Physics *Important question*

One concave & one convex lens both having focal lengths 4cm are placed very closed to each other. Their combined focal length is:
a) 2 cm
b) 4cm
c) 0
d) 1cm
e) Infinity

Question

Physics *Important question* 

One concave & one convex lens both having focal lengths 4cm are placed very closed to each other. Their combined focal length is:
a) 2 cm 
b) 4cm 
c) 0 
d) 1cm 
e) Infinity

anonymous · Answer

@ganeshie8 @mathslover @Mashy @secret66 @adrynicoleb @nincompoop @helpme1.2 @inkyvoyd @wolfe8 @Miracrown @ajeetkumarseo @Haseeb96

anonymous · Answer

@hartnn @iambatman

hartnn · Answer

http://www.schoolphysics.co.uk/age16-19/Optics/Refraction/text/Lenses_two_thin/index.html

hartnn · Answer

if there is some distance 'd' between them,
then we can use this formula
Effective focal length 
$EFD = \dfrac{f_1\times f_2}{f_1+f_2-d}$

hartnn · Answer

in our case,
f1 = 4, f2 = -4
d=0

hartnn · Answer

The first link gives you the derivation too :)

anonymous · Answer

let me solve it :)

anonymous · Answer

-16/0

hartnn · Answer

yeah, but i don't see $\large -\infty $ as one of the choice.

anonymous · Answer

e is infinity

hartnn · Answer

but its definitely not a finite number, so i'd go with +infinity

anonymous · Answer

So E is answer .

anonymous · Answer

Right ? @hartnn

hartnn · Answer

yes, it is :)

anonymous · Answer

Thank you so much ^_^ @hartnn 
Chocolate cake for you :P

hartnn · Answer

yaay! nomnomnom :D
welcome ^_^

anonymous · Answer

^_^