SNC2P Lesson 8

Topic 4.4What is the Law of Reflection and How Do Mirrors Form Images?

P.308-311

Today we will learn:• Review how to measure angles with a protractor.• About angles of incidence and angles of

reflection, and their relationship.• The proper names of a ray diagram. SUCCESS CRITERIAI will be successful if I can • Measure angles with a protractor.• Identify the parts of a ray diagram.• Know the angle of reflection, given the angle of

incidence.

Measuring Angles with a Protractor:• Place the vertex of the protractor on the vertex of

the angle. Be sure that the line that goes from 0° to 180°lies on one of the sides of your angle.

• Find the scale that starts with 0° .... it could be the inner or outer scale.

• Find the line on the protractor that lies on the second side of your angle. Read the angle.

CB

A

Now you try it!• Worksheet 4.4 A ... Using your protractor, what is

the measurement of angle XYZ in question 1?

CB

A

Drawing Angles using a Protractor:• Draw a side for one side of your angle.• Place the protractor on the line with the vertex of the

protractor on the end of the line.• Find the line on the scale representing the angle of your

choice. Make a dot beside the point of the scale indicating the angle you chose.

• Remove the protractor.• Place the ruler so it is aligned with the end of the first line and

the dot. Draw the second side of your angle along the ruler.

CB

A

Now you try it!• Worksheet 4.4 A ... Using your protractor and ruler,

draw and label angle DEF.

CB

A

- Lakes can be reflective when the water is calm and when it is wavy ... but the type of reflection is different. - When the surface is calm, the images are almost identical to the objects being reflected.- When the surface is wavy, spread out reflections result in scattering of light, and the images are fragmented.

What other reflections have you observed? What type of surfaces were they on —smooth, rough, curved, wavy?

WHAT REFLECTIONS DO YOU SEE IN THE WATER?

Remember from the previous section ...

All light behaves the same, regardless of its source.

Light travels in a straight line as long as it is moving through the same medium.

MEDIUM: The substance through which light travels.

DEFINITIONSREFLECTION: The change in direction of a light

ray when it bounces off a surface.RAY: A straight line with an arrowhead that

shows the direction in which light waves are travelling.

RAY DIAGRAMNormal: Dashed line perpendicular to a surface

such as a mirror. Incident Ray: The incoming ray of light travelling

towards the mirror or other surface.

Reflected Ray: The outgoing ray of light that has “bounced” off a reflecting surface. Begins at the point where the incident ray and normal meet.

Incident ray

Reflection ray

Reflecting Surface

Angle of Incidence: The angle between the incident ray and the normal. Labelled i.

Angle of Reflection: The angle between the normal and the reflected ray. Labelled r.

RAY DIAGRAM

Incident ray

Reflection ray

Reflecting Surface

1. The angle of reflection (r) is equal to the angle of incidence (i).

2. The reflected ray and the incident ray are on opposite sides of the normal.

3. The incident ray, the reflected ray, and the normal always lie on the same plane (flat surface).

LAWS OF REFLECTION

Incident ray

Reflection ray

Flat Reflecting Surface

• The Law of Reflection applies to every reflecting surface. When light reflects off a surface, the angle of incidence is always equal to the angle of reflection.

LAWS OF REFLECTION

Using the Law of Reflection

• The angle of incidence and the angle of reflection are always measured from the normal and not from the surface of the object.

• Why? Because some surfaces are curved, making it difficult to measure an angle from the surface.

EXAMPLE 1Consider the diagram.Which one of the angles (A,

B, C, or D) is the angle of incidence? ______

Which one of the angles is the angle of reflection? ______

B

C

Images in Plane Mirrors• The law of reflection also applies to looking

at objects in a mirror. • The image in a plane mirror is an exact

reflection of the actual image.

You need only two rays reflecting from a mirror to find the point where the reflection of an object is located.

RAY DIAGRAMS FOR PLANE MIRRORS

Object: The item in front of the mirror.Image: The reflection of the object in the mirror.Object distance: The distance from the mirror to the

object.

RAY DIAGRAMS FOR PLANE MIRRORS

Image distance: The distance from the mirror to the image.

Virtual image: An image that is located where no light rays ever meet. (This occurs when reflected rays are separating and must be extended backwards to find out where they meet.)

• TEXTBOOK– Page 310 Table 4.2 – provides step by step

directions in locating an image in a plane mirror using a ray diagram.

DRAWING RAY DIAGRAMS FOR PLANE MIRRORS

• CLASSWORK:– Complete Handouts 4.4A and 4.4B– If they are not completed in class, then please

complete them as homework tonight.