Rays and Plane Mirrors

• The line of particles on the crest of a wave is called a wave front

• Huygen’s Principle = a wave front can be divided into point sources. The line tangent to the wavelets from these sources marks the wave front’s new position.

Rays and Plane Mirrors

• Light traveling through a uniform substance always travels in a straight line.

• When light encounters a different substance, its path will change. It can be reflected and/or absorbed.

• Reflection = the change in direction of an EM wave at a surface that causes it to move away from that surface.

Rays and Plane Mirrors

• Two types of reflection:– Specular reflection = light reflected from a

smooth surface in one direction only (mirror or pond)

– Diffuse reflection = light reflected from a rough, textured surface in many different directions (paper, cloth, unpolished wood)

We will only be talking about specular reflection

Rays and Plane Mirrors

• If a straight line is drawn perpendicular to the reflecting surface at the point where the incident (incoming) ray strikes the surface, the reflecting ray is reflected at the same angle.

• Angle of incidence is the angle between a ray that strikes a surface and a perpendicular line to that surface.

• Angle of reflection is the angle formed by the perpendicular line to the surface and the direction in which a reflected ray moves.

Rays and Plane Mirrors

Angle of incidence = angle of reflectionΘ = Θ’

Rays and Plane Mirrors• Flat mirror = plane mirror• If an object is in front of a flat

mirror, the light rays that bounce off the object will spread out and reflect from the mirror’s surface. To an observer looking at the mirror, these rays appear to come from a spot on the other side of the mirror.

Rays and Plane Mirrors

• The new image formed is called a virtual image.

• Virtual image is an image from which light rays appear to diverge, even though they are not actually focused there.

p = object distanceq = image distance Object distance = image distance

Rays and Plane MirrorsImage location can be predicted using ray diagrams.1. Make a diagram showing the proportional

distances from object to mirror and image to mirror.

2. Pick a point on the object (usually the top)3. Draw a ray directly across to the mirror.

Because the angle of incidence is 0, it will be reflected directly back.

4. Draw another ray from the same point to the mirror at an angle.

5. Draw the reflected ray, remember to keep Θ and Θ’ equal.

6. Trace both reflected rays back to the point from which it appears to have originated. Use dotted lines!

7. This point is the image point, which corresponds to the same point on the object. Height and distance from mirror should also be the same.

Rays and Plane Mirrors

• This ray-tracing method works for any object placed in front of a plane mirror. The image is also reversed in the mirror.