Republic of Yemen
The Ministry of Education and Higher Education
Sana'a University
Faculty of Engineering
Mechatronics Engineering Department
Focal length of lenses
Done by:
Stu. Ebraheem Sameer Hanthel
00
Group:2
System:( PrivateExpense )
Supervise by:
Dr. Hossam AL-Khatip
1. Introduction
The experiment on the focal length of lenses is designed to help understand how a lens
converges or diverges light rays and how its focal length can be determined. The focal
length is the distance between the lens and the point where parallel light rays converge or
appear to diverge. This property is essential in applications like cameras, eyeglasses,
microscopes, and telescopes.
2. Object
The objective of this experiment is to determine the focal length of convex (converging)
and concave (diverging) lenses using different methods such as the lens formula and
focusing parallel light rays.
3. Experiment Tools
Convex lens
Concave lens
Optical bench or lens holder
Light source (e.g., a candle or distant light source)
Screen (to observe the image)
Ruler or measuring tape
Raybox (optional, for parallel rays)
Meter scale
4. Experiment Connection Diagram
A simple diagram would show:
A convex or concave lens placed on an optical bench.
A distant light source (like a candle or parallel rays from the sun or a raybox).
A screen placed on the optical bench to capture the focused or diverged light.
For a convex lens, the light rays will converge to a point on the screen. For a concave
lens, the rays diverge, and the focus is virtual.
5. Procedures
For a Convex Lens:
1. Set up the optical bench with the convex lens and a distant object (like a candle or
sunlit object).
2. Move the screen along the optical bench until a sharp image of the object is obtained.
3. Measure the distance between the lens and the screen, which gives the approximate
focal length.
4. Repeat the process for more accurate results and average the measurements.
For a Concave Lens:
1. Place the concave lens on the optical bench.
2. Use a convex lens of known focal length to create parallel light rays.
3. Observe the divergence of the rays through the concave lens.
4. Use the lens formula or graph method to calculate the focal length from the data
collected.
6. Results
Object Image 1 / 1 / cm- V+u cm
cm1 1v
distance distance u
v
u /
/ cm cm
17 24 0.059 0.042 41
15 28 0.067 0.036 43
12 44 0.083 0.023 56
20 19.3 0.050 0.052 39.3
25 16.3 0.040 0.061 41.3
30 15 0.033 0.067 45
35 14 0.029 0.071 49
33.33 14.28 0.03 0.07 47.61
12.5 50 0.08 0.02 62.5
100 11.11 0.01 0.09 111.11
7. Curve
1 / v (cm)
0.1
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0
u+v
120
100
80
60
40
20
0
120 100 80 60 40 20 0
8. Conclusion
The experiment successfully determined the focal length of the lenses. For convex
lenses, the focal length is the distance where parallel rays converge to a point. For
concave lenses, the focal length is calculated based on the virtual focus of diverging rays.
The results agree with the theoretical principles of lenses.
9. Discussion
This experiment demonstrates the fundamental properties of lenses and their ability to
manipulate light. The accuracy of the measurements may be affected by factors like lens
quality, alignment, and parallax error while reading distances. In real-life applications,
lenses with varying focal lengths are critical in designing optical instruments, from simple
magnifying glasses to complex telescopic systems.
