A convex lens forms a full image of the object on a screen. If half the lens is covered with an opaque object then
A. the image disappears
B. half the image is seen
C. full image of same intensity is seen
D. full image of decreased intensity is seen

Hint: By making half of the lens opaque, we are basically reducing the area of the lens through which light rays can pass. With reduction in this, a lesser number of light rays are able to pass through the lens but light from the whole object can still pass through the lens.

Detailed step by step solution:
Before solving this question, the student should be aware of the term ‘aperture of the lens’. The aperture of a lens can be defined as the area of the lens through which light rays can pass. It is basically the amount of surface of the lens exposed to the incoming light rays from an object.
Now the intensity of an image formed by a lens or even a mirror is defined by the number of light rays forming that image. It is the role of the aperture to catch the light rays which form the image and hence, decide the brightness of the image.
Now when the half of a convex lens is covered by an opaque object then we are just reducing the aperture of the lens. Due to reduction in aperture, the number of light rays passing through the lens reduces. The light rays are coming from the whole object and only half of them can pass through the lens. The result is that the image of the whole object is formed but the intensity of this image is reduced.
Hence, the correct answer is option D.

Note: Larger the aperture, greater number of light rays can pass through the lens and brighter will be the image formed by that lens. But if the aperture of the lens is reduced then, the number of light rays passing through the lens is also reduced which leads to decrease in the brightness of the image.