Question

The distance between an object and a convex lens is changing. It is noticed that the size of the image formed on a screen is decreasing. Is the object moving in a direction towards the lens or away from it?

Answer 1

Convex Lens:

• A convex lens is a lens that has at least one curved surface bulging outwards.
• It converges the rays falling on it.

Image formation by a convex lens:

• In a convex lens, if an object is placed at infinity the image formed is real, inverted, and smaller than the object.
• 
• If the object is placed exactly at twice the focal length $\left(2F\right)$, the image formed is real, inverted, and the same size as the object.
• If the object is between focal length $\left(F\right)$ and twice the focal length $\left(2F\right)$, the image formed is real, inverted, and larger than the object.
• This implies that, as the object move towards the lens, the image gets larger in size and as it moves away, the image gets smaller in size.
• Since the image is decreasing in size, the object must be moving away from the lens.

Hence, the object is moving away from the lens.