Apparatus:
Concave lens, concave mirror of short focal length, three uprights, optical bench, one and a knitting needle.
Observations and Calculations:
|
No. of obs. |
Position of | Observed | Corrected | f = p(-q) / p +(-q) | |||||
| Concave mirror M cm | Concave lens L cm | Needle at C cm | Needle at O cm | p = OL cm | q = CL cm | P cm | P cm | ||
| 1 2 3 |
|||||||||
Mean value of ‘f’=……..cm
Procedure
Find the approximate focal length of a concave mirror by directing it towards an open window and obtain a sharp image of any distant object on a screen (say paper). Measure distance b/w mirror and screen. This gives app. focal length of concave mirror. Hold it in front of light and the sharp image formed under a wall or white paper when it converges at a single point. Thus image formation takes place. Determine the index correction between lens, the needle and mirror and needle as usual with help of knitting needle. Mount the concave mirror. concave lens and Parallax needle uprights on the optical bench. Place the parallax needle in front of concave mirror at a distance of twice the focal length of concave mirror. Remove the parallax by changing the position of needle. The needle at this point will be at “C” Center of curvature of the mirror. Now insert the concave lens between the mirror and its center of curvature C without changing the position of the mirror. Now remove the parallax, between the object needle and its inverted image inside the mirror. This position ‘0’ of needle is further away from C.
Repeat the experiment three times by changing the position of the lens. Note the position of the lens ‘L’, object needle ‘O’ and center of curvature “C”. Measure OL as p and C as q. In order to get correct values of p and q, apply index correction. The distance ‘q’ should be taken as negative as the image is virtual. Find the value of focal length of lens using formula
f = p(-q) / p +(-q)
Precautions:
1. The concave mirror should be preferably of short focal length.
2. The position of the mirror should be kept fixed
3. Parallax should be removed carefully.
VIVA VOCE
Q.1. Why should be the concave mirror be of the short focal length?
Ans. To keep the combination mirror convergent over a wide range.
Q.2. What will be position of the image of an object places at the center of curvature of the concave mirror?
Ans. The image will also be formed at the center of curvature.
Q.3. What is the optical center of the lens?
Ans. It is point inside the lens with the peculiar property that any ray of light passing through it suffer no deviation in direction.
