Laboratory session in Physics II subject for September 2016-January 2017 semester in Yachay Tech University (Ecuador). Topic covered: optics, lenses, convergence, divergence, eye, abnormality Based on Bruna Regalado's work

1. PRACTICAL GUIDE 6:
GEOMETRIC OPTICS (LENSES)
CONTENT:
a-.) Definition of lens and its elements
b-.) The types of lenses
c-.) Imaging converging and diverging lenses
d-.) Abnormalities in vision and corrections
LEARNING OBJECTIVE:
1-.) Studying geometric optics by experimental demonstration of the phenomena occurring in
the light by using lenses.
2-.) To study the anomalies of vision and corrections.
2. THEORETICAL BASICS:
A lens is a transparent and homogeneous medium, defined by two or more refracting surfaces
curves. To be traversed by a light beam, this is, refracted.
The lenses alone or in combination, are generally used to form images by refraction in optical
instruments such as cameras, telescopes, microscopes, among others.
The lens are called thin when their thickness is sufficiently small (compared to the radii of
curvature of the boundary surfaces) as to be considered that any deviation of the rays passing
through the lens occurs in a single plane perpendicular to its optical axis (the lens passing
through its center, perpendicular to it).
Kinds of lenses (Figure 1):
a-) convergent glasses: are thicker in the center than at the edges.
b-) divergent lenses: are thinner in the center than at the edges.
Figure 1: divergent and convergent lenses.
Elements of a lens:
a-) Curvature centers C, C ': they are the geometric centers of the curved surfaces limiting
the transparent medium.
b-) Main axis: is the imaginary line joining the centers of curvature.
c-) Optical center (O): is the intersection point of the lens and the main axis
d-) Focus F, F ': is the point of the main axis through which the refracted ray lens, parallel
rays coming from the main axis.
e-.) Focal length f, f ': is the distance between the focus and the optical center.
Images produced by the lens:
The images generated by a lens can be: real or virtual. An image is real when it is formed in
PHYSICS
LABORATORY II

2. fact, that is, if a screen at the right point is placed, it will be formed on the image of the object
or light source. While a virtual image represents the position from which seems to be the light
that reaches the eye through the lens. However, the light actually never goes for that position
and if a screen is placed thereon no image will not be observed. Similarly, they have higher,
equal, smaller size images that bright object; while right or reversed from its original position.
For the construction of images on the lenses they are made by applying the following
three properties:
1.) A beam parallel to the main axis, is refracted through the focus.
2.) A ray passing through the optical center, not diverted.
3.) All ray passing through the focus, is refracted parallel to the main axis.
Equation lenses or Descartes Law:
1
−
1
=
1
′
Where: S0 or S = Distance to the measurement object from the center of the lens
Si or S '= distance to the image measured from the center of the lens
f = focal length image
Table 1: Summary imaging lenses
Object focus (F): is the point F, at a distance f in front of a converging lens so that light rays
from F out parallel to the optical axis after crossing the lens, Analogously, when a beam of
light rays sight point F located at a distance f behind a diverging lens, it will parallel to the
optical axis of the lens.
Image focus (F '): is the point F', at a distance f´’ behind where a converging lens converges
after passing through the lens, a bundle of rays incident parallel to the optical axis. If parallel
rays impinging on the optical axis a diverging lens, the beam seems to come from a refracted
point F'in front of the lens at a distance f.
Figure 2: Focus object and focus image of a converging lens.
Lateral Raise (A)
It refers to the size of the image obtained.

3. =
ℎ ℎ (ℎ )
ℎ ℎ (ℎ )
=
Lens power (P): is the inverse of the focal length, the value in the International System of
Units is expressed in diopters. The power measuring the degree convergence of the emerging
rays, i.e. higher power than greater convergence of rays. The power is positive for converging
and negative for a diverging lens
=
1
′
Sign criteria for distances lenses
1) Object distance (So): positive if the object is located on the side where the light originates;
negative if the object is located on the opposite side.
2) Distance image (Si): positive if the image is located on the side to which light passes;
negative if the image is located on the opposite side.
3) Focal length image (f '): Positive if the image is located on the side to which light passes;
negative if the image is located on the opposite side.
The lens and the human eye
Vision abnormalities:
1) Normal eye: the images are formed on the retina.
2) Myopia: Refractive defect where the image is created in front of the retina. I usually occurs
because the eye has more power than necessary. It is corrected with a diverging lens.
3) Sightedness: refraction defect where the image is created behind the retina. It usually
occurs because the eye does not have enough to focus the image and therefore power is
blurred. It is corrected with a converging lens.
Figure 3: Parts of the human eye.
3. LABORATORY MATERIALS:
Bottom with stem for light box
Halogen light box, 12V - 20 W and grids
Lens assembly slidingly +50
Lens assembly slidingly +100
Sliding lens mount - 50
Slide assembly for optical bench
Diaphragm support and / or slide
White sheets A4
Optical Riel
Slide Emperor Maximilian
DC power source from 0 to 12V, 2A / AC: 6V,
12V, 5A
White screen
Circular diaphragm d = 20mm
Tripods variables
Steel rods
Model of the human eye
4. EXPERIENCES:

