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) Incoming search terms:class 9 science sample paper for [...]

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 [...]