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A Numerical from Combination of thin lenses in contact

Two thin lenses of focal lengths f1=20cm and f2=60cm are placed in contact. find the focal length f3 of the combined lenses. find also the focal length f of a third lens placed in contact with these two that would result in overall focal length f11=-40cm. (Asked Young)

A light year is the distance light travels in one year . How many meters are there in one light year?

The speed of light in vacuum is 3 x 10^8 m/s. That means in one second it travels 300000000 metres.

As we know, in one year, there are 365 days; in each day there are 4 hours; in each hour there are 60 minutes and in each minute there are 60 seconds.

Therefore, the distance traveled by light in one year = 3 x 10^8 x 365 x 24 x 60 x 60 = 1 light year = 9.4605284 × 1015 meters

What are the characteristics of a good balance?

Angana Asked: “What are the characteristics of a good balance?”


Ans: “There are two important characteristics of a good (true) balance.

  1. The two arms must be of equal length
  2. The two pans must be of equal weight.
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