Animated Solution for Physics - Optics: A converging lens is used to form an image on a screen. When the upper half of the lens is covered by an opaque screen

Imagine you are standing in a dark room, projecting the image of a bright candle onto a screen using a simple converging lens. It’s a beautiful, crisp image. But then, someone asks a classic physics question: "What happens if I cover the top half of the lens with my hand?"

Your intuition might scream that the top half of the image will vanish. After all, if you cover half a window, you only see half the view outside, right? But lenses don't work like windows. Let's dive into the fascinating physics of why your intuition is wrong, and what actually happens.

To understand the answer, we need to look at how light travels. When light leaves a specific point on our object (say, the tip of the candle flame), it doesn't just travel in one straight line. It scatters in all directions.

A huge cone of these light rays hits the entire surface of the lens. Some rays hit the very top edge, some hit the dead center, and some hit the bottom edge. The magic of a converging lens is that it bends all of these diverging rays back together so they meet at a single, precise point on the screen.

This means that the top half of the lens is forming a complete image of the candle all by itself. The bottom half of the lens is also forming a complete image of the candle all by itself. They are just overlaying perfectly on the screen!

Now, let's bring in our opaque screen and cover the upper half of the lens.

What happens to the light rays? The rays that were trying to pass through the top half are blocked. They hit the screen and stop. But what about the rays hitting the bottom half? They are completely unaffected! They continue to pass through the lens, bend according to Snell's Law, and converge at the exact same location on the screen.

Because these lower rays still carry information from every part of the object (the top, middle, and bottom of the candle), they successfully reconstruct the entire image. So, the first major conclusion is: A complete image will be formed.

Where Did the Brightness Go?

While the image is complete, it won't look exactly the same. Think about the energy.

The brightness, or intensity ($I$), of the image depends directly on the total number of light rays that manage to reach the screen. By covering half of the lens, we have essentially blocked 50% of the light rays that were previously contributing to the image.

Less light means less energy. Less energy means a dimmer image. Therefore, our second major conclusion is: The intensity of the image will decrease.

This is a favorite concept for JEE and NEET because it perfectly tests whether you have memorized a formula or truly visualized the physics.

Covering a part of a lens never chops off a part of the image. It only reduces the aperture area, which in turn reduces the light-gathering power of the lens. The image remains whole, just a little less bright.

So, the next time someone tries to trick you by covering a lens, you can confidently tell them: "You can't hide the image, you can only dim the lights!"