Experiment to determine the focal length of a concave lens using a convex lens by forming a real image of a virtual object for accurate measurement.
Apparatus
Concave lens, convex lens, optical bench, and knitting needle.
Observation
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Length of the knitting needle (Lo) = …… cm
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Distance between the convex and concave lens (L) = …… cm
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Index correction for the convex–concave lens = Lo – L = …… cm
Procedure
First, place the needle at point P at a known distance from the lens. Without using the concave lens, carefully determine its conjugate point O by removing the parallax between the image of P and another needle placed at O.
Once found, keep P and L₁ fixed for the rest of the experiment. Then, insert the concave lens E between P and Oᵢ. This step makes the light rays from P less convergent. Now, remove the parallax again by moving the needle from O to I.
The light, which was initially converging to O, now converges to I. Therefore, I become the real image of the virtual object O. Measure LO = p and LI = v, and apply the index correction accordingly.
Next, keeping P and f₁ constant, shift f slightly and repeat the steps to take another set of readings.
Precautions and Sources of Error
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Check beforehand whether L and L₁ form a convex combination.
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Remove parallax carefully and apply the index correction with the correct sign.
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Keep the positions of P and I fixed while adjusting the lens.
Viva Voce
Q1. What is the particular advantage of the auxiliary lens method?
Ans: This method allows the convex lens to form a real image that acts as a virtual object for the concave lens. The image is then shifted away, helping in the calculation of the concave lens’s focal length.
Q2. How can you find the focal length of a concave lens?
Ans: Combine the concave lens with a convex lens to form a convex combination. The resulting setup helps determine the focal length of the concave lens.
Q3. Is the power of a concave lens positive?
Ans: No, the power of a concave lens is negative because its focal length is negative.
Q4. How can you distinguish between a real image and a virtual image?
Ans: A real image can be projected on a screen and is inverted, while a virtual image cannot be obtained on a screen and is erect.
Q5. What types of images are formed by concave and convex lenses?
Ans: A concave lens always forms a virtual image, while a convex lens usually forms a real image, except when the object lies within its focal length.