.A man wants to get a picture of a zebra. He photographed a white donkey after fitting a glass,
with black stripes, on to the lens of his camera. What photo will he get?
When you cover half the lens youget a fainter image. Effectively you are cutting out half the light rays, but they still produce an entire image. However half of the rays from all parts of the image are blocked, so that only half reach the image point, giving only half the intensity of the complete lens.
In our case the man cover someof lens area with black stripes. So, he obtain photo of the white donkey with less intensity.