4. Activity 1: Determination of the focal distance and obtaining the image in a
converging lens and a diverging lens with parallel rays
1.) Draw in the center of a white sheet, placed horizontally, the optical axis this place to 7cm
(from right to left sheet) the center of the converging lens. Then impinging the light beam for
the focus object and image focus. Obtaining an object image corresponding to 3.5 cm. The
article is placed in the following points (So): a distance on the left side of the center of the lens
after the focus identified at a distance located to the left of center of the lens after the
identified focus (as close as possible the focus), located at a distance between the center of the
lens and focus identified.
2.) Make the experience for a diverging lens, taking into consideration that 7cm where will be
located the center of the lens is from left to right and only a value of So.
Activity 2: Imaging in converging or diverging lens focal length known
Perform the following assemblies described below in the sequence indicated by the arrows:
1.) Light Source circular diaphragm (zero point of the optical rail) slide
emperor (at a distance x iris and lens) lens (+100)white screen (Image).
2.) Light Source circular diaphragm (zero point of the optical rail)slide emperor (at a distance
x iris and lens) lens(+50) placed in 10cm lens (+100) placed in 20cmwhite screen (image).
3.) Light Source circular diaphragm (zero point of the optical rail) lens (-50) placed in 5cm
slide emperor (at a distance x iris and lens) white screen.
4.) Light Source circular diaphragm (zero point of the optical rail)lens (+100) placed in 10cm
slide emperor (at a distance x iris and lens) white screen (image).
5-.) Light Source circular diaphragm (zero point of the optical rail)lens (+50) placed in 5cm
slide emperor (at a distance x iris and lens) white screen (image).
Activity 3: Study of the human eye by using lenses
1-.) Using the model eye locate this operation the normal eye.
2.) Do the experience again and move the lens so as to obtain a myopic eye.
3-.) Then, obtained through the lens movement, a hyperopic eye.
5. DATA EVALUATION:
ACTIVITY 1
Get the appropriate type of lens used to image and perform such calculations.
Lens
So
(cm)
Si
(cm)
hi
(cm)
A Imageobtained
Convergent
Divergent
ACTIVITY 2
Identify the type of lens used in each experiment and complete the following table:

5. N° Lens D1
(cm)
D2
(cm)
D3
(cm)
D4
(cm)
Image
1
2
3
4
5
D1:distance between the diaphragm and the source
D2:distance between the diaphragm and the slide
D3:distance between the slide and the lens
D4:distance between the lens and the screen (image)
ACTIVITY 3
1) Set the distances from the source to the lens to generate the eye: Normal, myopic and
farsighted.
2.) Set the distances between the lenses to generate the respective corrections
State of theeye Used Lens D1 Lens
(Correction)
D2
Normal
Myopic
Farsighted
D1:Distance from the source to the lens
D2:Distance from the lens to the model
Specific questions
1-.) In a lens, how the location of the object focus and the focus image is determined, and how
they differ?
2-.) What happens to the image if it moves away or white screen approaches the lens?
3-.) What kind of image you are obtained if the focal length is not adhered to locate the lens
on the optical rail?
4-.) According to the images obtained, when do you think is required to use each type of lens?
5-.) How important is power when selecting the type of lens to use?
6-.) What advantages does the use of lenses in correcting vision abnormalities?
4-.EXTRA HELP LITERATURE
• Jerry D. Wilson, Anthony J. Buffa and Bo Lou. Physical. Pearson Prentice Hall, 2007
• Paul A. Tipler and Gene Mosca. Physics for Science and Technology, 10th edition Editorial
Reverte, 2007
• Paul G. Hewitt. Conceptual Physics, 11th edition. Pearson Education, 2009
• Raymond A. and C. VuilleSerway. College physics. Cengage Learning, 2011
• Richard P. Feynman, Robert B. Leighton, and Matthew L. Sands. The Feynman Lectures on
Physics "vol. 1. Addison Wesley, 1989

6. Links:
Converging and diverging lenses
https://www.youtube.com/watch?v=7BQnCyutdWs
Converging and diverging lenses
https://www.youtube.com/watch?v=Hd8HmD2pGrE
Imaging in a Converging Lens
https://www.youtube.com/watch?v=bHEb8kqK7aA
Magnifying glasses, ALL CASES
https://www.youtube.com/watch?v=FdQpC3Itrio