How many focal points does a lens have ? How does the image distance and magnifi
ID: 1732189 • Letter: H
Question
How many focal points does a lens have?
How does the image distance and magnification give information about the type of image is produced by a lens or multiple lenses/
What types of optical instruments use lenses?
What is the angular magnification?
What types of lenses do a microscope, a magnifying glass, and a telescope use?
What is the objective lens?
What is the eyepiece?
Define spherical and chromatic aberrations for lenses?
How can the aberrations be reduced?
What is the circle of least confusion?
Explanation / Answer
1) A lens has 2 focal points namely first focal point f1 and the second focal point f2.
2) So we know that magnification m=Image distance (v)/object distance (o). Hence we can see that magnification is directly proportional to the image distance. Now if the image distance is negative that is it is on the same side of the object then the magnification is also negative. Hence it proves that the lens used or the multiple lenses used are the concave lens if the image distance is negative as the magnification factor is only negative for a concave lens.
If on the other hand, the image distance is positive that is it is in the opposite side of the object , hence the magnification factor is positive proving that the lens used is convex as convex lenses magnification factor is positive.
3)Angular magnification is defined as the ratio angle subtended by the image formed when viewed through the lens or any other optical device to the angle subtended by the object on the eye when viewed without the usage of an optical device or lens. It is a unitless physical quantity.
4)In a magnifying glass, we use the convex lens, similarly in a microscope as well we use a convex lens if it is a simple microscope or 2 convex lens if it is a compound microscope.
Telescopes are of 2 types first is the reflecting or astronomical telescope where instead of a lens we use a concave mirror in place of an objective lens to collect parallel rays from the object and form an image at the focal point. Then we use a convex eyepiece lens for the magnification for the viewer.
5)The lens found at the bottom of the magnifying instrument near the object to be magnified is called the objective lens. It has high magnifying power and hence a short focal length.
6)The lens attached nearer to the eye is called the eyepiece lens.It is also called the ocular lens.
7)The lenses used the optical instruments have two regions, one the center and the other the edges. Due to imperfections in the edges, the light striking those parts refracts more or irregular refraction occurs thus causing light to scatter providing imperfections in magnification. This phenomenon is called spherical aberration.
Chromatic aberration is the phenomenon where all the lights after refractions don't meet at the focus due to different wavelengths of the colors of light. This causes the image to get scattered and hence error occurs.
8)Chromatic aberration can be reduced if we can follow the main 3 things
a)Put the subject which you want to magnify or see in the center of the instrument.
b)Do an experiment under low contrast. Cut off as much as light as possible. Do the experiment with the minimum contrast or light required.
c)The most important thing.Adjust the focal lengths of the lens accordingly.
Spherical aberrations can be reduced by taking a thick lens instead of a thin one so that the light can get as much exposure to the center of the lens instead of the edges.
Another method is to place an aperture or a hole just after the lens so that it blocks the rays that make the image blur hence making the image more clear.
9)Now we can reduce the chromatic aberrations but can't make it to zero. So at the least chromatic aberration condition as well there will be wavelengths of lights that will not get focused properly. Now if we draw an imaginary circle around that area then that circle formed is the circle of least confusion as the chromatic aberrations can't be reduced below that.
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