools. Explain how optical tools are used to extend natural vision. Recognize how laser light is made and used in optical technology

Mirrors and Lenses can be more Powerful Optical Tools For example: Mirrors and lenses can be combined with each other, as they are in an overhead projector.

Microscopes Used to see small objects. Uses a combination of convex lenses- the eyepiece and the objective lenses. Most microscopes use a light or mirror to shine more light on the object. Page 613

Telescopes The objective lens gathers and focuses light from a distant object to form an image of the object. The eyepiece enlarges the image.

Telescopes Used to see objects to far away to see well with the naked eye. 2 Types: Refracting telescope is made by combining lenses. Reflection is made by combining lenses and mirrors.

Refraction Reflection

Lights, Camera, Action…………. Most film cameras focus images Like the eyes Read Page 614

Lasers A laser is a device that produces an intense, concentrated beam of light that is brighter than sunlight. A laser is made in a special tube called an optical cavity in which an energy source stimulates material to give off light waves.

Uses of Lasers Surveyors use them to measure distances and angles. Used to read bar codes, to scan images and pages of text and to create holograms

Fiber Optics Is Technology based on the use of light to send signals through transparent wires called optical fibers Very important in communications for telephones, TV wires, and broadband internet connections. Medical uses to see inside a patients body

Future Uses of Lasers Scientist plan to use them in nanotechnology—perform extremely fine operations. New ways of transferring energy. Supply energy for the spacecraft.