1. Chapter 20 Mirrors and Lenses

2. 20-1 The Optics of Mirrors
Plane Mirror – a mirror with a flat surface; a piece of glass that has a reflective coating on the front or back.
Virtual image – an image in which no light rays pass through the image; it appears “behind” the mirror, is upright, and appears as far behind the mirror as the object is in front of the mirror.

5. 20-1 The Optics of Mirrors
Concave Mirrors – curved inward like a spoon. The image formed by concave mirrors depends on how far the object is located from the mirror.
Optical axis – straight line through the center of the mirror.
Focal point – the point on the optical axis through which all reflected light rays pass.
Focal length – distance from the center of the mirror to the focal point.

6. 20-1 The Optics of Mirrors Concave Mirror Images
Real image – an image formed where the light rays meet and it can be projected onto a screen; forms when the object is located farther than the focal point from a concave mirror; it is also enlarged and inverted (upside down).
An object placed at the focal point reflects no image because the light rays do not converge (meet); ex> car headlights, flashlights, and spotlights use these mirrors.

7. 20-1 The Optics of Mirrors
Concave Mirror Images
Virtual image – appears behind the mirror, is upright and enlarged; formed when the object is between the focal point and the mirror. Make-up mirrors use this type of mirror.

8. 20-1 The Optics of Mirrors
Convex Mirrors – curve outward; the reflected rays never meet so the image is always virtual, upright, and smaller than the actual object.
Used in stores and factories to see large areas, and used for rear-view or side-view mirrors on autos to see a wide view of traffic. However, objects are always closer than they appear.

9. 20-2 The Optics of Lenses
Convex Lenses – thicker in the middle than at the edges.
Light rays are refracted toward the center of the lens and converge at the focal point; they are capable of forming real images that can be projected on a screen.
The amount of refraction depends on the change in the speed of light as it passes through the material and the shape of the object.
Thick lenses with very curved surfaces bend light more than ones with less curved surfaces.
The focal length of thick lenses is shorter than those of thin lenses.

10. 20-2 The Optics of Lenses
Cameras and the human eye lens view objects that are more than two focal lengths away so the real image is smaller and inverted. (The brain converts the image to upright.)
If the object is between one and two focal lengths from the lens, the real image is inverted and larger than the object. This is the method used by movie projectors and overhead projectors.

11. 20-2 The Optics of Lenses
Concave Lenses – thinner in the middle than at the edges.
Light rays diverge and never form a real image. The image is virtual, upright, and smaller than the object.
Concave lenses are usually used in combination with other lenses.
They are used with convex lenses in telescopes and cameras to spread out incoming light and extend the focal length – to see far-away objects.
They are also used to correct nearsighted vision.

12. 20-2 The Optics of Lenses
Lenses & Vision
Light enters your eye through the cornea (transparent covering), then passes through the pupil (opening).
The iris (colored part) adjusts the pupil size to control how much light reaches the lens.
The light converges to form an inverted image on the retina (back part of the eye).

13. 20-3 Lenses & Vision
The lens in the eye is soft, and flexible muscles in the eye can change its shape.
When you look at a distant object, you need a longer focal length, so your eye muscles adjust the lens to a less convex shape.
When you focus on a near object, the eye muscles increase the curvature of the lens to shorten the focal length.

14. 20-3 Lenses & Vision
Nearsighted – has difficulty seeing distant objects because the eyeball is too long or the corneas are too flat to allow the rays to converge on the retina. Concave lenses correct this problem by diverging the light rays before they enter the eye.
(see Fig on p. 520)
Farsighted - can’t focus clearly on nearby objects because the eyeball is too short or corneas are too flat to allow the rays to converge on the retina. The image is focused behind the retina. Convex lenses converge the rays to correct the image at the retina.

15. Methods to correct vision currently include: Eye glasses
20-3 Lenses & Vision
Astigmatism – causes blurry vision because the surface of the cornea is curved unevenly. These corrective lenses have an uneven curvature so as to “smooth out” the curves.
Methods to correct vision currently include:
Eye glasses
Contact lenses worn on the cornea.
Laser surgery to change the “pull” of the muscles that control the shape of the eyeball itself, making it less elongated or more elongated

16. 20- 4 Optical Instruments
Telescopes – devices designed to magnify objects far away.
early telescopes were made of mirrors and lenses.
In 1609, Galileo built and used his own telescope to discover the moons of Jupiter, the phases of Venus, and some details of the Milky Way.

17. 20- 4 Optical Instruments
Refracting telescopes use two convex lenses to gather and focus light from distant objects.
Refracting telescopes must have a very large, very heavy lens to see faraway stars & planets.
Reflecting telescopes use a concave mirror, a plane mirror and a convex lens to magnify distant objects.

18. 20- 4 Optical Instruments
Binoculars and terrestrial telescopes (used for bird-watching) use a third lens or reflecting prism to invert the upside down image so it appears upright.
Microscopes – use two convex lenses with short focal lengths to magnify very small, close objects.
The image is upside down and backwards.

19. 20 - 5 Applications of Light
Polarized light – transverse waves that vibrate in only one plane.
- Polarized filters – helps reduce glare; ex: sunglasses, camera lenses.
Lasers – are beams of light that do not spread out.
Total internal reflection – causes diamonds and other gems to sparkle due to the light reflecting inside the object.
Optical fibers – are transparent glass that is used in communications to pipe light from one place to another.