STD:- VIII PHYSICS SPHERICAL MIRRORS

Transcription

1 STD:- VIII PHYSICS SPHERICAL MIRRORS A spherical mirror is a mirror with a curved reflective surface, which may be either convex (bulging outward) or concave (bulging inward). There are two types of spherical mirrors. One is a CONCAVE MIRROR and the other is the CONVEX MIRROR. CONCAVE MIRROR A concave mirror is made by silvering the outer (or the bulging surface ) of the piece of a hollow sphere such that the reflection takes place from the inner surface. CONVEX MIRROR A convex mirror is made by silvering the inner surface of the piece of a hollow sphere such that the reflection takes place from the outer (or bulging) surface. 1

2 TERMS RELATED TO A SPHERICAL MIRROR 1. Centre of curvature (C): 2. Radius of curvature (R): The centre of the sphere of which the mirror forms a small part is called the centre of curvature. Any line that joins a point on the mirror with the centre of curvature is called the radius of curvature.the radial line is always perpendicular to the spherical mirror. 3.Pole (P): It is the centre of the reflecting surface of the spherical mirror. 4.Aperture (AB): It is the surface of the mirror from which reflection occurs. 5.Principal Axis: It is the line passing through the pole and the centre of curvature. 6.Focus or focal point (F): It is the point on the principal axis where the incident rays converge after reflection (as in the concave mirror ) or appear to diverge from, when produced backwards (as in the convex mirror). 7.Focal length (f): It is the distance between the pole and the focus of the spherical mirror. Relationship between focal length (f) and radius of curvature (R) : 2

3 Principal focus of a concave mirror Principal focus of a convex mirror A parallel beam of light after reflection from a concave surface converges at a point in front of the mirror.this point (F) is called the focus or the focal point of the mirror. A parallel beam of light after reflection from a convex surface diverges and rays do not meet.however, on producing backward, the rays appear to meet at a point behind the mirror.this point is called the focus or the focal point of the mirror. Thus, the principal focus of a concave mirror is a point in its principal axis to which all the light rays which are parallel to the principal axis converge after reflection from the concave mirror. Thus, the principal focus of a convex mirror is a point in its principal axis from which all the light rays initially parallel to the principal axis, appears to diverge after being reflected from the convex mirror. The focal point of a concave mirror is real as the incident rays actually meet after reflection. The focal point of a convex mirror is virtual. 3

4 RULES FOR REFLECTION There are certain rules for reflection by spherical mirrors. These are given below. RULE #1: - Ray of light parallel to the principal axis pass through the focus of the mirror. In case of convex mirror they appear to be diverging from the focus. RULE #2: - Ray of light passing through the focus become parallel to principal axis. 4

5 RULE #3: - Rays of light passing through the centre of curvature trace their path back after reflection. RULE #4: - Rays of light which are incident obliquely get reflected obliquely. RAY DIAGRAMS FOR FORMATION OF IMAGES IN A CONCAVE MIRROR 1.Position of object : At infinity 2.Position of object :Beyond C 5

6 Position of image : At F Position of image : Between F and C 3.Position of object : At C 4.Position of object : Between F and C Position of image : Beyond C Position of image : At C 5.Position of object : At F 6.Position of object : Between P and F 6

7 Position of image : At infinity Position of image : Behind the mirror Concave mirror: No. Object position Image position Size of the image Nature of the image 1. At infinity At F Highly diminished Real and inverted 2. Beyond C Between F and C Diminished Real and inverted 3. At C At C Same as that of the object Real and inverted 4. Between F and Beyond C Magnified Real and inverted C 5. At F At infinity Highly magnified Real and inverted 6. Between F and P Behind the mirror Magnified Virtual and upright RAY DIAGRAMS FOR FORMATION OF IMAGES IN A CONVEX MIRROR : 1.Position of object : At infinity 2.Position of object :At any position in front of the mirror Position of image : At F Position of image : Behind the mirror, between P and F 7

8 Convex mirror: No. Object position Image position Size of the image Nature of the image 1. At infinity At F Diminished to a point Virtual and upright 2. At any other position Between F and P Diminished Virtual and upright Uses of Mirrors: 1.Concave mirror: (i) Shaving mirror:used as a shaving mirror since it gives rise to an erect and enlarged image when the object is placed close enough,i.e., between the pole and the focus of the mirror. (ii)reflectors: Head lights of cars and search lights use concave mirrors to produce a strong parallel beam of light.in reflectors, the source of light is placed at the focus, which, after reflection forms a parallel beam. (iii)as a doctor s head mirror: If a parallel beam of light is incident on a concave mirror, it focuses the beam to a point.this fact enables the mirror to concentrate light beam on a small area to be examined like nose, throat,ear, teeth etc. (iv).in solar cooker: The parallel beam of the Sun s rays are converged and the energy that is focused at a point is sufficient to generate adequate heat for cooking. 2. Convex mirror : (i).as a rear view mirror : Convex mirror is used in vehicles to produce an erect and diminished image of the traffic approaching from behind. Although a plane mirror can also be used for the purpose, but a convex mirror provides a wider field of view than a plane mirror. (ii).in street lamps : A convex mirror is used in street lamp as a reflector so as to diverge the light over a larger area. DIFFERENCE BETWEEN A CONCAVE AND CONVEX MIRROR 8

9 CONCAVE MIRROR 1. A concave mirror is made by silvering the outer (or the bulging surface ) of the piece of a hollow sphere such that the reflection takes place from the inner surface. CONVEX MIRROR A convex mirror is made by silvering the inner surface of the piece of a hollow sphere such that the reflection takes place from the outer (or bulging) surface. 2. It is a converging mirror. 2. It is a diverging mirror. 3.The image formed is real except for the case when the object is between the focus and the pole. 4.The image can be magnified, of the same size or diminished depending upon the position of the object. 3. The image formed by it is always virtual. 4. the image formed is always diminished for all positions of the object in front of it. DIFFERENCE BETWEEN REAL AND VIRTUAL IMAGE REAL IMAGE 1.Real image is formed when two or more rays after reflection actually meet at a point. VIRTUAL IMAGE 1.Virtual image is formed when two or more rays after reflection do not actually meet but appear to meet at a point. 2. Real image can be obtained on a screen. 2. Virtual image cannot be obtained on a screen. 3.Real image is always inverted. 3.Virtual image is always erect. DISTINCTION BETWEEN A PLANE MIRROR, CONCAVE MIRROR AND CONVEX MIRROR: Case 1: If the image is upright, of same size and it does not change in size by moving the mirror, the mirror is plane. 9

10 Case 2: If the image is upright and diminished and it remains upright on moving the mirror away from the face, the mirror is convex. Case3: If the image is upright, magnified and becomes inverted on moving the mirror away from the face, the mirror is concave. 10