Mirrors and Lenses

Chapter 23

The Law of Reflection

• Light waves are electromagnetic waves.

• Light waves travel from their source in all directions

• Light is made up of rays that travel in straight lines.

• An arrow, called a ray, is used to show the path and direction of light

Mirrors

• Law of Reflection – states that the angle of reflection is equal to the angle of incidence.

• A ray diagram shows how rays change direction when they strike mirrors and pass through lenses.

Plane Mirror (Create Image)

• A mirror with a flat surface is a plane mirror.

• You see a reversed (R-L)

image

• Rays of light strike you and reflect. They strike the mirror and are reflected into your eyes.

Plane Mirror

• The image is virtual, right side up and reversed.

• Virtual Image- a copy of an object formed at the location from which the light rays appear to come. (Rays do not really come from behind the mirror)

Concave Mirrors• A concave mirror is

curved inward. They can produce both a virtual or a real image.

• Real images are in front of the mirror

• The point at which light rays meet is called the focal point.

Concave Mirror

Can make small objects appear larger

Ex: Make-up mirrors, shaving mirrors

Concave Mirror

• Real images occur when the object is further away from the focal point. Reflected rays meet in front of the mirror.

• Virtual images occur when the object is closer to the mirror than the focal point. Reflected rays spread out and appear to be coming from behind the mirror.

How do you locate the image in a Concave Mirror

Pick a point on the object (usually the one furthest from the principal axis), and then draw 2 intersecting rays that obey the following rules:

1. Any ray parallel to the principal axis is reflected through the focus.

2. Any ray through the focus is reflected parallel to the principal axis.

3. Any ray through the center of curvature is reflected back along the incident ray (back along itself)

Concave Mirrors

C F

Moving towards the focus, the image is REAL, inverted. It could be smaller, the same size, or larger than the image (depending on the object location)

“OUTSIDE“ the focus

Concave Mirrors – outside the focus

Concave Mirrors

C F

“INSIDE” the focus

Moving towards the mirror, the image is VIRTUAL, UPRIGHT, and gets smaller (although the image is ALWAYS larger than the object itself).

Concave Mirrors – inside the focus

Convex Mirror

• A convex mirror is bent outward. The object is virtual and appears smaller and upright.

• Convex mirrors spread out light.

Convex Mirrors

Can make large objects appear smaller (see a WIDE view)Ex: Security mirrors, driveway mirrors, car door mirrors

Convex Mirrors

F C

When an object gets closer to the mirror, its image is VIRTUAL, UPRIGHT, and keeps getting smaller (and the images are always smaller than the object).

C’ F’

Note: All rays want to pass through F, but

none do

Convex Mirrors

Refraction

• The index of refraction for a material is the ratio of the speed of light in a vacuum to the speed of the light in the material.

• When light enters a new medium at an angle, the change in speed causes the light to bend or refract.

Concave and Convex Lenses

• Lens- an object made of a transparent material that has 1 or 2 curved surfaces that can refract light.

• The curvature and the thickness affect the way it refracts light.

Concave Lens

• A concave lens is curved inward at the center and the thickest part at the outside edges.

• The light rays are spread out.

• Smaller, upright, virtual images are always formed. The image is formed at the point from which the refracted rays appear to come

Image Formation in Concave Lenses

F

F’

Concave Lenses

F

Convex Lens

• A convex lens is curved outward at the center and is thinnest at the outer edge.

• Convex lenses form either real or virtual images.

• The real image is upside down.

Convex Lenses

• Real images are produced when an object is further away from the mirror than the focal point (The object is also inverted.)

• Virtual images are produced when an object is closer to the lens than the focal point. ( The object is upright and larger.)

Image Formation in Convex Lenses

F

F’

Convex Lenses

F’

F

Type Real / Virtual Upright/ Upside-down

Smaller/Larger

Plane Mirror Virtual Upright(Reversed)

Same

Concave Mirror(Behind focal pt)

Real Upside down Depends on location

Concave Mirror(In front of focal pt)

Virtual Upright Depends on location

Convex Mirror Virtual Upright Smaller

Concave Lens Virtual(b/twn focus & lens)

Upright Smaller

Convex Lens(Behind focal pt)

Real Upside down Depends on location

Convex Lens(In front of focal pt)

Virtual Upright Larger

23.2 Light & Sight The Human Eye

Cornea: Protective “Window” of

eye

Iris: Colored part that acts like a

camera shutter.

Pupil: Hole in the middle of the

iris.

Lens: has adjustable focal length.

Retina: Where image is formed.

Optic nerve: Sends image to

brain where it is flipped

upside down.

Muscles that “tense” the lens

Eye Problems

• Nearsightedness - The eyeball is too thick, causing the image to focus in front of the retina. A person can't see distant objects, but can see near objects well. A concave lens can be used to correct this problem.

• Farsightedness - The eyeball is too thin, causing the image to focus behind the retina. A person can see distant objects clearly, but has difficulty with near objects. A convex lens can be used to correct this problem.

Cameras ……

….. and the human eye

BOTH use converging lenses with the object far beyond the focus

and both form INVERTED images.

Film

23.3 Light and Technology

Magnifying Glasses

2F’ F’ F 2F

Object that you are looking at (under the lens)

Larger, virtual image that you see

Film ProjectorsW

hite screen