Apparatus:
Concave lens, convex lens/needles, optical bench, knitting needle.
Observation:
Length of the knitting needle =Lo =…….cm
Distance between the convex lens and concave lens =L =…….cm
Index correction for the convex concave lens = Lo – L = …..cm
Procedure:
Put a needle at P at a distance from t_{1} and without using L, determine its conjugate point O very carefully, by removing the parallax between the image of P and a needle placed at 0.
P and L{1} should not be disturbed for the rest of the experiment. Place the concave lens E between & and O_{i} it will make the light from P less convergent and the parallax will now be removed by taking the needle from 0 to 1. Light which was converging to O now converges to 1. Hence 1 is the real image of the virtual object O. Measure LO = p and LI = v and apply the index correction.
Keeping P and f_{1} fixed, shift f and get other sets of observations.
Precautions and Sources of error:
1. We should test beforehand whether and l{1} form a convex combination or not.
2. Parallax must be removed very carefully and index correction applied taking due care of its sign.
3. Position of P and I must remain unchanged when the position “t” is being adjusted.
VIVA VOCE
Q.1. What is the particular advantage of the auxiliary lens method?
Ans. Its particular advantage lies in the fact that the convex lens forms a real image of an object which acts as virtual object for concave lens and is shifted away to act as real image for it. It helps in calculation of focal length of concave lens.
Q.2. How will you find the focal length of a concave lens?
Ans. Focal length of concave lens may be found by combining it with a convex lens to get a convex combination.
Q.3. Is the power of concave lens is positive?
Ans. No. power of concave lens is negative because its focal length is negative.
Q.4. How will you distinguish between a real image and virtual image?
Ans. A real image can be obtained on a screen and a virtual image cannot be obtained on the screen. Moreover, a real image is always inverted whereas a virtual image is always erect.
Q.5. What type of image is formed by concave and convex lens?
Ans. A concave lens always forms virtual image but a convex lens forms a real image except when the object lies within the focal length.
