Measurement of focal length of the combination of the two lenses separated by a distance
1.https://www.researchgate.net/publication/344172103_CALCULATION_OF_THE_FOCAL_LENGTH_OF_A_TWO-LENS_SYSTEM_-_AN_EDUCATIONAL_EXPERIMENT
2.https://www.cmu.edu/biolphys/smsl/teaching/IntermedOptics/IntOptics_data/lab%20manuals/Lab2%20-%20Lenses.pdf
3. http://e-jst.teiath.gr/issues/issue_51/Zekou_51.pdf
4. https://www.doubtnut.com/question-answer-physics/a-determine-the-effective-focal-length-of-the-combination-of-the-two-lenses-in-exercise-if-they-are--20692932
5. http://satish0402.weebly.com/uploads/9/4/6/7/9467277/nodal_slide.pdf
6.https://www.veerashaivacollege.org/images/stories/modified_Opt.Instrumentnew.pdf