Aim
Apparatus
Light source (incandescent bulb);
Rail;
Red filter;
Blue filter;
Screen (image);
Reference grid (object);
Stands;
Light blockers.
Introduction
Chromatic aberration results from the condition of state in which a lens fails to focus all colors passing through it at the same focal point. The failure occurs when the lens has got more than one refractive indices meaning that there also exists more than one focal point. Reason being that the focal point of a lens depends on its refractive index.
There exist two types of chromatic aberrations one being axial and the lateral. The axial type of aberration occurs when there exist different wavelengths of light all focused at different distances from the lens. For instance say they occur at different points on the optical axis. The second type of aberration occurs due to focusing of the different wavelengths at different positions on the focal plane.
Additionally, the two types of aberration may at times occur together; the axial type can be reduced by a process known as stopping down. Transverse which occurs at the center mainly is however not deductible through stopping down.
1f=1q+1p
The above formula is the formula for calculating the focal length of a wave, where f stands for the focal length; q is the object distances and p the image distance. Since refractive index of refraction indexed by letter n, solely depends on the wavelength of the light of a lens differs with every different colors of light. We can say that color blue for instance, has a shorter focal length on a lens compared to reed light.
C=fred-f bluefwhite×100.
(Where white is used as the reference focal length)
Conversely, the focal points of rays going through the outer part of the lens coupled with those going through the center differ and hence, the effect is known as spherical aberration. Denoted with letter ( s)
S=fcentre-fperimeterfuncovered×100
Which involves a comparison between the focal length of the lens at central part which is blocked to the focal length of the lens with the outer part of blocked with the answer given as a percentage.
Procedure
We familiarized with the equipment then set up all the different components as instructed.
We then turned on the light source
After the above preliminary steps, we started the measurement procedure. First we positioned the lens at a distance from the object then took the position measurement of the lens
We then positioned the screen to produce the sharpest image of the object on the screen.
At this point we took the measurements of the image
After measurements, we determined the focal length by repeating the procedure for ten times then getting the average using the different image positions for different lenses.
After determining the focal length, we then determined the chromatic aberration. We began by placing the red filter on an empty stand and placed the stand in front of the stand holding the object. We repeated this procedure ten times also using different lenses and image positions and an average calculated.
The red light was then replaced with a blue light and the procedure repeated.
We then determined spherical aberration by applying a blocker to the stand holding the lens and made sure the blocker was centered. Measurements were then carried out again ten times.
Finally, we applied the light blocker to the periphery ten times with different lenses and image positions.
The light source was then turned off.
Data analysis
Screen distance lens distance
1f=1q+1p
f =30.25cm
Inner filter at 15.2cm for all
Screen distance lens distance
1f=1q+1p
f=36.2
Outer
Screen distance lens distance
1f=1q+1p
f= 34.41
Red screen from light = 19.9 cm
red ƛ= 6200- µp
1 ƛ=1.60- down ×106 m-1
Screen distance lens distance
1f=1q+1p
f=32.37cm
Blue from light = 26.0
Blue : ƛ=4400-49ooAo
1ƛ= 2.3-2.0 ×106 m-1
Screen distance lens distance
1f=1q+1p
f=31.11
C= fred-f bluefwhite×100
32.37-31.1130.25×100 =4.1625%
S=fcentre-fperimeterfuncovered×100
Sources of errors
The images formed were not clear enough due to diffraction. Diffraction limits the ability of the lens to form better picture resolutions which is fundamental in image focusing and clearance. There are several categories in which aberrations fall into. These categories limit us towards getting the desired results in the experiments. They include lens flare, distortion, ghosting, spherical aberration, coma and diffractions. We could not get accurate aberration analytics due to an overlap in some of the above mentioned light interferences.
Additionally, the lens surfaces are never smooth enough for a quick and easy focusing on the screen. We realize that the image formed on the screen is not very stable and keeps varying widely with a switch on lenses.
One more source of error during the experiment arose from the lack of proper measurement tools. The length of the image was more of an estimate since the figures were to two decimal places.
Recommendation
There needs to be a proper way method of digitally measuring aberration instead of using lenses and repeating a single procedure more than ten times for accuracy on average.
Conclusion
Work cited
1. Serway, and Jewett, Physics for Scientists and Engineers. 6th edition. Thomson- Brooks/Cole, 2004
