Ray Optics And Optical Instruments

The image formed by a concave mirror is always real when the object is placed between focal point and infinity. But when the object is placed between focal point and pole, then the image is always virtual But when the object is virtual ie, object is placed far away from the focal point, then the image is certainly real Hence, the required answer is certainly real if the object is virtual

Different focal length for different colours causes it

In chromatic aberration the image formed by a lens has coloured fringes, because the refracive index for different colours is different and hence the focal length of lens for different colours is different. So, the cause of chromatic aberration is the variation of focal length with colour

It is observed if deviation produced is minimum.
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When light enters from one medium to another, the wavelength and velocity of the light changes though the frequency remains same.