Civil Engineering - Surveying - Discussion
Discussion Forum : Surveying - Section 1 (Q.No. 22)
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
Discussion:
38 comments Page 3 of 4.
Avinash said:
8 years ago
1/60 = 1/15 + 1/f2
if we solve, it is coming 20.
Then how it is 30? Anyone explain me.
if we solve, it is coming 20.
Then how it is 30? Anyone explain me.
Amit ami sharmA said:
9 years ago
Yes, you are correct @Lokesh.
Siddh said:
9 years ago
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.
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.
R@ghv said:
9 years ago
It's a three lens combination so I agree with you @Sammer Kashmiri.
Katakamsaketh said:
9 years ago
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.
Sammy said:
9 years ago
1/f = 1/f1 + 1/f2.
1/60 = 1/15 + 1/f2.
So, f2 = 30.
1/60 = 1/15 + 1/f2.
So, f2 = 30.
Sameer kashmiri said:
9 years ago
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.
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.
Lokesh said:
1 decade ago
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.
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.
Priya said:
1 decade ago
Please explain again.
Suresh Awasthi said:
1 decade ago
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.
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.
Post your comments here:
Quick links
Quantitative Aptitude
Verbal (English)
Reasoning
Programming
Interview
Placement Papers