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IF HALF PART OF A CONVEX LENS IS COVERED,THE FOCAL LENGTH ____________CHANGE,BUT THE INTENSITY OF IMAGE____________.
The convex lens will form complete image of an object, even if its one half is covered. It can be understood by the following two cases.
Case I
When the upper half of the lens is covered
In this case, a ray of light coming from the object will be refracted by the lower half of the lens. These rays meet at the other side of the lens to form the image of the given object.

Case II
When the lower half of the lens is covered
In this case, a ray of light coming from the object is refracted by the upper half of the lens. These rays meet at the other side of the lens to form the image of the given object.

The focal length remains the same but the intensity will decreases.


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