GEO - Reflection and Mirrors

Reflection and Mirrors

Introduction

In this unit we will be examining how light behaves when it strikes the boundary between two different media. This can be difficult if we consider the transverse shape of an electromagnetic wave, so we instead simplify the wave into a light ray. If we assume light travels in a straight-line path we can represent that path with a straight arrow pointing in the direction of motion of the wave. This model of light is very useful for describing reflection and refraction.

Reflection

When light strikes the boundary between two different media it can be absorbed, transmitted, or reflected. When a light ray is incident on a reflective surface we measure its angle of incidence relative to a line that is normal (perpendicular) to the surface. The resulting reflective ray will leave the surface at the same angle relative to the normal. This is known as the law of reflection.

Often this will be written mathematically as:

You've seen the physical equivalent of this if you've ever played billiards. When you hit the cue ball into a rail, it will bounce off the rail at the same angle, just on the other side of the normal.

When light strikes a surface, it behaves as a large group of rays, each ray reflecting at an angle consistent with the law of reflection. If the surface is completely smooth, the normal lines used to measure each ray will be parallel. The resulting reflected rays will, therefore, be parallel. This type of reflection is known as a specular reflection. Every time you look in a mirror you are looking at a specular reflection. If, however, light strikes a rough surface the normal for each incident ray will be different from the other rays. This means that each of the reflected rays will go off in a different direction. This is known as a diffuse reflection. The following video explains more. Click on the icon to begin.

Forming Images in a Plane Mirror

When you place an object in front of a mirror, you see its image reflected back. This video walks you through the steps of how the image is formed in the mirror. 

Drawing rays in order to locate and measure the image is a powerful tool. We will learn how to draw these ray diagrams in the next lesson. Let's take a moment to learn the types of measurements we will need to understand in our ray diagrams. In this example, object A is placed in front of a plane (or flat) mirror producing image A. Light rays leave the object and travel in all directions. Some of those rays will strike the mirror's surface at such an angle that they will be reflected into your eye. When you see the reflected rays, they appear to be diverging from a single position. That position is where your brain perceives the location of the image. The distance from the mirror to the image location is known as the image distance. Likewise, the distance from the mirror to the object is known as the object distance.

For plane mirrors, the object distance equals the image distance.

In the case of a plane mirror, the object will always appear behind the mirror, as we saw in the video. This is because the reflected rays do not converge in front of the mirror, but behind. When the convergence point does not exist in real space (i.e. behind the mirror where light is not actually traveling to your eye) the image is referred to as a virtual image. To see the image we must trace the reflected rays back, behind the mirror to their convergence point. If the convergence point were in front of the mirror a screen could be placed at that point and we would see the image projected on the screen. Such images are referred to as real images. We'll see examples of real images in the next lesson.

For plane mirrors, the images are always virtual.

Two other useful measurements that can be taken are the height of the image and the height of the object. They measure exactly what you think they measure. As was shown in the video, the height of the image is the same as the height of the object for a plane mirror.

The ratio of image height to object height gives us the magnification of the image. When the absolute value of the magnification is greater than 1 it indicates the image is larger than the object. A magnification with absolute value between 0 and 1 results from an image that is smaller than the object.

For plane mirrors, the magnification always equals 1.

The last thing to note about the plane mirrors image is that it has the same vertical orientation as the object (i.e. if the object is upright, the image is upright). We will see examples of inverted images in the next lesson.

For plane mirrors, the image is always upright.

As we start to put objects in front of different types of mirrors we will always be interested in determining whether the image is: real or virtual, magnified or reduced, or upright or inverted.

IMAGES AND VIDEOS SOURCED FROM PUBLIC DOMAIN