question_answer

During Physics practical period, Mr. John Rohit's Physics teacher gave the students three different lenses of power 0.5 D, 4D and 10D; and asked them to identify which of these three lenses are suitable for constructing a good astronomical telescope. As the topic of optical devices were already taught in the previous class, so Rohit among all students could identify the lenses.

Read the above passage and answer the following questions:

(i) Which lenses are used as objective and which one as eyepiece?

(ii) Among two convex lenses of same focal length but of aperture \[{{A}_{1}}\] and \[{{A}_{2}}\] \[({{A}_{2}}<{{A}_{1}}),\] used as objective lens in astronomical telescope, which will be preferred and why? (Provided they have identical eyepiece)

(iii) What values are shown by Rohit?

Answer:

(i) Out of these three lenses, Rohit suggested to use a lens of power 0.5D for objective and lens of power 10D as the eyepiece, (ii) As resolving power of telescope, \[{{R}_{P}}=\frac{A}{122\lambda }\] \[\therefore {{A}_{2}}>{{A}_{1}}\] \[\therefore {{R}_{2}}>{{R}_{1}}\] Larger the aperture of objective, higher the resolving power of telescope as well as gathering of light to form image and hence brighter image will be obtained. (iii) Subject knowledge, presence of mind and intelligence.