Final answer:
No, the image cannot be formed on a screen with a concave lens, as concave lenses produce virtual images which cannot be projected onto screens.
Step-by-step explanation:
To determine whether an image can be formed on a screen by a concave lens, we can use the lens formula which is given by 1/f = 1/do + 1/di where f is the focal length of the lens, do is the distance of the object from the lens, and di is the distance of the image from the lens. In this case, Rahul is using a concave lens with a focal length of -20 cm (negative because concave lenses have a negative focal length) and the object is placed 25 cm away from the lens. Plugging the values into the formula, we have 1/-20 = 1/25 + 1/di, which simplifies to 1/di = -1/20 - 1/25 = -1/20 - 4/100 = -5/100 = -1/20. This means di = -20 cm, implying that the image is located 20 cm in front of the lens on the same side as the object.
Concave lenses produce virtual, upright, and diminished images. Since virtual images cannot be projected on a screen (because the light rays do not actually converge), the answer to the student's question is (b) No, the image cannot be formed on a screen.