Lectures and laboratories activities on the nature of Physics and

concepts and models in optic:

2. From the mechanism of vision to

the rectilinear propagation of light

Alberto Stefanel

Research Unit in Physics Education

University of Udine

(da Galili, Hazan 2001).

The process at the base of vision in Hystory of physics

(da Galili, Hazan 2001).

In the diffent

cases:

What do you see?

•Which of the following figures describe the mechanism of

vision?

a) b)

c)d)

bulbbulb

bulb

object

object

object

object

eyeeye

eyeeye

lightlight

lightlight Visual rays

bulb

Inside a black box there are:a mirrora withe tablea puppet

When we close the box with the top….

What do you expect to see, looking from the hole?

Inside a black box there are:a mirrora withe tablea puppetAn hole in the side permit to see inside the box when we cover it with the top

I will see:…… I do not will see

The shadow formation

Explanation

according to the

model of

rectilinear

propagation

Refraction

Reflection Diffusion

Absorption

Transmission

What happens when the light of a source

illuminate an object?

Light and matter

In these cases the phenomenon

is the reflection:

The light shot an object and

return in the same medium with

a well defined angle

A. Reflection and diffusion

reflection

A. Riflessione e diffusione

reflection

Also in these cases light shot the systems and return in the same medium….but in all direction…..

Diffusion by a reflection process

Diffusion by reflessionei r

Riflection

i r

From “Sunday Afternoon on the Island of La Grande Jatte”

Georges Seurat

(1884-1886)

Oil on canvas

... Diffused riflection by the particles

Of pigment suspended in a transparent

medium

What are the laws underlying the reflection

phenomena?

How work reflection when the surface is curved

(not flat)?

How it is possible to construct the image of an

object reflected by a flat or spherical mirror?

The parallax method

Front vision

The line from the black pin to the red

one individuate the incident direction

Lateral vision

Incident

direction

Find the position where to put the blue

pin to obtain the visual alignement of the

black, red and the image of the blue pins

reflected

direction

mirror

From this point of visualization the black pin, the red

pin and the image of the blue are aligned

The direction of the line from the red and

the blue individuate the reflected direction

The parallax method

Measurement

AH (cm) BH (cm) BH/AH

1.…2,5………………… ………2,3………… ……0,92…

2.…3,5………………… ………3,5………… …....1........

3.…4,5………………… ………4,5………… .……1……..

4.…………………… …………………………… .……………..

5.…………………… …………………………… .……………..

6.…………………… …………………………… ………………

Incident

direction

reflected

direction

mirror

A H B

Incident

direction

reflected

direction

mirror

1 A H B

Incident

directionreflected

direction

mirror

2 A H B

Incident

directionreflected

direction

mirror

3 A H B

A. Stefanel - Ottica 1

Me

diu

m 2

The laws of reflection

SIncident ray

Reflected ray

Medium 1

θi

θr

normal

A) -The incident ray

- The reflected ray

- normal to the surface of

separation the two beams

ARE ON THE SAME PLANE

B) -The angle θi,

- the angle θr

ARE EQUAL

θi = θr

A. Stefanel - Ottica 1

Reflection from a non-flat surface

θi

θr

Applying the laws of reflection points by

points

Normal: direction orthogonal to the

tangent plane to the surface in the

incidence point

Mezzo 1

Mezzo 2

A. Stefanel - Ottica 1

Reflection from a spherical mirror

S

C

P

S’ V

h 1 1 1

----- = ---- + ----

(r/2) p q

Se p � ∞ 1/p � 0 1/q = 1/(2r)

Focal distance : f = r/2

1 1 1

----- + ----- = ----

p q f