Mirror and Lens

Thursday, March 26:

Quiz on Mirrors

Thursday, April 2:

Test on Mirrors and Lenses

Reflection

Reflection occurs when a wave reaches a boundary and it bounces back into the first media as it strikes that boundary.

It may be:

Total - all of the wave’s energy reflects back. Ex: light hitting aluminum, chrome, other metals

Partial – some of the wave’s energy reflects back. Ex: light waves hitting water or glass

Law of Reflection

Waves bounce back at the same angle at which they arrived. Direction of waves are shown using straight line rays. Incident and reflected rays make equal angles perpendicular to a surface called the normal.

Angle of incidence = angle of reflection (as measured from the normal)

Normal

Diffuse Reflection Normal reflection is called specular reflection.

Diffuse reflection is when light is reflected in

many directions from a rough surface.

Most things around us are seen by diffuse reflection, such as the light which reflects from a book you read, because the surface of ordinary paper as seen through a microscope in is actually quite rough.

Terminology

An object is a source of diverging

light rays. The object can send out its

own light rays (like a light bulb) or can

reflect light.

The term upright mean “right side up”

and inverted means “upside down”.

Mirrors

3 main types:

Plane (flat mirror like in the bathroom)

Concave (inwardly curved like make-up and

shaving mirrors)

Convex (outwardly curved like car and

grocery store mirrors)

Plane Concave Convex

How Parallel Light Rays

Reflect

When parallel light rays are incident on a

mirror, they can reflect with 3 possible

options:

Remain parallel (only happens with a plane

mirror)

Converge (all rays eventually meet at a

single point) happens with a concave mirror

Diverge (all rays spread away from each

other) happens with a convex mirror

Parallel, Converge, or

Diverge?

Parallel Converge Diverge

Luminous vs. Illuminated

Luminous Objects

give off light

Illuminated Objects

reflect light

Characteristics of an Images

For all images, you must be able to determine the following relative to the object:

Size • Reduced

• Same Size

• Enlarged or Magnified

Orientation • Upright

• Inverted

Type of Image • Real – light rays converge

• Virtual – light rays diverge

Plane Mirror Objects seen appear to be

somewhere behind mirror. Eye sees the reflected light and extrapolates its path back to a point behind the mirror where it appears to originate.

What you see is called a VIRTUAL IMAGE.

Plane mirrors cannot produce a REAL IMAGE because parallel light rays that strike the mirror always reflect parallel to each other.

Reflected light rays must intersect in order to form a REAL IMAGE.

Reflection is

on top of

incident light

For a Plane Mirror…

object size = image size

Image orientation is the same as

object (no inversion). All mirrors

exhibit a left to right flip.

object distance in front of the mirror =

image distance behind the mirror

Convex vs. Concave Mirrors

Concave Mirrors

If object is placed beyond the focal point of concave mirror, all light rays from a single point on object intersect at a single point upon reflection.

Light rays converging on a single point in real space will produce a REAL IMAGE because the light rays appear to be radiating from that point as they continue onward.

Concave Mirrors also…

Can create a VIRTUAL IMAGE:

If object is placed closer to mirror than focal point, reflected light rays diverge. Observer would see a virtual image located somewhere behind the mirror because the light appears to originate from that point. Virtual images formed by concave mirrors are larger and farther away from the mirror than the object is.

NO IMAGE is created at all when an object is placed exactly at the focal point, the reflected light rays run parallel to each other. Ex: Instead, would see a wide beam of parallel light like that of flashlight or car headlights or a completely blurry object.

Convex Mirrors

Always produce virtual images

Virtual images formed by convex

mirrors are smaller and closer to the

mirror than the object is. The image is

always oriented the same as the

object.

Lens/Mirror Equation

(units of length MUST match)

f stands for focal length

di is the distance from the mirror/lens to the image

do is the distance from the mirror/lens to the object

f (di1 do

1)1

di ( f1 do

1)1

do ( f1 di

1)1

Image Height

Magnification of an image can be

found by a series of ratios between

the distances of objects and their

images and their respective heights

as shown below.

hi

hodi

do

Helpful Hints for Mirror Probs

In the formula, the numbers can tell you the characteristics of the image:

Size • Reduced ( |hi| < ho )

• Same Size ( |hi| = ho )

• Enlarged ( |hi| > ho )

Orientation • Upright(hi positive)

• Inverted (hi negative)

Type of Image • Real (di positive, in front of mirror)

• Virtual (di negative, behind the mirror)

Refraction of Light

Light changes direction (bends) as it crosses a

boundary between 2 media in which the light

moves at different speeds.

