SPHERICAL MIRRORS

1. SPHERICAL MIRRORS Free powerpoints at http://www.worldofteaching.com

2. Aim: To understand reflection in curved mirrors. You will learn: • About concave mirrors and how light is reflected by them • The four rays needed to locate an image of an object in a concave/convex mirror in diagrams • How an image is formed in a concave/convex mirror and types of image depending on distance of the object from the mirror • Real and virtual images • Formulas for calculating magnification and focal length for mirrors. • Uses of concave and convex mirrors • Experiment to measure the focal length of a concave mirror

3. What is a spherical mirror? Spherical mirrors are mirrors whose surfaces form a part of a hollow sphere, The spheres can be made of glass or any polished metal surfaces Types of spherical mirrors : 1. Convex Mirror 2. Concave Mirror

4. Convex mirror :. If the reflecting surface of the mirror curves outwards Concave mirror :If the reflecting surface of the mirror curves inwards.

6. Terms used to describe spherical mirrors The centre of the spherical mirror is called the pole Pof the mirror. The centre of the sphere from the mirror is part of is called C the centre of curvature. The straight líne joining P to C is called the principal axis. The halfway point between C and P is called the focus F The distance between F and P is called the focal length. The laws of reflection apply to concave and convex mirrors

7. Light rays used to find an image in a concave mirror • A ray which strikes the pole is reflected and an equal angle with the axis i r

8. Light rays used to find an image in a concave mirror • A ray which passes though the centre of curvature is reflected back along its own path.

9. Light rays used to find an image in a concave mirror • A ray which strikes the mirror parallel to the axis is reflected through the focus.

10. Light rays used to find an image in a concave mirror • A ray which passes though the focus and strikes the mirror is reflected out parallel to the principal axis.

11. Focal length of a concave mirror : In a concave mirror, the rays appear to converge at a point F. The distance PF is called the focal length and F is called the focal point. The focus is in front of the mirror.

12. For a concave mirror: If the object is outside the focus the image is real and in front of the mirror. If the object is inside the focus the image is virtual and behind the mirror. (pass concave mirrors and around get students to try this) Real Image A real image is one formed by the actual intersection of light rays. Real images can be formed on a screen. The image of a distant object in a concave mirror arrives as a parallel beam and a real image is formed at the focus. In the next slides we see where the images can be formed in concave mirrors.

13. Formation of images in a concave mirror 1) When the object is inside the focus F. The image is virtual and erect.

14. Formation of images in a concave mirror 2) When the object is beyond C. The image inverted and diminished.

15. Formation of images in a concave mirror 3) When the object is between C and F The image is magnified and inverted.

16. Formation of images in a concave mirror 4) When the object is at C. The image is the same size as the object and inverted.

17. Formation of images in a concave mirror 5) When the object is at F. The image is at infinity.

18. f Formulas for Concave mirrors f = the focus u = object distance v = image distance If the image is real v is positive and if the image is virtual v is negative. hi or v M = ho u u m = magnification ho = height of object v hi = height of image

19. f Formulas for Concave mirrors f = the focus u = object distance v = image distance If the image is real v is positive and if the image is virtual v is negative. hi or v M = ho u u m = magnification ho = height of object v hi = height of image

20. Concave mirror problem An object is placed 40 cm in front of a concave mirror of focal length 30 cm. Find the position and nature and magnification of the image. Solution U = 40, f = 30 v = ? hi or v Now M = ho u 1 4 3 v 120 = - M = = v 120 120 u 40 M = 3 1 1 1 1 1 1 1 1 = The image is 120 cm in front of the mirror, real, inverted and 3 times the size of the object. = + = + 30 40 v v 120 V = 120 cm 1 1 1 = - v 30 40

21. Uses of concave mirrors Searchlights and floodlights as bulb at the focus reflects light out paralell. Shaving and makeup mirrors because the image is magnified and upright. Dentists to see around your mouth . Projectors – to make a brighter image.

22. Focal length of a concex mirror :- In case of convex mirrors, the parallel rays, on reflection, appear to diverge from a point F. the focal length. The focus, in this case is behind the mirror. The image is always virtual erect and diminished

23. Light rays used to find an image in a convex mirror • A ray which strikes the pole is reflected and an equal angle with the axis i i r

24. Light rays used to find an image in a convex mirror • A ray which passes though the centre of curvature is reflected back along its own path.

25. Light rays used to find an image in a convex mirror • A ray which strikes the mirror parallel to the axis is reflected as if through the focus.

26. Light rays used to find an image in a convex mirror • A ray which passes though the focus and strikes the mirror is reflected out parallel to the principal axis.

27. Finding the image in convex mirror Using any two rays form the 4 previous slides the location of the image in convex mirror may be found. The image is always virtual and behind the mirror and always diminished. v u f f is negative v is negative

28. Formulas for Convex mirrors f = the focus u = object distance v = image distance For convex mirrors f and v are always negative as they are behind the mirror. ho or v M = hi u v m = magnification u ho = height of object f hi = height of image

29. Uses of convex mirrors • Car door mirrors. • Concealed entrances to see oncoming traffic. • In shops to deter shoplifters. • In railway stations. Used because they give a wide field of view and an errect image always. The image is always diminished which is a disavantage for car door mirrors. Why?