Law of Reflection and Mirrors

How Light can be Redirected

The Law of Reflection

• When an object is smooth, the light that reflects off of the surface of that object will form an image– An image is a reproduction of an object

produced by an optical device• An optical device is anything that uses light

• Light rays bounce off of a mirror in a similar way that a pool ball bounces off the bumpers of a pool table

Law of Reflection (cont’d)• When an incoming ray of light, called the

incident ray, hits a mirror at a specific angle, called the angle of incidence, it will reflect that ray off at precisely the same angle, called angle of reflection.

• The diagram on the next slide will give you a better idea of what this looks like

Law of Reflection (cont’d)Notice how the angle of incident, labelled i, and the angle of reflection, labelled r , are measured from a line drawn perpendicular to the plane of the mirror, called the normal.

i r

The Law of Reflection

• The law of reflection can be written using the Greek letter theta, θ.– This symbol is commonly used to represent an

angle

• Putting subscripts next to theta helps to identify what angle the symbol represents

θi = θr

• Once again remember that θ is measured from the normal line.

Plane Mirrors• The law of reflection applies to both flat mirrors and curved

mirrors.• Any mirror that has a flat reflective surface is called a plane

mirror.• Images in a plane mirror appear as far behind the mirror as the

object being imaged is in front of the mirror• The image appears to be behind the mirror, but it would not be

possible to catch this image on a piece of paper if it were put behind the mirror because no light form the object reaches this point.

• When no light comes from where an image appears to be, we call this a virtual image.

Plane Mirror (cont’d)

Mirr

or

ObjectReflected Virtual

Imagedo di

For a plane mirror, the distance of the object from the mirror is equal to the distance the image appears to be in the mirror. In other words, do = di.

Plane Mirrors Exercise• Take a Post-It note and put the initial of your first name

on in dots on the far left, as shown:

• Then draw a line down the middle using a ruler as shown

• Finally measure the distances the dots are from the line and make a dot an equal distance from line on the other side of the line as seen above

Plane MirrorsExercise(cont’d)

• What did you notice about your letters?• Were the two letters exactly the same?• If they were exactly the same try the same

exercise with the letter R.• This shows why letters appear backwards in

images in mirrors.• You’ll see examples of how we try to correct for

such problems on the front of ambulances (see above).

Questions

• Do Plane Mirror Reflection Quick Lab on page 428 of the textbook.– Answer questions 7 and 8 a & b for the lab.

• Do questions 1, 2 and 9 from page 433 in the textbook. Put your answers to these questions in the title page document that coincides with this PowerPoint presentation.