Plane MirrorsSNC2P – Optics

Plane Mirrors• Mirrors reflect in predictable ways. As the angle of incidence

(i) increases, the angle of reflection (r) also increases, they are equal to one another

• The incident ray, reflected ray, and normal are all on the same plane (flat surface)

• These are the laws of reflection

Laws of Reflection

① The angle of reflection equals the angle of incidence (θr = θi)

② The incident ray, normal, and reflected ray all lie in the same plane

Regular Reflection• When parallel rays of light strike a smooth surface, like a

mirror, the light rays move away from the mirror parallel to each other. This is known as regular reflection. This type of reflection produces clear images

Diffuse Reflection• If light is reflected off a dull or rough surface the parallel light

rays coming in will strike at various different angles of incidence. The reflected rays bounce off at different angles. This is known as diffuse reflection. We are not able to see a clear reflection when this occurs.

Characteristics of an Image• An image is a picture of an object formed using light rays.• Mirrors make the light rays meet, or appear to meet.

Characteristics of an ImageImage Characteristics (SALT)• S: Size (larger, smaller, same)• A: Attitude (upright or upside down)• L: Location (position relative to the mirror surface)• T: Type (virtual or real)

Real Image• Formed when light rays actually meet (i.e. real rays)• Can be seen (captured) on a screen

Virtual Image• Formed where light rays do not actually reach (i.e. virtual rays)• Cannot be seen on a screen (must look into the device)

Locating Images in Plane MirrorsAn image in a plane mirror is always a virtual image. It looks as though it is behind the mirror. The only way to see a virtual image is to look at the device. We can draw this using the law of reflection and geometry.

Using the Law of Reflection:a) Incident rays from the object

hit the surface of the mirror and are reflected back to the observer (θr = θi). Note that these are “real” rays being shown.

b) The reflected rays enter the observers eyes.

c) The brain interprets the location of the image as behind the mirror. The brain unconsciously extends the light rays through the plane mirror forming a virtual image behind the mirror. These are “virtual” rays as they don’t actually exist and are shown as dashed lines so we can tell the difference.

Questions1. Where will the image of the pen appear in the following?

Locating Images Using Geometrya) A straight line is drawn from the tip of the object,

perpendicular to the mirror, and extended behind the mirror. This is called an “object-image” line

b) The distance from the object to the mirror (i.e. the “object” line) is the measured.

c) The “image” line is made the same length as the “object” line. The end of the “image” line is the location of the virtual image.

Images in Plane Mirrors - SummaryImages in Plane Mirrors• Are always:• The same size as the object• Upright• Located behind the mirror• Virtual

Check Your Learning

1. Why is the word “ambulance” spelled in reverse on the front of an ambulance vehicle?