Understanding Image Formation by a Convex Lens

A convex lens forms an image by refracting light rays from an object. Rays originating from a point on the object pass through different parts of the lens and converge at a corresponding point in the image plane (for a real image) or appear to diverge from a point (for a virtual image).

Consider light rays from a single point on the object. These rays spread out and strike the entire surface of the lens (or at least the part of the lens through which they can pass). The lens then refracts these rays such that they converge to form the image of that specific point.

Effect of Blocking Part of a Convex Lens

When the lower half of a convex lens is made opaque, it means that light rays that would have passed through the lower half are now blocked. However, rays from every point on the object can still pass through the upper half of the lens.

• Light from each point on the object still reaches the upper, unblocked part of the lens.
• These rays are refracted by the upper half of the lens and still converge to form the image of that point.
• Since rays from all parts of the object can still pass through the remaining part of the lens, a complete image of the object is still formed.

Analyzing the Impact on Image Properties

While the entire image is still formed, blocking a portion of the lens reduces the total amount of light that passes through the lens and reaches the image plane. The brightness or intensity of an image is directly proportional to the amount of light energy received per unit area.

Since fewer light rays contribute to forming the image, the image will be less bright compared to when the entire lens is unblocked. This reduction in brightness is described as a reduction in the intensity of the image.

Let's examine the given options:

• Option 1: No change in image - This is incorrect. Blocking part of the lens does affect the image, specifically its intensity.
• Option 2: Image will show only half of the object - This is incorrect. As explained, light from all parts of the object passes through the remaining part of the lens and forms a complete image. Blocking a part of the lens affects the quantity of light, not which part of the object is imaged.
• Option 3: Intensity of image gets reduced - This is correct. Blocking the lower half reduces the effective aperture area of the lens, meaning less light energy from the object reaches the image plane. This results in a dimmer image, i.e., reduced intensity.
• Option 4: (B) and (C) only - This is incorrect because Option (B) is incorrect.

Therefore, when the lower half of a convex lens is made opaque, the intensity of the image gets reduced, but the entire image is still formed.

Revision Table: Convex Lens Properties

Additional Information: Lens Aperture and Intensity

The intensity of the image formed by a lens is related to the amount of light collected by the lens from the object. The effective area of the lens that allows light to pass through is called the aperture. When a part of the lens is blocked, the effective aperture area is reduced.

Mathematically, the intensity of the image is proportional to the square of the diameter of the lens aperture (or the area of the aperture). If half the lens is blocked, the area through which light passes is roughly halved (assuming uniform blocking). This reduction in the amount of light leads to a dimmer image.

It's important to remember that every part of the lens contributes to forming the image of every point on the object. This is why blocking a part of the lens doesn't remove a corresponding part of the image, but rather makes the entire image less bright.