Analyse the following observation table showing variation of image-distance (v) with object-distance (u) in case of a convex lens and answer the questions that follow without doing any calculations : S. No. 1 2 3 4 5 6 Object-Distance u (cm)–100–60 –40–30–25 –15Image-Distance v (cm)+25+30+40+60+100+120(a) What is the focal length of the convex lens? Give reason to justify your answer.(b) Write the serial number of the observation which is not correct. On what basis have you arrived at this conclusion?(c) Select an appropriate scale and draw a ray diagram for the observation at S.No. 2. Also find the approximate value of magnication.
Answers
1.From the observation 3, we can say that Radius of curvature is 40cm so foal length of the convex lens is 20cm.[f=R/2]
2. Observation 6 is incorrect , as the object is in between P and F, the image formed will be virtual image on the same side of object, image distance should be negative.
3.Ray diagram for observation 3: Please refer attachment : object beyond C
4.magnification=m=v/u m=30/-60=-1/2= -0.5so the image is real, inverted and diminished.
Answer:
1.From the observation 3, we can say that Radius of curvature is 40cm so foal length of the convex lens is 20cm.[f=R/2]
2. Observation 6 is incorrect , as the object is in between P and F, the image formed will be virtual image on the same side of object, image distance should be negative.
3.Ray diagram for observation 3: Please refer attachment : object beyond C
4. magnification=m=v/um=30/-60=-1/2= -0.5so the image is real, inverted and diminished.
Explanation: