Class X Physics CBSE
1. (a) What is spherical aberration in spherical mirrors? How can it be reduced? (b) What are parabolic mirrors? List two uses of parabolic mirrors. 2. State the principle of reversibility of light. Use this principle to show that the refractive index of medium 2 with respect to medium 1 is reciprocal of the refractive index of medium 1 with respect to medium 2.
Answers
Answer:
1. (a) Spherical aberration in light microscopy
The curved surface of a lens is most often spherical (this shape is easy to make). When light refracts through a lens with a spherical surface, the rays that pass through the outer diameters of the lens will focus at a different point from rays that pass through the center. This causes the resulting image to be blurred.
When light is refracted from the outer vs. inner portions of the curved surface of a spherical converging lens (A) or diverging lens (B), the resulting image is blurred along the optic axis (i–ii).
The simplest method of reducing spherical aberration is to place an aperture, hole or "stop", in front of or after the lens. The aperture blocks out rays that blur the image. The disadvantage is, however, a greatly reduced light throughput of the lens.
This is an objective from a microscope from the 1760s. The objective lens is at the bottom. To reduce spherical aberration, the manufacturer used the small aperture (black, left) so that only light that passes through the center of the lens would be used in image formation.
(b) A parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves. Its shape is part of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis. The parabolic reflector transforms an incoming plane wave traveling along the axis into a spherical wave converging toward the focus. Conversely, a spherical wave generated by a point source placed in the focus is reflected into a plane wave propagating as a collimated beam along the axis.
2. According to principle of reversibility of light, if the path of the light is reversed after suffering a number of reflections and refractions, then it retraces its path. This means that if a light ray travels from medium 1 to medium 2 and has angle of incidence and angle of refraction as i and r respectively, then if the light is incident from medium 2 at an angle r, then the angle of refraction in medium 1 will be i.
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