Laws of Reflection• From the Activity you performed, when you

shine an incident light ray at a plane mirror, the light is reflected off the mirror and forms a reflected ray.

• This behaviour leads to the 2 laws of reflection

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

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

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Incident ray

Normal

Reflected ray

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Reflecting Light Off Surfaces

When a series of parallel incident rays strike a flat

reflective surface

• the incidence angles are all identical

• angle of reflection will also be identical reflected raysIncident rays

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If parallel incident rays were directed at an irregular

surface, they will have different incidence angles.

The reflected rays will be reflected in many directions.

This is known as diffuse reflection

Incident rays Reflected rays

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Reflection and Dyslexia

People with dyslexia have difficulty reading print

• complain about the glare off white paper

• there is too much reflected light

• condition can be helped by the use of coloured filtered glasses

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Using Light Rays to Locate an Image

Light rays and the laws of reflection help determine how and where an image is formed in a plane mirror.

REFLECTION is the bouncing back of light rays from a surface

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We all know that light travels in a straight line.

When your eyes detect reflected light from plane mirrors, your brain projects the light rays back in a straight line.

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Types of Images

Real Images- mirrors can produce images that can be projected on a screen. A real image is ALWAYS inverted and appears in front of the mirror.

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Virtual Image- mirrors can also produce images that cannot be projected on a screen. A virtual image is ALWAYS upright and appears behind the mirror.

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Locating images in a Plane Mirror

object

mirror

Image of object

reflected

incident

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Locating Images in a Plane Mirror

• From the top of the object draw 2 incident rays

• From mirror draw 2 reflected rays

• extend reflected rays behind mirror

• where the extended lines cross, behind the mirror, the image forms

• repeat above steps for bottom of object

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Locating Images in a Plane Mirror

Is there any easier way? Of course...

mirror

2cm 2cm

3.5cm 3.5cm

object

Draw line & measure Repeat lines behind mirror

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Locating Images in a Plane Mirror

•From top of object, draw a line perpendicular to mirror (along normal)•measure the distance of this line and extend the line, the same distance, behind the mirror•repeat this step from the bottom of the objectfor complicated objects, draw more than two lines

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2 cm

2 cm

3.2 cm

• 1. A ray that strikes perpendicular to the mirror surface, reflects perpendicular to the mirror. The reflected ray is extended beyond the mirror.

• 2. A ray that strikes the mirror at any angle reflects so the angle of incidence equals the angle of reflection. The reflected ray is extended beyond the mirror.

Steps for drawing Plane Mirror Ray Steps for drawing Plane Mirror Ray Diagrams:Diagrams:

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Perpendicular ray

Incident ray

Reflected

ray

Ray Diagram Using 2 Points:

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• Example:

– Let’s draw the letter P together on the board

P ?17

• Using the information provided, draw these 2 images using the principles of reflection off a PLANE MIRROR.

• Use at least 3 points off of the object.

Lets Practice!!

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1. Plane Mirror- a flat mirror that reflects light rays in the same way that they approach the mirror.

2. Concave Mirror- a converging mirror where light rays that strike the mirror surface are reflected so they converge or “come together”, at a point.

Types of Mirrors

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3. Convex Mirrors- a diverging mirror where light rays that strike the mirror surface are reflected so that they diverge, or “go apart” and they never come to a point.

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• SALT is used to describe images formed by mirrors.

1. S- Size: compared to original object is it same size, smaller or bigger?

2. A- Attitude: which way the image is oriented compared to the original object (upright or inverted).

3. L- Location: location of the image (in front or behind the mirror).

4. T- Type: is the image a real image or virtual image?

S.A.L.T

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• Characteristics of a plane mirror image:

1. Object size= Image Size

2. Object distance from mirror= image distance from mirror

3. Attitude (orientation) is ALWAYS upright

4. ALWAYS forms a virtual image

5. Image is reversed- left to right

PLANE MIRRORS

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• The image in a plane mirror appears to be backwards compared to how we view the object directly.

• This is why the word on the front of an AMBULANCE is written backwards- so it can be read when seen in a rear-view mirror.

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References

• kilby.sac.on.ca/faculty/gbrennagh

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