A lens has power 4D. Find the focal length that lens.

A student focuses the image of a candle flame, placed at about 2 m from a convex lens of focal length 10 cm, on a screen. After that he moves gradually the flame towards the lens and each time focuses its image on the screen.

1. In which direction does he move the lens to focus the flame on the screen?
2. What happens to the size of the image of the flame formed on the screen?
3. What difference is seen in the intensity (brightness) of the image of the flame on the screen?
4. What is seen on the screen when the flame is very close (at about 5 cm) to the lens?

Draw ray diagrams showing the image formation by a convex mirror when an object is placed:
At infinity.

A boy with a mouth 5cm wide stands 2m away from a plane mirror. Where is his image and how wide is the image of his mouth?

With the help of a ray diagram, determine the position, nature and size of the image formed of an object placed at the centre of curvature of a concave mirror.

How will you distinguish between a plane mirror, a concave mirror and a convex mirror without touching them?

List four characteristics of the images formed by plane mirrors.

Describe with the help of a diagram, the nature, size and position of the image formed when an object is placed beyond the centre of curvature of a concave mirror.

State two factors on which the lateral displacement of the emergent ray depends.

When a light ray passes from air into glass, what happens to its speed? Draw a diagram to show which way the ray of light bends.