CHAPTER 25: THE REFLECTION OF LIGHT: MIRRORS

WAVEFRONTS AND RAYS

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.

TYPES OF REFLECTION

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

THE REFLECTION OF LIGHT

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, Ɵi , 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, Ɵr , is the angle that the reflected ray makes with the normal.

LAW OF REFLECTION

The incident ray, the reflected ray, and the normal to the surface all lie in the same plane, and the angle of reflection Ɵr equals the angle of incidence Ɵi

Ɵr = Ɵi

CHECK YOUR UNDERSTANDING

Consider the diagram below. Which angle (A, B, C, or D) is the angle of

incidence? ______ Which angle is the angle of reflection? ______

CHECK YOUR UNDERSTANDING

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?

PLANE MIRRORS

When you look into a plane (flat) mirror, the image you see has 3 properties:

1. The image is upright2. The image is the same

size you are3. The image is located as

far behind the mirror as you are in front of it.

The image is also reversed left to right

PLANE MIRRORS

When you look into a plane (flat) mirror, the image you see has 3 properties:

1. The image is upright2. The image is the same size you are3. The image is located as far behind the mirror

as you are in front of it.

PLANE MIRRORS

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.

PLANE MIRRORS

do = distance from object to mirror

di = distance from image to mirror

ho = height of object

hi = height of image

For a plane mirror: do = di and ho = hi do di

ho hi

IMAGES

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.

IMAGES

The image is called virtual because it does not really exist behind the mirror

CHECK YOUR UNDERSTANDING

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?

ASSIGNMENT

Check Understanding: p 778 #1-3

Focus: p 792 #2, 4, 7 Problems: p. 793 #3-6