What is the power of a concave lens of focal length $25\, cm$?

How is the refractive index of a medium related to the speed of light? Obtain an expression for the refractive index of a medium with respect to another in terms of the speed of light in these two media.

Two adjacent walls and the ceiling of a rectangular room are mirror surfaced. The number of images of himself that an observer sees is

The laws of reflection hold good for

You are given a convex lens of focal length $10\, cm$. Where would you place an object to get a real,inverted and highly enlarged image of the object? Draw a ray diagram.

$A$ 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 $= 12.0\, cm$
Position of convex lens $= 50.0\, cm$
Position of the screen $= 88.0\, cm$
$(i)$ Where will the image be formed if he shifts the candle towards the lens at a position of $31.0\, cm$?
$(ii)$ What will be the nature of the image formed if he further shifts the candle towards the lens?
$(iii)$ Draw a ray diagram to show the formation of the image in case $(ii)$ as said above.