Saved Bookmarks
| 1. |
Two identical glass (mu_g=3/2) equiconvex lenses of focal length f each are kept in contact. The space between the two lenses is filled with water mu_w=4/3. The focal length of the combination is |
|
Answer» Solution :From lensmaker.s formula In `1/f=(mu_g-1)((1)/(R_1)-(1)/(R_2)),R_1=R,R_2=-R` `1/f=(mu_g-1)(2/R)` `=(3/2-1)(2/R)` `therefore 1/f=1/R` `therefore 1/f=1/R` `therefore f=R` … (1)  FOCAL length of concave lens of water, In `(1)/(f.)=(mu_w-1)((1)/(R_1)-(1)/(R_2)),R_1=-R,R_2=R` `(1)/(f.)=(4/3-1)(-2/R)` `therefore (1)/(f.)=-(2)/(3R)` `therefore f.=-(3f)/(2) [becauseR=f` given] Now for lenses in contact EQUIVALENT focal length be f. `1/F=1/f+(1)/(f.)+1/f` `=2/f+(1)/(f.)` `=2/f+(1)/(f.)` `=(6-2)/(3f)` `=(4)/(3f)` |
|