Image FormationImage Formation

We will use geometrical optics: light propagates in straight lines until its direction is changed by reflection or refraction.

When we see an object directly, light comes to us straight from the object.

When we use mirrors and lenses, we see light that seems to come straight from the object but actually doesn’t.

Thus we see an image (of the object), which may have a different position, size, or shape than the actual object.

Image Formation

Images Formed by Plane Mirrors

(the ray reaching your eye doesn’t really come from the image)

object image

virtual image

But…. the brain thinks the ray came from the image.

When we use mirrors and lenses, we see light that seems to come

straight from the object but actually doesn’t. Thus we see an

image, which may have a different position, size, or shape than

the actual object.

Images Formed by Plane Mirrors

You can locate each point on the image with two rays:1. A ray normal to the mirror

Image is reversedfront to back

object image

Images Formed by Plane Mirrors

You can locate each point on the image with two rays:1. A ray normal to the mirror2. The ray that reaches the observer’s eye

Image is reversedfront to back

object image

Images Formed by Plane Mirrors

object image

You can locate each point on the image with two rays.

Images Formed by Plane Mirrors

object image

The distance from the image to the mirror equals the distance from the object to the mirror: d = d

d d

Also, the height of the image equals the height of the object

Images Formed by Plane Mirrors

Properties of Mirror Images Produced by Plane Mirrors

• The distance from the image to the mirror equals the distance from the object to the mirror: d = d

• The mirror image is upright but reversed right to left

• The mirror image is reversed front to back

• The mirror image is the same size as the object

A flower has a height h, and is at a distance d, from a plane mirror.An observer is at table level, a distance d behind the flower.Find y as a function of h

Parabolic Mirrors

• Shape the mirror into a parabola of rotation (In one plane it has cross section given by y = x2).

• All light going into such a mirror, parallel to the para- bola’s axis of rotation, is reflected to pass through a common point - the focus.

• What about the reverse?

•

• These present the concept of a focal point - the point to which the optic brings a set of parallel rays together.

• Parallel rays come from objects that are very far away (and, after reflection in the parabolic mirror, converge at the focal point or focus).

• Parabolas are hard to make. It’s much easier to make spherical optics, so that’s what we’ll examine next.

Parabolic Mirrors

Spherical Mirrors

Concave Convex

Spherical Mirrors

Convex Mirror

A ray incident parallel to the axis reflects as if coming from the focus.

The focal distance fis given by f = - R/2

Concave Mirror

A ray incident parallel to the axis reflects passing through the focus.

The focal distance f is given by f = R/2

To analyze how a spherical mirror works we drawsome special rays, apply the law of reflection wherethey strike the spherical surface, and find out wherethey intersect.

Spherical Mirrors

A ray parallel to the mirror axis reflects through the focal point fA ray passing through the focus reflects parallel to the axisA ray that strikes the center of the mirror reflects symmetricallyA ray passing through the center of curvature c, returns on itself

cf

Image Formation

c

f

Trace one ray incident parallel to the axisTrace a second ray incident through the focusThe image is at the intersection of the two rays.Repeat for every point in the image.In practice only ‘head’ and ‘tail’ are needed.

When the object is beyond c, the image is: real (on the same side as the object), reduced, and inverted.

Spherical Mirrors - Concave

c

f

c

f

Object between c and f.

Spherical Mirrors - Concave

Image is real, inverted, magnified

Object between f and the mirror.

Spherical Mirrors

cf

Image is virtual, upright, magnified

The Mirror Equation

0 0 0 0 and i i i i

h d h d R

h d h R d

0 0Then, i i

d d R

d R d

0

0 0

1Next, 1 2

1

1 1 1

ii

i

Rd R R

R d dd

d d f

c

The Mirror Equation

f

l’l

'

1 1 1f l l

f = R / 2

cf

Magnification

h

h’

The magnification is given by the ratio M = h’ / h = - l’/ l

Curved Mirrors

mirror equation

focal length

magnification

1

l 1

l'1

f

f R / 2

M h'h

l'l

Curved Mirrors

Sign conventions:

Distance in front of the mirror positiveDistance behind the mirror negativeHeight above center line positiveHeight below center line negative

Concave Convex

Image With a Convex Mirror

c

f

Here the image is virtual (apparently positioned behindthe mirror), upright, and reduced. Can still use the mirror equations (with negative distances for f, c=R, and l’).