King Saud University College of Science Physics & Astronomy Dept.

Transcription

1 King Saud University College of Science Physics & Astronomy Dept. PHYS 111 (GENERAL PHYSICS 2) CHAPTER 36: Image Formation LECTURE NO. 9 Presented by Nouf Saad Alkathran

2 36.1 Images Formed by Flat Mirrors Image formation by mirrors can be understood through the behavior of light rays as described by the wave under reflection analysis model. We begin by considering the simplest possible mirror, the flat mirror.

3 The object distance, p : is the distance between the object P and the flat mirror. Diverging light rays leave the source and are reflected from the mirror. Upon reflection, the rays continue to diverge. The dashed lines in Figure are extensions of the diverging rays back to a point of intersection at P`. The diverging rays appear to the viewer behind the mirror, (virtual image). The image distance, q: is the distance between the image p` and the flat mirror. The image of an object seen in a flat mirror is always virtual.

4 Images are classified as real or virtual. A real image is formed when all light rays pass through and diverge from the image point. A virtual image is formed when most if not all of the light rays do not pass through the image point but only appear to diverge from that point. Real images can be displayed on a screen (as at a movie theater), but virtual images cannot be displayed on a screen.

5 lateral magnification M Let us define lateral magnification M of an image as follows: When M = 1 When M > 1 When M < 1 image height = object height image height > object height image height < object height

6 Properties of image created by flat mirrors The object distance = The image distance p = q The object height = The image height h = h` The image of an object seen in a flat mirror is always virtual. A flat mirror produces an image that has an apparent left right reversal.

7 36.2 Images Formed by Spherical Mirrors CONCAVE SPHERICAL MIRRORS CONVEX SPHERICAL MIRRORS If the inside surface of the spherical mirror is polished, it is a concave mirror. If the outside surface is polished, is it a convex mirror. R is the radius of curvature of the mirror. The principal axis of the mirror is a straight line drawn through the center of curvature C and the midpoint of the mirror.

8 36.2 Images Formed by Spherical Mirrors

9 The mirror equation The image point in this special case is called the focal point F, and the image distance is called the focal length f, where The mirror equation can be expressed in terms of the focal length:

10 Ray Diagrams for Mirrors Ray 1 is drawn from the topof the object parallel to the principal axis and is reflected through the focal point F. Ray 2 is drawn from the top of the object through the focal point (or as if coming from the focal point if p < f ) and is reflected parallel to the principal axis. Ray 3 is drawn from the top of the object through the center of curvature C (or as if coming from the center C if p < 2f ) and is reflected back on itself.

11 CONCAVE MIRRORS Concave mirror could create a real and virtual images. If the object is placed between F and C, the image is real, inverted and larger magnified.

12 If the object is placed at a distance greater than C from the mirror, the image is real, inverted and smaller reduced in size.

13 When an object is placed between the focal point F and concave mirror, The image is virtual, upright, and larger magnified.

14 CONVEX MIRRORS For convex mirrors the image of an object is always virtual, upright, and smaller reduced in size.

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18 An automobile rearview mirror shows an image of a truck located 10.0 m from the mirror. The focal length of the mirror is m.

19 36.3 Images Formed by Refraction

20 36.4 Images Formed by Thin Lenses The Type of Lenses 1. Converging lenses. 2. Diverging lenses. Lenses refract light in such a way that an image of the light source is formed.

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22 Rules for Converging Lenses 1. Any incident ray traveling parallel to the principal axis of a converging lens will refract through the lens and travel through the focal point on the opposite side of the lens. 2. Any incident ray traveling through the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. 3. An incident ray which passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

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24 Rules for Diverging Lenses 1. Any incident ray traveling parallel to the principal axis of a diverging lens will refract through the lens and travel in line with the focal point (i.e., in a direction such that its extension will pass through the focal point). 2. Any incident ray traveling towards the focal point on the way to the lens will refract through the lens and travel parallel to the principal axis. 3. An incident ray which passes through the center of the lens will in effect continue in the same direction that it had when it entered the lens.

25 The Shape of Lenses Converging and diverging lens come in a variety of shapes depending on their application.

26 Ray Diagrams Here are some useful rays in determining the nature of the images formed by converging and diverging lens. Since lenses pass light through them (unlike mirrors) it is useful to draw a focal point on each side of the lens for ray tracing.

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28 Image Formation by A Converging Lens If p> 2F: When the object is placed further than twice the focal length from the lens, the image is real, inverted and smaller. This is the configuration for a camera.

29 Image Formation by A Converging Lens If 2F> p >F: When the object is placed between F and 2F, the image is real, inverted and larger. This is the configuration for a projector. Since you normally want the real image on the screen to be upright, the object (film or slide) is placed upside down in the projector.

30 Image Formation by A Converging Lens If p < F: When the object is placed between F and the lens, the image is virtual, upright and larger. This is the configuration for a magnifying glass. The magnifying glass must clearly be positioned so that the object distance is less than its focal length.

31 Image Formation by A Diverging Lens For all p: A diverging lens always forms an upright, virtual, and smaller diminished image.

32 36.7 The Eye In many respects, the human eye is similar to the camera. Light enters through the transparent covering, the cornea. The amount of light that enters is regulated by the iris, the colored part of the eye that surrounds the pupil. The pupil is the opening through Which light passes. Light passes through the pupil and lens and is focused on a layer of tissue at the back of the eye the retina. Different parts of the retina receive light from different directions.

33 Conditions of the Eye When the eye suffers a mismatch between the focusing range of the lens cornea system and the length of the eye. The near point of a normal eye is approximately 25 cm. 1. Farsightedness (or hyperopia) 2. Nearsightedness (or myopia)

34 Farsightedness (or hyperopia): A farsighted person can usually see faraway objects clearly but not nearby objects. The eyeball is too short and images form behind the retina. The remedy is to increase the converging effect of the eye by wearing eyeglasses or contact lenses with converging lenses. Converging lenses converge the rays sufficiently to focus them on the retina instead of behind the retina.

35 Nearsightedness (or myopia) A nearsighted person can see nearby objects clearly, but does not see distant objects clearly. Distant objects focus too near the lens, in front of the retina. The eyeball is too long. A remedy is to wear lenses that diverge the rays from distant objects so that they focus on the retina instead of in front of it.

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41 Summary Images Formed by Flat Mirrors Images Formed by Convex Mirrors Images Formed by concave Mirrors 36.4 Images Formed by Thin Lenses Converging lenses Diverging lenses Thy Eye Conditions of the Eye

42 The End