Light and Optics

Light is an Electromagetic Wave.

Let’s consider light and how we see it everyday. It can come from objects that emit their own light (luminous) or can be reflected from non-luminous objects (illuminated).

The Sun = luminous light source The Moon = illuminated object

No matter whether the source of light is luminous or illuminated, light is emitted in straight lines called Rays.

Light Rays travel, or propagate, as waves in direct linear paths.

When we see the light emitted from an object, we see it because it is in our line of sight.

The Line of Sight:

In order to view an object, you must sight along a line at that object; and when you do light will come from that object to your eye along the line of sight.

Even though a luminous object emits light in all directions, we only see the light rays that propagate in our line of sight.

Line of Sight

Emitted Light Rays

LuminousObject

Reflection When a travelling wave hits a fixed boundary, it will reflect or change its direction of propagation.

The Law of Reflection:When an incoming ray of light (incident light) hits a surface it will

reflect (reflected light)

EQUALS

Normal Line

The angle of Incident Lightto the Normal Line

The angle of Reflected Lightto the Normal Line

Normal Line

ӨiӨR

Mirrors

- Mirrored glass serves as a reflecting boundary for light rays.

Plane Mirrors (Flat) Spherical Mirrors

Concave Mirror Convex Mirror

Seeing a reflection:

Reflected images are visible ONLY where reflected light rays converge – where they come together and intersect.

In the case of a Plane Mirror, the reflected waves diverge – they spread out and never intersect. But, if you trace the reflected rays back behind the mirror, they converge and form an image there.

Tracing light rays as they reflect from a mirror is called: Ray Diagrams

Ray Diagrams help us visualize where reflected images are formed and how they appear to us.

1. ALL reflections from mirrors – no matter what the shape - conform to the Law of Reflection.

2. The reflected image can be seen clearly where all reflected rays converge to a focus.

3. If the point where the reflected image forms is on the same side of the mirror as the object, then it is called a Real Image.

4. If the point where the reflected image forms is on the opposite side of the mirror as the object, then it is called a Virtual Image.

Basic Rules for understanding Ray Diagrams for ALL types of mirrors:

Image formed by a Plane (flat) mirror

The image seen is located where all rays from the object converge.

- This image is behind the mirror. (virtual)

- The image is the same distance behind the mirror as the object is in front

of the mirror.

- The image is oriented in the same direction as the object. (it is Upright)

- The size of the image in the mirror is the same size as the object in front of

the mirror. (there was NO Magnification)

Ray Diagram for a Plane Mirror

Physical Properties of a Concave Mirror:

- A Concave Mirror is a spherical reflecting surface.

Let’s do a Ray Diagram of distant light incident on the mirror . . .

Wherever the incident rays strike the mirror, the reflected rays MUST conform to the Law of Reflection: Өi = Өr relative to the Normal Line.

When parallel rays of light strike a concave mirror, they converge and intersect at the focus.The distance from the mirror to the point of focus is called the mirror’s focal length.

The focal length is ½ the radius of the curvature of the mirror.

The two basic rules of reflection for concave mirrors:

•Any incident ray traveling parallel to the principal axis on the way to the mirror will pass through the focal point upon reflection.

•Any incident ray passing through the focal point on the way to the mirror will travel parallel to the principal axis upon reflection.

 

Step #1:

Pick a point on the top of the object and draw three incident rays traveling towards the mirror.

Draw one ray so that it passes exactly through the focal point on the way to the mirror.

Draw the second ray such that it travels exactly parallel to the principal axis. Place arrowheads upon the rays to indicate their direction of travel.

Draw a third ray from the object through the center of the mirror’s curvature. If the object is farther from the mirror than the center of curvature, the ray will reflect directly back. 

Step-by-Step Method for Drawing Ray Diagrams – Convex Mirrors

Step #2

Once these incident rays strike the mirror, reflect them according to the two rules of reflection for concave mirrors.

- The ray that passes through the focal point on the way to the mirror will reflect and travel parallel to the principal axis.

- The ray that traveled parallel to the principal axis on the way to the mirror will reflect and travel through the focal point.

Place arrowheads upon the rays to indicate their direction of travel. Extend the rays past their point of intersection.  

Step-by-Step Method for Drawing Ray Diagrams – Convex Mirrors

Step #3

Once these incident rays strike the mirror, reflect them according to the two rules of reflection for concave mirrors.

- The ray that passes through the focal point on the way to the mirror will reflect and travel parallel to the principal axis.

- The ray that traveled parallel to the principal axis on the way to the mirror will reflect and travel through the focal point.

Place arrowheads upon the rays to indicate their direction of travel. Extend the rays past their point of intersection.  

Step-by-Step Method for Drawing Ray Diagrams – Convex Mirrors

Step #4

Mark the part of the image located at the intersection. In this case it is the head of the arrow.

Repeat as needed for any other parts of the object.

If the base of the object is located on the principal axis, the reflected line is also on the principal axis. 

Step-by-Step Method for Drawing Ray Diagrams – Convex Mirrors

If the reflected rays NEVER converge, then trace the reflected rays behind the mirror.

If the traced rays (virtual rays) converge, then a focused image will be formed at the intersection.

If neither the reflected real rays nor the traced virtual rays never meet, there will Not be a focused image.

How to find the focused image:

What does the reflected image look like?

If the object is between the mirror and the focus, then the image produced is virtual, upright and larger than the object. “make-up mirror”

If the object is located farther from the mirror than the focus, then the image produced is real, inverted, and its size gets smaller the further it is from the mirror.

Lab Problem #5

Do Now:On your sheet of graph paper, draw a ray diagram to determine the image of an object in front of a concave mirror.

• In a carnival attraction, a 6ft tall man stands 5ft in front of a concave mirror. The mirror has a radius of 2ft. Draw and describe the reflected image he sees.

• A 5ft tall woman stands 3ft in front of a concave mirror. The mirror has a radius of 10ft. Draw and describe the reflected image she sees.

• Pocket cigarette lighters are available that will ignite a cigarette with a concave mirror. Incident rays of light from the Sun strike a concave mirror with a radius of 10cm.

Where should the tip of the cigarette be placed to ignite it? What would happen if the cigarette is placed somewhere else?

Image formed by a convex mirror

1. When incident light is reflected from a Convex mirror, NO rays will converge. - ALL rays DIVERGE

2. When incident rays reflect, they obey the Law of Reflection.

3. If the incident rays are parallel, the reflected rays will be real and directed AWAY from the mirror’s focus. They can be traced backwards to the mirror’s focus.

- This is due to the spherical shape of the mirror

This produces a virtual image.

Cf

The SAME RULES apply for Convex Mirrors as for Concave Mirrors:

Incident Ray: Reflects out:1) Parallel through Focus2) Center Center3) Focus Parallel

The image you see will be smaller and Upright.Due to the nature of the diverging reflected rays,You will see a wide field-of-view.