1. Chapter 20
Mirrors
and
Lenses

2. 20:1 The Optics of Mirrors Learning Target:
To understand how mirrors work.
Success Criteria:
I can…
explain how an image is formed in two types of mirrors.
Identify examples and uses of plane, concave, and convex mirrors.

3. 20:1 The Optics of Mirrors Plane Mirrors:
Any smooth object that reflects light to form an image is a mirror.
A mirror that has a flat surface is known as a plane mirror.
When looking in a plane mirror, your image should be upright and the same size as you.

4. How does a plane mirror work?
Light reflects off of you, then off of the mirror and into your eyes.
In a plane mirror, the image formed is erect and appears as far behind the mirror as the object is in front of it.
This image that you see is a virtual image because no light rays pass through it.

5. What do you see when you look into a spoon?
Concave Mirrors:
What do you see when you look into a spoon?
Concave mirrors are curved inward, like the inside of a spoon.
The way an image forms in a concave mirror depends on the position of the object in front of the mirror.

6. A concave mirror has an optical axis, which is a line drawn through the center of the mirror.
Because of the mirror’s curvature, light rays parallel to the optical axis are all reflected to pass through a single point on the axis, called the focal point.
The distance from the center of the mirror to the focal point is called the focal length.

7. Making Ray Diagrams:
-Drawing the lines:
1. Draw the first ray from the top of the
object, parallel to the principal axis.
2. The second ray starts at the top of the
object and goes directly through the focal
point (F).
3. Draw a third ray from the top of the object, directly
through the center of curvature (C)
4. Draw the object where the first two rays
meet.
-Describe how to get LOST in the image:
What is the location? (closer to/farther from the mirror)
What is the orientation? (upright or inverted)
What is the size? (bigger or smaller)
What is the type? (virtual or real)

8. Let’s practice making our own ray diagrams.

9. If an object is just farther from a concave mirror than the focal point:
The image will be real, enlarged, and upside down.
A real image is one where the rays of light really meet at the image, so you can hold a screen there and see it.

10. When an object is placed at the focal point of a concave mirror, no image can be seen.
All reflected rays are parallel to the axis and do not converge to make an image.
Lights placed at the focal point of a concave mirror produce a beam.
How might we take advantage of this?

11. When an object is closer to the mirror than the focal point, a virtual image that is upright and enlarged can be seen.

12. Convex mirrors curve outward.
The image is always virtual, upright, and smaller than the actual object because the reflected rays never meet.
Because of the shape of the mirror, you can see a large field of view.
Unfortunately, the objects also appear to be farther away than they actually are…
Have we seen this warning somewhere?

13. More ray diagrams… MUAHAHAHA!!!

14. Learning Checkpoint:
Describe the image formed by a concave mirror when the object is between one and two focal lengths.
What are convex mirrors used for? Explain.
Contrast the differences between the surfaces of plane, concave, and convex mirrors.
4. Apply: What kind of mirror would you use to
focus light entering a telescope? Why?
5. Some measuring instruments used in chemistry have a
plane mirror behind the needle. How does this help a
chemist make correct measurements?

15. 20:2 The Optics of Lenses
Learning Target: -To understand lenses. Success Criteria: I can… -describe the types of images formed with convex and concave lenses. -cite examples of how these lenses are used. -explain how lenses are used to correct vision.

16. 20:2 The Optics of Lenses Convex Lenses:
A convex lens is thicker in the middle than at the edges.
Light rays approaching the lens parallel to the optical axis will be refracted toward the center of the lens.
They converge at the focal point on the other side to create a real image.
The thickness and curvature of the lens determines the focal point of the image.

17. Convex lenses are capable of producing many kinds of images, depending on the focal length of the lens and the position of the object.
What happened with objects #7 & 8?

19. Concave lenses are thinner in the middle and thicker at the edges.
The rays diverge and never form a real image.
The image is always virtual, upright, and smaller than the actual object.
Concave lenses are usually used with other lenses.
They can spread light and extend focal length to make a clear image of a faraway object.
They are also used to correct nearsightedness.

20. Lenses and Vision: How does my eye work?
Muscles change the shape of a convex lens to adjust the focal length of an image.
The image should be focused on the retina.
In a nearsighted eye, the eyeball is too long or the cornea bulges out, so the image is focused in front of the retina.
Fix with concave lenses.
In a farsighted eye, the eyeball is too short or the cornea too flat, so the focal point is behind the retina.
Fix with convex lenses.

21. Learning Checkpoint: Distinguish between the characteristics of
convex and concave lenses.
When using a slide projector, why must the slides be inserted in the projector upside down?
What type of lens would you use to examine a tiny spider on your desk?
If you have difficulty reading the board from the back row, what is most likely your vision problem? How could it be corrected?
Why must the lens in your eye be convex, rather than concave? Explain.

22. 20:3 Optical Instruments Learning Target:
To understand how optical instruments function.
Success Criteria:
I can…
compare refracting and reflecting telescopes.
explain how a camera creates an image.

23. 20:3 Optical Instruments Telescopes:
An optical instrument is one that assists the human eye in making observations.
A telescope is designed to magnify an object that is very far away.
They use mirrors and/or lenses to view far away objects.

24. Refracting telescopes typically use two convex lenses to gather and focus light from distant objects.
Issues:
To get a bright image, the objective lens must be large.
This is heavy, causing the telescope to swing down or sag.
These lenses are quite costly.
See any problems with the image produced?

