# Lesson 3 10.2 Properties of Light and Reflection P.411-413 - PowerPoint PPT Presentation

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Lesson 3 10.2 Properties of Light and Reflection P.411-413. Today we will learn: A bout reflection. How to predict the location of an object using rays. How to draw a ray diagram for light reflecting off a smooth surface. SUCCESS CRITERIA I will be successful if I can

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Lesson 3 10.2 Properties of Light and Reflection P.411-413

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## Lesson 310.2 Properties of Light and ReflectionP.411-413

Today we will learn:

• How to predict the location of an object using rays.

• How to draw a ray diagram for light reflecting off a smooth surface.

SUCCESS CRITERIA

I will be successful if I can

• Explain what a ray is.

• Draw a ray diagram.

• Explain the laws of reflection.

### Think-Pair-Share:What can be done so that the villagers will receive sunlight all year round?

• Viganella is a small village in Italy.

• Due to the mountains, the town does not receive direct sunlight from November 11th – February 2nd.

• The villagers love their home and do not want to move.

### The Use of Mirrors

• In 2006 they spent 100,000 EUROS on a 5m x 8m mirror that reflects light down to the village in the winter.

• The mirror is controlled by computers that follow the Sun across the sky.

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.

REFLECTION: The change in direction of a light ray when it bounces off a surface.

MEDIUM: The substance through which light travels.

RAY: A straight line with an arrowhead that shows the direction in which light waves are travelling.

• A shadow occurs when an opaque object blocks the direct light from a light source

Ray diagrams can be used to show how the size of the shadow depends on the objects distance from the light source.

• A larger object will produce a larger shadow

• An object closer to the light source will produce a larger shadow

FERMAT’S PRINCIPLE

• Predicts the path light will take after reflecting from a surface or passing through more than one medium.

• Light follows the path that will take the least time.

• i.e. When light reflects from a surface and remains in one medium, its speed is constant.

### RAY DIAGRAM DEMONSTRATING THE LAW OF REFLECTION

Normal: Dashed line drawn perpendicular to the mirror at point of reflection.

Incident Ray: The incoming Ray of light from a light source towards a surface.

Reflected Ray: The outgoing Ray of light. Begins at the point where the incident ray and normal meet.

Reflection ray

Incident ray

Reflecting Surface

RAY DIAGRAM DEMONSTRATING THE LAW OF REFLECTION

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

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

Reflection ray

Incident ray

Reflecting Surface

### LAWS OF REFLECTION

• The incident ray, the reflected ray, and the normal always lie on the same plane.

• The angle of reflection is equal to the angle of incidence.

### LAW OF REFLECTION

• When light reflects off a surface, the angle of incidence is always equal to the angle 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 1

Consider 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

### EXAMPLE 2

A ray of light is incident towards a plane mirror at an angle of 30 degrees with the mirror surface. Determine the angle of reflection.

r

90 – 30 = 60 degrees

i

Law of reflection states:

Angle of incidence = Angle of reflection

⦟ i = ⦟ r

Therefore, the angle of reflection will be 60°

### EXAMPLE 3

A ray of light is approaching a set of three mirrors as shown in the diagram. The light ray is approaching the first mirror at an angle of 45-degrees with the mirror surface. Trace the path of the light ray as it bounces off the mirror. Continue tracing the ray until it finally exits from the mirror system.

How many times will the ray reflect before it finally exits?

• CLASSWORK: Measuring Angles Handout

• HOMEWORK: Textbook page 414 # 2-3