Rap Optics-2 Flashcards
thin lens
A thin lens is a transparent optical medium
bounded by two surfaces; at least one of which should be spherical.
power of a lens
Power of a lens is a measure of the convergence or divergence, which a lens introduces in the light falling on it.
The power P of
a lens is defined as the tangent of the angle by which it converges or diverges a beam of light parallel to the principal axis falling at unit distance from the optical
centre
primary focus
when an object is placed on the principal axis of a lens so that after refraction, the image is formed at infinity, then that point is called the first principal focus or object focus of lens.
secondary focus
when an object is infinity, the poisition of the image formed by the lens after refraction on the principal axis is called the second principal focus or image focus of the lens.
when a lensis surrounded by the same media on both sides, both the focal lengths are same. if they have different media then the focal lengths are different.
prism
it is transparent medium bound by plane surfaces, such that the two refracting surfaces are not parallel to each other.
conditions of minimum deviation
the refracted ray is parallel to the base of prism
i=e
r1=r2
relation b/w i and deviation
You can see that, in general, any given value of d, except for
i = e, corresponds to two values i and hence of e. This, in fact, is expected
from the symmetry of i and e in Eq. (9.35), i.e., d remains the same if i and e are interchanged. Physically, this is related
to the fact that the path of ray in Fig. 9.21 can be
traced back, resulting in the same angle of
deviation.
main considerations with an astronomical telescope
The main considerations with an astronomical telescope are its light
gathering power and its resolution or resolving power. The former clearly
depends on the area of the objective. With larger diameters, fainter objects
can be observed. The resolving power, or the ability to observe two objects
distinctly, which are in very nearly the same direction, also depends on
the diameter of the objective.
The largest lens objective
in use has a diameter of 40 inch (~1.02 m). It is at the Yerkes Observatory
in Wisconsin, USA. Such big lenses tend to be very heavy and therefore,
difficult to make and support by their edges. Further, it is rather difficult
and expensive to make such large sized lenses which form images that
are free from any kind of chromatic aberration and distortions.
advantages of cassegrain telescope
i) there are no chromatic aberrations
ii) spherical aberrations can be minimised easily using a paraboloid shape mirror
iii) mirrors are easier to support than lenses
iv) image is brighter compared to refracting type.
v) high resolving power
It has the
advantages of a large focal length in a short telescope. The largest telescope
in India is in Kavalur, Tamil Nadu. It is a 2.34 m diameter reflecting
telescope (Cassegrain). It was ground, polished, set up, and is being used
by the Indian Institute of Astrophysics, Bangalore. The largest reflecting
telescopes in the world are the pair of Keck telescopes in Hawaii, USA,
with a reflector of 10 metre in diameter.
compound microscope
consists of 2 converging lens
i) an objective of small focal length and aperture which is towards the object
ii) an eye piece lens of comparatively larger focal length and aperture towards eye of observer.
the distance between the two can be adjusted by rack and pinion method
astronomical telescope
it’s a refracting type telescope consisting of 2 converging lens
i) objective lens of large focal length and aperture, towards object to gather large amts of light from abject at infinity
ii) eye piece lens of small focal length and aperture towards eye of observer.
telescope factors
a good telescope should posses large light gathering and high resolving power.
both depend on aperture of objective lens , but lens of a large aperture has certain defects like spherical and chromatic aberrations.
telescope factors
a good telescope should posses large light gathering and high resolving power.
both depend on aperture of objective lens , but lens of a large aperture has certain defects like spherical and chromatic aberrations.
