how do you find the focal length of lens experimentally?
Answers
Answer:
Aim : To determine the focal length of a convex lens.
Apparatus : Convex lens, meter scales, V-stand, screen, candle.
Procedure :
i) Place the convex lens on a ‘V’ stand. .
ii) Light a candle and take it far away from the lens along the principal axis.
iii) Adjust the screen on the other side of the lens to get a clear image on it.
iv) Measure the distance between the ‘V’ stand and the screen which gives a rough idea of focal length of the lens.
U-V Method : (Displacement method)
Now place the candle at a distance of 60 cm from the lens, such that the flame of the candle lies along the principal axis of the lens.
Adjust the screen t© get a clear image and measure the values of object distance (u) and image distance (v) and note them in a table.
Repeat the above step for various distances like 50 cm, 40 cm, 30 cm etc., note the corresponding (v) values and note them in the table.
Calculate the focal length for each case, you find that the focal length remains constant for a given lens.
Take a v-stand and place it on a long table at the middle. (2) Place a convex lens on the v-stand. Imagine the principal axis of the lens. (3) Light a candle and ask your friend to take the candle far away from the lens along the principal axis. (4) Adjust a screen (a sheet of white paper placed perpendicular to the axis) which is on other side of the lens until you get an image on it. (5) Measure the distance of the image from the v-stand of lens (image distance 'v') and also measure the distance between the candle and stand of lens (object distance 'u'). Record the values in the table Object distance 'u' Image distance 'v' Focal length 'f' (6) Now place the candle at a distance of 60 cm from the lens, try to get an image of the candle flame on the other side on a screen. Adjust the screen till you get a clear image. (7) Measure the image distance 'v' and object distance 'u' and record the values in table. (8) Repeat the experiment for various object distances like 50 cm, 40 cm, 30 cm, etc. Measure the image distances in all cases and note them in table. (9) Using the formula 1/f = 1/v - 1/u find f in all the cases. We will observe the value 'f' is equal in all cases. This value of 'f' is the focal length of the given lens.Read more on Sarthaks.com - https://www.sarthaks.com/533591/how-do-you-find-the-focal-length-of-a-given-lens-experimentally-in-your-lab