Physics 2011 JAMB Past Questions - Question 32: An image in a convex lens is upright of the lens is 15cm, what is this object distance ?

Correct Answer

C

Explanation

1/u - 1/v = 1/f =1/u - 1/3u =1/15=15 - 5 = u/15u therefore u = 10cm
To determine the object distance in a convex lens, we can use the lens formula:

1/f = 1/v - 1/u

Where:
f is the focal length of the lens,
v is the image distance,
u is the object distance.

In this case, since the image formed by the convex lens is upright, the image distance (v) will be positive. The focal length (f) can be determined using the lens maker's formula or given in the problem statement.

Let's assume the focal length (f) of the convex lens is 15 cm. We can substitute the values into the lens formula and solve for the object distance (u):

1/15 = 1/v - 1/u

Since the image is formed on the same side as the object (upright image), the image distance (v) will be positive. Let's assume the image distance (v) is also 15 cm:

1/15 = 1/15 - 1/u

Simplifying the equation:

1/15 - 1/15 = -1/u

0 = -1/u

Since the left side of the equation is zero, the right side must also be zero. Therefore, -1/u = 0, which implies that the object distance (u) is infinity.

Hence, if the image formed by the convex lens is upright and the lens has a focal length of 15 cm, the object distance is infinity.