Formation of images by lenses

Position of object	Position of image	Size of image	Nature of  image
Convex Lens
At infinity	At Focus	Highly diminished	Real, inverted
Beyond 2 F	Between F and 2F	Diminished	Real, inverted
At 2F	At 2F	Of same size	Real, inverted
Between F and  2F	Beyond 2F	Enlarged	Real, inverted
At F	At infinity	Highly enlarged	Real, inverted
Between optical centre and F	The same side as in the object	Enlarged	Virtual and erect
Concave Lens
At infinity	At focus	Highly diminished	Virtual and erect
Between lens and infinity	Between lens and F on the same side	Diminished	Virtual and erect

 Dispersion of Light: When a ray of while light (or a composite light) is passed through a prism, it gets spitted into its constituents colours. This phenomenon is called dispersion of light. The coloured Patten obtained on s screen after dispersion of light is called spectrum.

·         The dispersion of light is due to different deviation suffered by different colours of light. The deviation is maximum for violet colour and minimum for red colour of light. The different colours appeared in the spectrum are on the following order, violet, indigo, blue, green, yellow, orange and red. (VIBGYOR)

·         The dispersion of light is due to different velocities of light of different colorus in medium. As a result, the refractive index of a medium is different from different of light.

·         The velocity of light in a medium is maximum for that colour for which refractive index is minimum. Clearly, the velocity of violet colour of light is minimum is a medium and retroactive index of that medium is maximum for red colour and refractive index of that medium is minimum for red colour.