A student placed a candle flame at different distances from a convex lens and focused its image on a screen. He recorded his observation in tabular…

A student placed a candle flame at different distances from a convex lens and focused its image on a screen. He recorded his observation in tabular form as given below :

S.No.	Distance of flame from the lens (cm)	Distance of the image from the lens
$\displaystyle 1$	-$\displaystyle 90$	+ $\displaystyle 18$
$\displaystyle 2$	-$\displaystyle 60$	+$\displaystyle 20$
$\displaystyle 3$	-$\displaystyle 40$	+ $\displaystyle 24$
$\displaystyle 4$	-$\displaystyle 30$	+ $\displaystyle 30$
$\displaystyle 5$	-$\displaystyle 24$	+ $\displaystyle 40$
$\displaystyle 6$	-$\displaystyle 20$	+ $\displaystyle 60$
$\displaystyle 7$	-$\displaystyle 18$	+ $\displaystyle 90$
$\displaystyle 8$	-$\displaystyle 12$	+ $\displaystyle 120$

Analyse the observation table and on the basis of your analysis only, answer the following questions (without doing any calculations) :

What is the focal length of the convex lens used ? Give reason to justify your answer.

Which one of the sets of observations is not correct and why ?

Draw ray diagram to show image formation for any correct set of observation.