Explanation:

1.)First two characteristics are usual for convex lenses - images would be real and inverted, given that object is placed not within focal point and center of lens.

If the object is placed at twice the focal length, from 1/u - 1/v = 1/f, we get

1/2f - 1/f = 1/v

or v = 2f, which means the image will be formed at twice the focal length. Note that both u and v are 2f and that means magnification is 1.

If the object distance is made to be higher, gradually the image will keep getting closer to focal point. Finally when the image is very far, it would be like parallel incident rays and the image will be at the focal point.

2.)Real images are formed when light rays after reflection or refraction converge at a point before a mirror or lens. While virtual images are formed when the light rays diverge after reflection or refraction. Real images appear on the screen as against virtual images never appear on the screen.