What are the units of refractive index?

Which of the following statements is true?

At what distance should an object be placed to obtain its real,inverted and of same size image as the object by a convex lens?

The image of a candle flame formed by a lens is obtained on a screen placed on the other side of the lens. If the image is three times the size of the flame and the distance between the lens and the image is $80 \, cm$,at what distance should the candle be placed from the lens? What is the nature of the image at a distance of $80 \, cm$ from the lens (in $, cm$)?

The figure below shows a lens treated as a combination of two prisms and a glass block. $(i)$ Name the lens formed by the combination. $(ii)$ Is the lens converging or diverging? $(iii)$ Draw the refraction of two incident rays to illustrate the property of the lens mentioned by you in part $(ii).$

Light waves are said to travel in a straight line because ...