Conventional mirrors do not transmit light, and nothing can be seen through them. If Y is a concave lens, then a virtual image of X is seen when the eye is placed on the other side of the lens, this is the conventional case. This is not obvious,hiwever,for the case when Y is a convex lens. Here,X is at twice the focal lenght from the lens. A real image is brought to a focus on the other side of the lens and 20 cm. from the lens. But there is no screen there,and the light keeps tavelling on and will diverge as though an object were there. To the eye it will be the samevas though an object were 25 cm. away from it.