- Trending Categories
- Data Structure
- Networking
- RDBMS
- Operating System
- Java
- MS Excel
- iOS
- HTML
- CSS
- Android
- Python
- C Programming
- C++
- C#
- MongoDB
- MySQL
- Javascript
- PHP
- Physics
- Chemistry
- Biology
- Mathematics
- English
- Economics
- Psychology
- Social Studies
- Fashion Studies
- Legal Studies

- Selected Reading
- UPSC IAS Exams Notes
- Developer's Best Practices
- Questions and Answers
- Effective Resume Writing
- HR Interview Questions
- Computer Glossary
- Who is Who

# A concave lens of 20 cm focal length forms an image 15 cm from the lens. Compute the object distance.

Focal length of concave lens, $f$ = $-$20 cm (focal length of a concave lens is always taken negative)

Image distance, $v$ = $-$15 cm (image distance is taken negative because image formed by a concave lens is on the left side of the lens)

To find: Object distance $(u)$.

Solution:

From the lens formula, we know that-

$\frac {1}{v}-\frac {1}{u}=\frac {1}{f}$

Substituting the given values in the formula, we get-

$\frac {1}{(-15)}-\frac {1}{u}=\frac {1}{(-20)}$

$-\frac {1}{15}-\frac {1}{u}=-\frac {1}{20}$

$\frac {1}{u}=\frac {1}{20}-\frac {1}{15}$

$\frac {1}{u}=\frac {3-4}{60}$

$\frac {1}{u}=\frac {-1}{60}$

$u=-\frac {1}{60}$

$u=-60cm$

Thus, the object is at a distance of 60 cm from the concave lens, and the negative sign implies that it is on the left side of it.