Amount of refraction of light depends on

properties of media (material type,

temperature or density) and angle at which it

hits the boundary.

Examples of Light Refraction

Pond or pool looks

shallower than it actually

is

Straw or spoon in a glass

appears bent

White light comes out of

prism as rainbow

Air above hot stove seems

to shimmer

Stars twinkle

More on Refraction of Light

Light waves travel faster in air than in water and slower in glass than water.

More dense = slower light

When light enters a different medium, speed changes and it bends.

Bending of light due to change in speed = REFRACTION

Fiber Optics

Fiber optics use

total internal

reflection.

Light is totally

internally reflected

over and over

many times due to

refraction.

Advantages of Fiber Optic

Technology

Used to get light to inaccessible places such as car engines, inside a patient’s body, and in communications transmitting telephone messages – replacing electrical circuits and microwave links in communication technology

Can carry more info in high frequencies of visible light than in lower frequency electrical current

Thin glass fibers replace bulky expensive copper cables – more practical in weight, size, cost

Lenses

A lens is made of transparent material, such

as glass or plastic, with an index of refraction

larger than that of air, causing light to bend

(refract) as it passes through it.

A lens has a curved surface on one or both

sides.

Plano-convex

Double-convex

Plano-concave

Double-concave

Types of Lenses

Convex vs. Concave Lenses

A convex lens causes parallel light rays to

eventually converge and a concave lens

causes parallel light rays to eventually diverge.

Convex Lens: Beyond 2F

Image is real, inverted, and reduced.

Check Line

Convex Lens: @ 2F

Image is real, inverted, and same size.

Check Line

Convex Lens: Between FP and 2F

Image is real, inverted, and enlarged.

Check Line

Convex Lens: @FP

No image is formed.

Check Line

Convex Lens: Between FP and Lens

The refracted light rays diverge. The image forms on the same side of the lens at the object. The image is virtual, upright, and enlarged.

Check Line

The Concave Lens

Rays diverge after they hit the lens. Image will

always be virtual, upright, and reduced.

Check Line

Applications that Use Lenses

Hand lenses/Magnifying glasses

Projectors

Refracting telescopes

Binoculars

Cameras

Microscopes

Corrective Eyeglasses and Contact lenses

Nearsightedness

Nearsightedness

(Myopia) occurs when

the eyeball is too long,

so focal length is too

short.

Image forms in front of

the retina; causes

distant objects to be

blurry.

Corrected by a concave

lens that forces light to

diverge to a farther point

on back of retina.

Farsightedness

Farsightedness (Hyperopia) occurs when the eyeball is too short so focal length is too long, also happens with aging as muscles holding the shape of lens relax and allow it to flatten.

Image forms behind wall of the retina; causes objects located close to the eye to become blurry.

Corrected by convex lens that forces light to converge at a closer point on the back of retina.

Mirror Image Chart

Object

Placement/Mirror

Size of image

compared to size of

object

Real or Virtual?

Sign of di? Upright or Inverted?

Sign of hi?

Plane Mirror same virtual so -di upright so +hi

Btwn Concave

Mirror and F larger virtual so -di upright so +hi

At F of Concave

Mirror NO IMAGE IS FORMED

Beyond F of

Concave Mirror same, smaller, or

larger real so +di inverted so -hi

Convex Mirror smaller virtual so -di upright so +hi

Lens Image Chart

Object

Placement/Lens

Size of image

compared to size of

object

Real or Virtual?

Sign of di? Upright or Inverted?

Sign of hi?

Btwn Convex Lens

and F larger virtual so -di upright so +hi

At F of Convex

Lens NO IMAGE IS FORMED

Beyond F of

Convex Lens same, smaller, or

larger real so +di inverted so -hi

Concave Lens smaller virtual so -di upright so +hi