Reflection and Mirrors Chapter 17 1 The Law

Reflection and Mirrors Chapter 17 1

The Law of Reflection • When light strikes a surface it is reflected. • The light ray striking the surface is called the incident ray. • A normal (perpendicular) line is then drawn at the point where the light strikes the surface. • The angle between the incident ray and the normal is called the angle of incidence. • The light is then reflected so that the angle of incidence is equal to the angle of reflection. • The angle of reflection is the angle between the normal and the reflected light ray. 2

Normal Incident Ray Angle of Incidence Reflection Reflected Ray Mirror 3

The incident ray, normal, and reflected ray are all in the same plane. 4

Regular reflection occurs when light is reflected from a smooth surface. When parallel light rays strike a smooth surface they are reflected and will still be parallel to each other. 5

Diffuse reflection occurs when light is reflected from a rough surface. The word rough is a relative term. The surface is rough at a microscopic level. For example, an egg is a rough surface. When parallel light rays strike a rough surface, the light rays are reflected in all directions according to the law of reflection. 6

• Light reflected from the surface of a painted wall would be an example of ____ reflection. • A. Regular • B. Diffuse 7

Types of Mirrors Convex mirrors are made from a section of a sphere whose outer surface was reflective. Convex mirrors are also known as diverging mirrors since they spread out light rays. They are typically found as store security mirrors. Concave mirrors are made from a section of a sphere whose inner surface was reflective. Concave mirrors are also known as converging mirrors since they bring light rays to a focus. Concave They are typically found as magnifying mirrors 8

Plane Mirrors have a flat surface. The mirror hanging on the wall in your bathroom is a plane mirror. 9

Real images are images that form where light rays actually cross. In the case of mirrors, that means they form on the same side of the mirror as the object since light can not pass through a mirror. Real images are always inverted (flipped upside down). Virtual images are images that form where light rays appear to have crossed. In the case of mirrors, that means they form behind the mirror. Virtual images are always upright. 10

Original image • What type of image is shown above? – A. Real – B. Virtual 11

Plane Mirror In a plane mirror the object is the same size, upright, and the same distance behind the mirror as the object is in front of the mirror. 12

Images in a plane mirror are also reversed left to right. 13

Original • Image What type of mirror created this image? A. Plane B. Convex C. Concave 14

Curved Mirrors The center of curvature also known as radius of curvature (C) of a curved mirror is located at the center of the sphere from which it was made. The focal point (f) is located halfway between the mirror’s surface and the center of curvature. C = 2 f The principle axis is a line that passes through both the center of curvature (C) and the focal point (f) and intersects the mirror at a right angle. 15

• A concave mirror has a radius of curvature of 15 cm. What is the focal length of this mirror? • A. 15 cm • B. 30 cm • C. 7. 5 cm 16

Concave Mirrors Principle Axis C f Light source Convex Mirrors Principle Axis f C Light source 17

Rules for Locating Reflected Images 1. Light rays that travel through the center of curvature (C) strike the mirror and are reflected back along the same path. 2. Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through the focal point (f). 3. Light rays that travel through the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 18

All three of these light rays will intersect at the same point if they are drawn carefully. However, the image can be located by finding the intersection of any two of these light rays. 19

Locating images in concave mirrors 20

Concave Mirror with the Object located beyond C 21

Concave Mirror Object beyond C Light rays that travel through the center of curvature (C) hit the mirror and are 22 reflected back along the same path.

Concave Mirror Object beyond C Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through 23 the focal point (f).

Concave Mirror Object beyond C Light rays that travel through the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 24

Concave Mirror Object beyond C Image: Real Inverted Smaller Between f and C The image is located where the reflected light rays intersect 25

Concave Mirror with the Object located at C 26

Concave Mirror Object at C Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through 27 the focal point (f).

Concave Mirror Object at C Light rays that travel through the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 28

Concave Mirror Object at C Image: Real Inverted Same Size At C The image is located where the reflected light rays 29 intersect

Concave Mirror with the Object located between f and C 30

Concave Mirror Object between f and C f C Light rays that travel through the center of curvature (C) hit the mirror and are 31 reflected back along the same path.

Concave Mirror Object between f and C f C Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through 32 the focal point (f).

Concave Mirror Object between f and C f C Light rays that travel through the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 33

Concave Mirror Object between f and C Image: Real Inverted Larger Beyond C f C The image is located where the reflected light rays 34 intersect

Concave Mirror with the Object located at f 35

Concave Mirror Object at f Light rays that pass through the center of curvature hit the mirror and are reflected back 36 along the same path.

Concave Mirror Object at f Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through 37 the focal point (f).

Concave Mirror Object at f No image is formed. All reflected light rays are parallel and do not cross 38

Concave Mirror with the Object located between f and the mirror 39

Concave Mirror Object between f and the mirror Light rays that travel through the center of curvature (C) hit the mirror and are 40 reflected back along the same path.

Concave Mirror Object between f and the mirror Light rays that travel through the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 41

Concave Mirror Object between f and the mirror Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through 42 the focal point (f).

Concave Mirror Object between f and the mirror Image: Virtual Upright Larger Further away The image is located where the reflected light rays 43 intersect

Locating images in convex mirrors 44

Convex Mirror with the Object located anywhere in front of the mirror 45

Convex Mirror Object located anywhere f C Light rays that travel through the center of curvature (C) hit the mirror and are 46 reflected back along the same path.

Convex Mirror Object located anywhere f C Light rays that travel parallel to the principle axis, strike the mirror, and are reflected back through 47 the focal point (f).

Convex Mirror Object located anywhere f C Light rays that travel through (toward) the focal point (f), strike the mirror, and are reflected back parallel to the principle axis. 48

Convex Mirror Object located anywhere f C Image: Virtual Upright Smaller Behind mirror, inside f The image is located where the reflected light rays 49 intersect

Original Image • What type of mirror could have created the image shown above? – A. Plane – B. Concave – C. Convex 50

Original image • Where would the object need to be placed to create this image? – A. – B. – C. – D. At C Between f and C Beyond C Between f and the mirror 51

Original image • Where would this image form? – A. – B. – C. – D. At C Between f and C Beyond C Between f and the mirror 52

Mirror Equation (1/f) = (1/do) + (1/di) f = focal length do = object distance di = image distance 53

Mirror Magnification Equation M = -(di / do) = (hi / ho) M = magnification di = image distance do = object distance hi = image height ho = object height 54

Mirror Sign Conventions F + for Concave Mirrors - for Convex Mirrors di + for images in front of the mirror - for images behind the mirror do + always hi + if upright image - if inverted image ho + always M + if virtual - if real image Magnitude of magnification <1 if smaller =1 if same size >1 if larger 55

• If the focus of a convex mirror is 60 cm from the mirror, what is the radius of curvature? • A. 120 cm • B. -120 cm • C. 30 cm • D. -30 cm 56

• A concave mirror of radius 60 cm is placed so that a luminous object is 35 cm in front of the mirror. Where does the image form? • A. 0. 0048 cm • B. 210 cm • C. -0. 012 cm • D. -84 cm 57

• A man 2 m tall stands 10 m in front of a convex mirror which has a radius of curvature of 5 m. How tall is the image? • A. 0. 4 m • B. -0. 7 m • C. 0. 7 m • D. 2 m 58