1. C HAPTER 25: T HE R EFLECTION OF L IGHT : M IRRORS

2. W AVEFRONTS AND R AYS In describing the propagation of light as a wave we need to understand: wavefronts : a surface passing through points of a wave that have the same phase and amplitude. rays : a ray describes the direction of wave propagation. A ray is a vector perpendicular to the wavefront.

3. T YPES OF R EFLECTION Specular reflection- when parallel light rays strike a smooth, plane surface, the reflected rays are parallel to each other. (mirrors do this) Diffuse reflection – a rough surface reflects the light rays in all directions

4. T HE R EFLECTION OF L IGHT Most objects reflect a certain portion of the light falling on them. For a ray of light incident on a mirror The angle of incidence, Ɵ i, is the angle that the incident ray makes with respect to the normal The normal is a line drawn perpendicular to the surface at the point of incidence. The angle of reflection, Ɵ r, is the angle that the reflected ray makes with the normal.

5. L AW OF R EFLECTION The incident ray, the reflected ray, and the normal to the surface all lie in the same plane, and the angle of reflection Ɵ r equals the angle of incidence Ɵ i Ɵ r = Ɵ i

6. C HECK YOUR U NDERSTANDING Consider the diagram below. Which angle (A, B, C, or D) is the angle of incidence? ______ Which angle is the angle of reflection? ______

7. C HECK YOUR U NDERSTANDING A ray of light is incident towards a plane mirror at an angle of 30-degrees with the mirror surface. What will be the angle of reflection?

8. P LANE M IRRORS When you look into a plane (flat) mirror, the image you see has 3 properties: 1. The image is upright 2. The image is the same size you are 3. The image is located as far behind the mirror as you are in front of it. The image is also reversed left to right

9. P LANE M IRRORS When you look into a plane (flat) mirror, the image you see has 3 properties: 1. The image is upright 2. The image is the same size you are 3. The image is located as far behind the mirror as you are in front of it.

10. P LANE M IRRORS A mirror is an object that reflects light. A plane mirror is simply a flat mirror. Plane mirrors are ground to be flat – the flatter the more expensive. (Typically good ones have no hills or valleys larger than 500nm). Consider an object placed at point P in front of a plane mirror. An image will be formed at point P´ behind the mirror.

11. P LANE M IRRORS d o = distance from object to mirror d i = distance from image to mirror h o = height of object h i = height of image For a plane mirror: d o = d i and h o = h i dodo didi hoho hihi

12. I MAGES An image is formed at the point where the rays of light leaving the object either actually intersect or where they appear to originate. If the light rays actually do intersect, then the image is a real image. If the light only appears to be coming from a point, but is not physically there, then the image is a virtual image.

13. I MAGES The image is called virtual because it does not really exist behind the mirror

14. C HECK Y OUR U NDERSTANDING To save expenses, you would like to buy the shortest mirror that will allow you to see your entire body. Should the mirror be (a) half your height (b) two-thirds your height, or (c) equal to your height? Does the answer depend on how far away from the mirror you stand?

15. A SSIGNMENT Check Understanding: p 778 #1-3 Focus: p 792 #2, 4, 7 Problems: p. 793 #3-6