Question
Thu March 08, 2012 By: Neha Singh

in online test question was.. Q.Which one of the following is the use of concave lens? ans given was objective lens of a microscope and a telescope sir but convex lens is used in microscope...i hve not understood...plsz explain

Expert Reply
Wed March 14, 2012
A refracting or refractor telescope is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope)
optical diagram of Galilean telescope y - Distant object ; - Real image from objective ; y’’ - Magnified virtual image from eyepiece ; D - Entrance pupil diameter ; d - Virtual exit pupil diameter ; L1 – Objective lens ; L2 - Eyepiece lens e - Virtual exit pupil - Telescope equals [2]

The original design Galileo Galilei came up with in 1609 is commonly called a Galilean telescope. It uses a convergent (plano-convex or bi-convex) objective lens and a divergent (plano-concave or bi-concave) eyepiece lens. Galilean telescopes produce upright images.

Galileo’s best telescope magnified objects about 30 times. Because of flaws in its design, such as the shape of the lens and the narrow field of view, the images were blurry and distorted. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. The Galilean telescope could view the phases of Venus, and was able to see craters on the Moon and four moons orbiting Jupiter.

Parallel rays of light from a distant object (y) would be brought to a focus in the focal plane of the objective lens (F' L1 / y’). The (diverging) eyepiece (L2) lens intercepts these rays and renders them parallel once more. Non-parallel rays of light from the object traveling at an angle ?1 to the optical axis travel at a larger angle (?2 > ?1) after they passed through the eyepiece. This leads to an increase in the apparent angular size and is responsible for the perceived magnification.

The final image (y’’) is a virtual image, located at infinity and is the same way up as the object.

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