A concave mirror and a convex lens are held separately in water. What changes, if any, do you expect in the focal length of either?

Draw ray diagrams showing the image formation by a convex lens when an object is placed:At twice the focal length of the lens.

If the magnification of an image formed by a mirror is positive, what does it mean?

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?

Explain with the help of a diagram, why the convex lens is also called a converging lens.

Identify the device used as a spherical mirror or lens in following cases, when the image formed is virtual and erect in each case.Object is placed between infinity and device, image formed is diminished and between focus and optical centre on the same side as that of the object.

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

If the image formed by a spherical mirror for all positions of the object placed in front of it is always erect and diminished, what type of mirror is it? Draw a labelled ray diagram to support your answer.

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.

What is observed when 2mL of dilute hydrochloric acid is added to 1g of sodium carbonate taken in a clean and dry test tube? Write chemical equation for the reaction involved.