25. Reflecting telescopes use a concave mirror, a plane mirror, and a convex lens to magnify objects.

26. Microscopes:
A microscope uses two convex lenses with short focal lengths to magnify very small, close objects.
A virtual, enlarged image is created because the object is between 1 and 2 focal lengths from the lens.

27. Cameras:
A camera gathers light through a lens and projects an image on light-sensitive film.
The image is real, inverted, and smaller than the actual object.
The size of the image depends on the focal length of the lens and how close the lens is to the film.

28. Depending on the lens of the camera, you can get different results for an image.
Wide-angle lenses have short focal lengths that produce small images, but include much of an object’s surroundings.
Telephoto lenses have longer focal lengths and are located farther from the film, making the image appear larger and closer.

29. Learning Checkpoint:
Compare and contrast reflecting and refracting telescopes.
If you wanted to photograph a single rose on a rosebush, what kind of lens would you use? Explain why you chose this lens.
Microscopes and refracting telescopes both use two sets of convex lenses. How can they be used for different purposes?
Which optical instrument- a telescope, a microscope, or a camera- forms images in a way most like your eye? Explain.
Why are some maps of the moon upside-down?

30. 20:4 The Hubble Space Telescope
Learning Target:
To understand lens and mirror use in the Hubble Space Telescope.
Success Criteria:
I can…
describe the development and goals of the Hubble Space Telescope.
evaluate the need for a space telescope.

31. 20:4 The Hubble Space Telescope
Is it worth it?:
There is a problem with trying to observe space from Earth’s surface…
The atmosphere blocks out some light that we would like to collect for observation.
The atmosphere bends/distorts the light we do get, which influences our images.
The Hubble Space Telescope removes the flaws of images produced elsewhere.

32. The Hubble Space Telescope uses mirrors to collect and focus light.
It’s expensive!
The initial building and launch costs were around $1.5 billion. (There have been a lot of repairs since its launch in 1990).
The Hubble will be retired shortly, to be replaced by an even larger telescope, the James Webb Space Telescope.

33. What has the Hubble done for us?
Learned about dark matter, other
galaxies, and universe formation.

34. Learning Checkpoint:
Explain whether the Hubble Space Telescope is designed more like a refracting or a reflecting telescope.
What knowledge do we hope to gain from the Hubble Space Telescope?
Why can an orbiting telescope form clearer images than one on Earth?

35. 20:5 Applications of Light
Learning Target:
To understand various ways that we can apply, utilize, and manipulate light for our benefit.
Success Criteria:
I can…
describe polarized light and the uses of polarizing filters.
explain how a laser produces coherent light and how it differs from incoherent light.
apply the concept of total internal reflection to the uses of optical fibers.

36. 20:5 Applications of Light
Polarized Light:
When discussing light, what does “polarized” mean?
Light is typically emitted from a source so that its waves vibrate in many directions.
Polarizing filters can be used to polarize light, which eliminates some of the directions that light can vibrate in.
In polarized light, the transverse waves vibrate in only one plane.

37. How do we take advantage of polarized light? Sunglasses!
Light reflected from horizontal surfaces, such as a lake, is partially polarized horizontally and is called a glare.
Polarizing sunglasses are made with vertically polarizing filters to block out most of the glare.
Camera lenses
Same idea as sunglasses.
3-D movies
In one kind of 3-D glasses, each eye piece contains a polarizing filter (one vertical and one horizontal).
The movie is shown through two projectors, one with a horizontally polarizing filter and the other with a vertically polarizing filter.

38. A laser is a device that produces a beam of coherent light.
Lasers
A laser is a device that produces a beam of coherent light.
Coherent light is a beam of light in which all rays travel parallel to one another, so that the beam spreads out very little.
The electromagnetic energy travels with only one wavelength and all of the crests and troughs are aligned.
Incoherent light spreads out because it contains more than one wavelength of energy and the waves don’t travel in the same direction.

39. Lasers are inefficient.
No more than 1% of energy input is converted into light…
Lasers are useful. What do we use them for?
Pointing -Scanning -Surgery
Cutting and Welding -Building
Fiber-optic communications -Games

40. Optical Fibers:
Total internal reflection occurs when light striking a surface between two materials reflects totally back into the first material.
Have you ever pulled your feet from the water and noticed that they sometimes disappear as you raise them to the surface?

41. As you raise your feet, light reflecting from your feet strikes the surface between the water and the air and reflects back into the water.
The reflecting light never reaches your eyes…
Total internal reflection depends on the speed of light in the two materials and the angle at which the light strikes the surface.
The speed of light must be less in the first material.
The angle at which the light strikes the surface must be large.

42. Total internal reflection makes light transmission in optical fibers possible.
Optical fibers are transparent glass fibers that can pipe light from one place to another.
Light entering one end of the fiber is continuously reflected from the sides of the fiber until it emerges from the other end.
Optical fibers are used in:
Communications
Body exploration
Entertainment

43. Learning Checkpoint: What is polarized light?
Distinguish between coherent and incoherent light.
Explain how an optical fiber transmits energy.
Which of the following materials could possibly be substituted for the glass in an optical fiber: clear plastic, wood, or water? Explain your answer.
Geologists and surveyors often use lasers for aligning equipment, measuring, and mapping. Explain why.