NEW SIGN CONVENTION OF CONVEX AND CONCAVE LENS.

 New sign convention for convex and concave lenses. 

1. The optical centre of the lens matches with the origin of  two dimensional cartesian system ( the point of intersection of x-axis and y-axis).

2. The object should be placed at the left side of the lens( towards negative x-axis).

3. The distances are measured from the optical centre of the lens( the origin). Assign the signs according to the signs of the segments of the axes.(x or y axis).

For CONVEX LENS.

4. The  object distance (u) is always negative.

5. The image distance (v) is positive for real images.

6. The image distance (v) is negative for virtual image.

7. The focal length (f) is positive. (Second local length)

For CONCAVE LENS.

8. The object distance (u) is always negative.

9. The image distance (v) is always negative as it forms virtual image.

10. The focal length (f) negative. (Second local length)

For OBJECT and IMAGE  lengths.

11. The length of the image is positive as it is always kept erect.

12. The length of the real image is negative  as it is inverted.

13. The length of the virtual image is positive. 

For MAGNIFICATION, (m=Image  length÷ object  length)

14. m is negative for real images.

15. m is positive for virtual image.

For NUMERICALS,

16.  Do not assign  any sign for unknown quantities. 

LENS FORMULA 

17.  Magnification (m)= Size of the image÷size of the object.

18. m= image distance  ÷object distance.

19.  Lens formula: 1/f= 1/v-1/u.