A student focussed the image of a candle flame
Here, we will measure all distance with respect to the potion of convex lens.
the object distance will be, u = 12.6 cm - 51.8 cm = -39.2 cm
the image distance will be, v = 91.0 cm - 51.8 cm = 39.2 cm
so, the focal length will be calculated using the lens formula
1/f = 1/v - 1/u
1/f = (2 / 39.2)
thus, the focal length fot he lens is
f = 19.6 cm
the image distance will be, u = 32.2 cm - 51.8 cm = -19.6 cm
and focal length, f = 19.6 cm
1/v = 1/f + 1/u
1/v = 1/19.6 - 1/19.6
v = infinity, the image will be formed at infinity as the object is kept on the point of focus of the lens.
The object is shifted towards the lens, the object distance thus decreases and so we would see a variation in image characteristics. We suggest you look at the ray diagrams in lesson notes for more...