Draw ray diagram showing the image formation by a concave lens when an object is placed between focus and twice the focal length of the lens.

Draw ray diagram showing the image formation by a concave lens when an object is placed beyond twice the focal length of the lens

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

Draw ray diagram showing the image formation by a convex mirror when an object is placed at finite distance from the mirror.

Define power of a lens. What is its unit? One student uses a lens of focal length $50 \mathrm{cm}$ and another of $-50 \mathrm{cm}.$ What is the nature of the lens and its power used by each of them?

Note: To make the question academically more meaningful use lens in the last sentence instead of the word length.

A student focussed the image of a candle flame on a white screen using a convex lens. He noted down the position of the candle, screen and the lens as under

position of candle $=12.0 \mathrm{cm}$

position of convex lens $=50.0 \mathrm{cm}$

position of the screen $=88.0 \mathrm{cm}$

What is the focal length of the convex lens?

A student focussed the image of a candle flame on a white screen using a convex lens. He noted down the position of the candle, screen and the lens as under

position of candle $=12.0 \mathrm{cm}$

position of convex lens $=50.0 \mathrm{cm}$

position of the screen $=88.0 \mathrm{cm}$

Where will the image be formed, if he shifts the candle towards the lens at a position of $31.0 \mathrm{cm}$?