03 – LAW OF REFLECTION & PLANE MIRRORS

PW2 - Reflection

RECALL THE LAW OF REFLECTION

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

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

RECALL THE LAW OF REFLECTION

incident ray - a ray approaching the surface

point of incidence - the point where the incident ray strikes the surface.

normal - a perpendicular constructed where the incident ray strikes the surface.

reflected ray - a ray that leaves the surface at the point of incidence.

angle of incidence - the angle between the incident ray and the normal.

angle of reflection - the angle between the reflected ray and the normal.

TYPES OF REFLECTION

Regular (specular) reflection: is when all the rays in a beam of light are reflected at the same angle occurs in smooth surfaces (i.e.: mirrors).

Diffuse reflection: occurs when the rays in a beam of light are scattered off a surface. occurs in rough surfaces (which is much more common)

TYPES OF REFLECTION

This is why you see increased glare when driving in the rain or icy conditions.

For the purposes of this unit, we will focus on REGULAR REFLECTION

PLANE MIRRORS

Plane Mirror: a flat smooth surface

that reflects light in a regular or specular way.

Most common, found in bathrooms, dressing rooms, automobiles, etc.

PLANE MIRRORS – DESCRIBING IMAGES Virtual Images:

The images formed in a plane mirror appear to be behind the mirror. This means that the light appears to be coming from behind the mirror.

We call images formed in this fashion virtual images. A virtual image is an image that light does not pass through, but only appears to pass through.

PLANE MIRRORS – DESCRIBING IMAGES The four (4)characteristics of images in a plane mirror

are: 1) same size as object

The dimensions of the image are equal to the dimensions of the object. Magnification is 1.0.

If a 1.6 metre tall person stands in front of a mirror they will see an image that is 1.6 m tall.

2) erect (upright) 3) virtual (behind the mirror and cannot be projected on a

screen) The object distance, do, is equal to the image distance, di, and the

image is directly across from the object (perpendicular to mirror). do = di

4) laterally inverted (right side of object is left side of image)

FINDING IMAGES – POINT OBJECT

Draw a perpendicular line from the object to the mirror.

Extend the perpendicular line behind the mirror.

Measure the perpendicular distance (do) from the object to the mirror.

Make the perpendicular distance to the image (di) equal to do.

FINDING IMAGES – HOW EYES SEE A POINT OBJECT

Draw a perpendicular line from the object to the mirror.

Extend the perpendicular line behind the mirror.

Find the image using do = di.

FINDING IMAGES – HOW EYES SEE A POINT OBJECT

Draw a straight line from the image to the top of the eye. (Be sure to use a dotted line one the image side of the mirror and a solid line on the object side of the mirror.)

Draw another line from the image to the bottom of the eye.

Where the “image to eye lines” strike the mirror, draw straight lines from the mirror back to the object.

Finally, draw arrows on the “object to eye ray lines” indicating the direction that light travels.

FINDING IMAGES – HOW EYES SEE LARGE OBJECTS

Draw perpendicular lines from the top and bottom of the object to the mirror.

Extend the perpendicular lines behind the mirror.

Find the image using do = di. Use dotted lines to draw the image.

FINDING IMAGES – HOW EYES SEE LARGE OBJECTS

Draw a straight line from the top of the image to the top of the eye. Be sure to use a dotted line on the image side of the mirror and a solid line on the object side of the mirror.

Draw another line from the bottom of the image to the bottom of the eye.

FINDING IMAGES – HOW EYES SEE LARGE OBJECTS

Where the “image to eye lines” strike the mirror, draw straight lines from the mirror back to the object.

Finally, draw arrows on the “object to eye ray lines” indicating the direction that light travels.

TIPS FOR DRAWING RAY DIAGRAMS

Use a ruler Use a protractor Use a pencil For rays of light, always indicate the direction by using

arrows Label the mirror clearly, or add thatched lines For rays behind the mirror, used a dotted line Recall that the angle of incidence is the angle between

the normal and the incident ray, not the mirror! Likewise, the angle of reflection is the angle between

the normal and the reflected ray, not the mirror!