Chapter 24 Geometrical Optics. Copyright 2010 Pearson Education, Inc.

Transcription

1 Chapter 24 Geometrical Optics

2 Lenses convex (converging) concave (diverging)

3 Mirrors

4 Ray Tracing for Mirrors We use three principal rays in finding the image produced by a curved mirror. The parallel ray (P ray) reflects through the focal point. The focal ray (F ray) reflects parallel to the axis. The center-of-curvature ray (C ray) reflects back along its incoming path.

5 Ray Tracing for Mirrors This diagram shows how the three rays are used to find the image formed by a convex mirror. The image is located where the projections of the three rays cross. The size of the image can also be determined. In convex mirrors, the image is always virtual, right side up, and minified.

6 Ray Tracing for Mirrors The process is similar for a concave mirror, although there are different results depending on where the object is placed.

7

8

9

10 Ray Tracing for Lenses Lenses are used to focus light and form images. There are a variety of types; we will consider only the symmetric ones, the double concave and the double convex.

11

12 Ray Tracing for Lenses The three principal rays for lenses are similar to those for mirrors: The P ray or parallel ray approaches the lens parallel to its axis. The F ray is drawn toward (concave) or through (convex) the focal point. The midpoint ray (M ray) goes through the middle of the lens. Assuming the lens is thin enough, it will not be deflected. This is called the thin-lens approximation.

13 Ray Tracing for Lenses As with mirrors, we use these principal rays to locate the image. Here is a concave lens: In concave lenses, the image is always virtual, right side up, and minified.

14 Ray Tracing for Lenses The convex lens forms different image types depending on where the object is located with respect to the focal point:

15

16 eye or camera photocopier projector magnifying glass

17 Lens Animation

18 The Camera The simplest camera consists of a lens and film in a light-tight box: The camera lens cannot change shape; it moves closer to or farther away from the film in order to focus.

19 The Human Eye Light passes through the cornea of the human eye and is focused by the lens on the retina. The ciliary muscles change the shape of the lens, so it can focus at different distances. The vitreous and aqueous humors are transparent. Rods and cones on the retina convert the light into electrical impulses, which travel down the optic nerve to the brain.

20 The Human Eye The eye produces a real, inverted image on the retina. Why don t things look upside down to us? The brain adjusts the image to appear properly.

21 The Dress

22

23 The Human Eye The ciliary muscles adjust the shape of the lens to accommodate near and far vision.

24 Lenses in Combination and Corrective Optics In a two-lens system, the image produced by the first lens serves as the object for the second lens.

25 Lenses in Combination and Corrective Optics A nearsighted person has a far point that is a finite distance away; objects farther away will appear blurry. This is due to the lens focusing too strongly, so the image is formed in front of the retina.

26 Lenses in Combination and Corrective Optics To correct this, a diverging lens is used. Its focal length is such that a distant object forms an image at the far point:

27 Lenses in Combination and Corrective Optics A person who is farsighted can see distant objects clearly, but cannot focus on close objects the near point is too far away. The lens of the eye is not strong enough, and the image focus is behind the retina.

28 Lenses in Combination and Corrective Optics To correct farsightedness, a converging lens is used to augment the converging power of the eye. The final image is past the near point:

29 The Compound Microscope A compound microscope has, in its simplest form, two converging lenses. One, the eyepiece, is close to the eye, while the objective lens is close to the object.

30 The Compound Microscope

31 The Refracting Telescope Objective lens Eyepiece object at infinity F o F o F e F e first image final image The refracting telescope is similar to the microscope, but its eyepiece is small while its objective lens is large.

32 The Refracting Telescope It is desirable to have the objective of a telescope be as large as possible, so that it may collect as much light as possible. Each doubling of the diameter of the objective gives four times as much light. Very large lenses are difficult to handle; they are thick and heavy, must have two precision surfaces, and absorb more of the light the thicker they are.

33 The Reflecting Telescope Therefore, large telescopes are now made as reflectors the objective is a mirror rather than a lens. The mirror has only one surface, can be made very thin, and reflects almost all the light that hits it.

34 Homework Continue to work on projects, which are due May 26.