Civil Engineering - Surveying - Discussion

22. 

Two concave lenses of 60 cm focal length are cemented on either side of a convex lens of 15 cm focal length. The focal length of the combination is

[A]. 10 cm
[B]. 20 cm
[C]. 30 cm
[D]. 40 cm

Answer: Option C

Explanation:

No answer description available for this question.

Pawan Kumar said: (Nov 30, 2013)  
1/f = -1/60 -1/60 +1/15 = 1/30/

f = 30.

Karan Dumbare said: (Jul 25, 2014)  
1/f = 1/F1+1/F2.
1/60 = 1/15+1/F2.
F = 30.

Manasa Reddy said: (Sep 14, 2014)  
If we substitute d values in above formula am getting f2=20.

Nitin said: (Sep 20, 2014)  
1/f = 1/f1 + 1/f2.
1/f = 1/60 + 1/60.

= 1/30. So combination is 30.

M.Lokeswara Rao said: (Oct 7, 2014)  
1/f = 1/f1+1/f2.

1/60 = 1/15+1/f2.

f2 = 30.

Ravi said: (Nov 2, 2014)  
1/f = 1/f1+1/f2.

1/60 = 1/15+1/f2.

1/60-1/15 = 1/f2.

1/f2 = 1/30.

f2 = 30.

Aswin said: (Dec 20, 2014)  
How is this calculated? Please explain.

Saubhagya Laxmi said: (Jan 24, 2015)  
1/60-1/15 = -1/20 then how the answer z 30?

Suresh Awasthi said: (Apr 3, 2015)  
Formula: 1/f = 1/f1 + 1/f2.

According to the problem 60 cm on the either side i.e. f1 = f2 = 60 & focal length combination f = ?

Now, 1/f = 1/f1 + 1/f2.

1/f = 1/60 + 1/60.

= 1/30. So combination is 30.

Priya said: (Jul 4, 2015)  
Please explain again.

Lokesh said: (Jul 19, 2015)  
Convex is magnifying glass, makes image larger. So its behavior is making big +ve. Concave is indicated as -ve.

Now three lenses are combined to make single lens. To get the property of new lens we have to find out its focal length, it can be calculated by 1/f = 1/f1+1/f2+1/f3.

f1 = f2 = 60, f3 = 15. You will get f = 30.

Sameer Kashmiri said: (May 7, 2016)  
1/f =- (1/f1) - (1/f2) + (1/f3).
1/f =- (1/60) - (1/60) + (1/15).
1/f =- (1/30) + (1/15).
1/f = 15/450.
1/f = 1/30.

So, f = 30.

Sammy said: (May 9, 2016)  
1/f = 1/f1 + 1/f2.

1/60 = 1/15 + 1/f2.

So, f2 = 30.

Katakamsaketh said: (Jul 20, 2016)  
We know that the focal length of the concave lens is negative and convex lens is positive then the focal length of the combination can be found out by using lens formula.

R@Ghv said: (Aug 14, 2016)  
It's a three lens combination so I agree with you @Sammer Kashmiri.

Siddh said: (Sep 16, 2016)  
Two concave lens = 60cm and convex lens = 15cm,
then 1/f = -1/f1 - 1/f2 + 1/f3
= -1/60 - 1/60 + 1/15
= -1/60 - 1/60 + 4/60
= 4 - 2/60
= 2/60
= 1/30.
So, the combination is 30 cm.

Amit Ami Sharma said: (Dec 15, 2016)  
Yes, you are correct @Lokesh.

Avinash said: (Mar 28, 2017)  
1/60 = 1/15 + 1/f2
if we solve, it is coming 20.

Then how it is 30? Anyone explain me.

Shubham Nimbal said: (Apr 6, 2017)  
How come 1/30?

Manu said: (Apr 18, 2017)  
1/60 = 1/15 + 1/f2.

if we solve, it is coming 20.

Pankaj Yadav said: (May 1, 2017)  
1/f = 1/60+1/60,
1/f = 1/30.

The combination of concave f = 30
add with convex,
1/f = 1/30+1/15,
1/f =1/10,
f = 10 cm of combination.

Pankaj Yadav said: (May 3, 2017)  
1/f= -1/60-1/60.
1/f= -1/30.
1/f' = -1/30+1/15.
1/f'= 1/30,
f'=30.

Ratan Pal said: (Jul 6, 2017)  
I think 1/f=1/15 -1/60 -1/60.

Vamsi said: (Sep 7, 2017)  
Explain this sum cleary.

Priya said: (Oct 6, 2017)  
Explain it clearly.

Rinku Singh said: (Oct 20, 2017)  
f1=f2=-60.
1/F1= 1/-60 +1/-60 =1/-30,
F1 =-30 and given F2 = 15,
1/F = 1/-30 +1/15 =1/30,
F =30 ans.

Akash said: (Nov 17, 2017)  
1/f = 1/f1+1/f2.
1/f = 1/60+1/60.
1/f = 1/30 answer.

Rafi said: (Jan 15, 2018)  
1/f = -1/f1+1/f2-1/f3
1/f= -1/60+1/15-1/60
1/f = 4/60 -2/60
1/f = 2/60
f = 30cm.

Akshay Gupta said: (Jan 18, 2018)  
Formula: 1/f = 1/f1 + 1/f2.

According to the problem 60 cm on the either side i.e. f1 = f2 = 60 & focal length combination f = ?
Now, 1/f = 1/f1 + 1/f2.
1/f = 1/60 + 1/60.
= 1/30. So combination is 30.

Julia Roberts said: (Jan 21, 2018)  
1/f=1/f1+1/f2.
concave=-ve.
convex=+ve.
1/f=1/15-(1/60+1/60).
.: f=30.

Hariom said: (Feb 15, 2018)  
Yes. It is 30.

Sammy said: (Apr 29, 2018)  
Simple.

1/f = -1/f -1/f + 1/f.
1/ f= -1/60 -1/60 + 1/15.
1/f= -1/30 + 1/15.
1/f= 1/30.
F=30.

Rashmi said: (Jul 10, 2018)  
Please explain me the answer.

Tskd said: (Jan 5, 2019)  
Since two cancave is sandwitched with one convex ie.,
1/f = 1/60 + 1/60 - 1/15,
1/f = 1/30.

Dev Kumavat said: (Mar 14, 2019)  
Thank you @Sameer Kashmiri.

Naveen said: (Feb 8, 2020)  
1/f = 1/F1+1/F2.
1/60 = 1/15+1/F2.
F = 30.

Poj said: (Mar 5, 2021)  
Concave (-) and convex (+).

Given:
Two concave i.e F1 = -1/60.
F2 = -1/60.

One convex lens i.e F3 = 1/15.
combined focal length = (F1 + F2) + F3.
= { ( -1/60) + ( -1/60) } + 1/15,
= (-2/60) + 1/15,
= -1/30 + 1/15,
= -1+2/30,
= 1/30 = 30.

Neeraj Kumar Singh said: (Aug 19, 2021)  
1/f = 1/15 - 1/60 - 1/60.
1/f = 1/30,
F = 30.

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