Sudha finds out that the sharp image of the window pane of her science laboratory is formed at a distance of $15 \mathrm{cm}$ from the lens. She now tries to focus the building visible to her outside the window instead of the window pane without disturbing the lens. In which direction will she move the screen to obtain a sharp image of the building? What is the approximate focal length of this lens?

Under what condition in an arrangement of two plane mirrors, incident ray and reflected ray will always be parallel to each other, whatever may be angle of incidence. Show the same with the help of diagram.

Draw a ray diagram showing the path of rays of light when it enters with oblique incidence from air into water;

Draw a ray diagram showing the path of rays of light when it enters with oblique incidence from water into air.

Draw ray diagram showing the image formation by a concave mirror when an object is placed between pole and focus of the mirror.

Draw ray diagram showing the image formation by a concave mirror when an object is placed between focus and centre of curvature of the mirror.

Draw ray diagram showing the image formation by a concave mirror when an object is placed at centre of curvature of the mirror.

Draw ray diagram showing the image formation by a concave mirror when an object is placed a little beyond centre of curvature of the mirror.