MATERIAL REQUIRED
Convex Lens, Optical Bench
THEORY
A lens is a piece of transparent material which concentrates or disperses light rays when passes through them by refraction. According to the shape and purpose of the lens, they are classified into two types convex lens and concave lens.
A lens that is thicker at its center than at the edges is called a concave lens. It is also called conversing lens. The point at which all the ray of light coming from infinity meet is called focus. The distance between optical focus and principal axis is called focal length.
If u, v and f represents object distance, image distance and focal length of a lense respectively, the relation between them is given by;
1/f =1/u+1/v
PROCEDURE
- Mount convex lens at the middle of optical bench with lens holder.
- Mount one needle as object at one end of optical bench generally at 0 cm.
- Mount another needle at the another end of optical bench with needle holder taken as image.
- Look at he object needle behind the lens from image needle in front of the lens.
- The object needle must be seen as inverted.
- Try to make height of image an object image is equal.
- Try to conside tip of object and image needle by moving either image needle or lens.
- Then remove parallax error by moving head and image needle together.
- If parallax error was removed then tip of object and image needle fixed with moving head i.e: they are not separated.
OBSERVATION
| S.N. | Position of Object(O) | Position of Lens(L) | Position of Image(I) | Object distance (u)=L-O | Image distance (v)=I-L | m=v/u | 1/u | 1/v | 1/f=1/u+1/v | f | Mean f =Σf/n |
| 1 | 0 cm | 45 cm | 86.6 cm | 45 cm | 41.6 cm | 0.92 | 0.02223 | 0.02404 | 0.04627 | 21.413 cm | 64.359÷3 |
| 2 | 2.5 cm | 45 cm | 88.5 cm | 42.5 cm | 43.5 cm | 1.02 | 0.02353 | 0.02299 | 0.04652 | 21.496 cm | =21.453 cm |
| 3 | 7 cm | 48 cm | 93 cm | 41 cm | 45 cm | 1.09 | 0.02439 | 0.02223 | 0.04662 | 21.450 cm |
RESULT
The focal length of given convex lens was ………………….. cm.
CONCLUSION
Hence focal length of convex lens was calculated by optical bench method
PRECAUTION
- Parallax error must be minimized.
- Distance must be measured accurately.