An astronomical telescope has an objective of diameter 20 cm with a focal length

Question

An astronomical telescope has an objective of diameter 20 cm with a focal length of 180 cm. The telescope is used with an eyepiece of focal length 30 mm. What is the angular magnification of this telescope?
A) 6 x B) 180 x C) 60 x D) 360 x E) 540 x .                                                                                                                                                                                   
In a double slit experiment, if the separation between the two slits is 0.050 mm and the distance from the slits to a screen is 2.5 m, find the spacing between the first-order and second-order bright fringes when coherent light of wavelength 600 nm illuminates the slits.
A) 3.0 cm B) 1.5 cm C) 9.0 cm D) 6.0 cm E) 4.5 cm

Explanation / Answer

the condition for constructive interference is

d sin theta = m L

for first order

d   ( y1/D)= m L

for second order

d   ( y2/D)= (m+ 1)L

subtract both equation

(y2- y1) d/D = L( m+1 - m)

(y2- y1) d/D = L

y2- y1 = 600 nm ( 2.5 m)/ 0.050 * 10 ^-3 m

           = 3.0cm

option (A) is correct answer

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