This is because two independent sources of light cannot be coherent, as their relative phases are changing randomly.

Heat waves can be polarised because these are transverse waves, whereas sound waves cannot be polarised because these are longitudinal waves.
Transverse waves can oscillate in the direction perpendicular to the direction of its propagation but longitudinal waves like sound waves oscillate only along the direction of its propagation. So, longitudinal waves cannot be polarised.

No,it is because in lens maker's formula,
\(\frac { 1 }{ v } -\frac { 1 }{ u } =(\mu -1)(\frac { 1 }{ R_{ 1 } } -\frac { 1 }{ R_{ 2 } } )\)
\(\ =-(\mu -1)(\frac { 1 }{ R_{ 1 } } -\frac { 1 }{ R_{ 2 } } )\)
on reversing the lens values and signs get reversed.

Angle of prism, \(A={ 5 }^{ 0 },{ \mu }_{ R }=1.641,{ \mu }_{ B }=1.659\)
As, we know that, \(\delta =(\mu -1)A\) 
\(So,{ \mu }_{ B }=({ \mu }_{ B }-1)A \ and \ { \mu }_{ R }=({ \mu }_{ R }-1)A\)
\(So,{ \mu }_{ B }-{ \mu }_{ R }=({ \mu }_{ B }-{ \mu }_{ R })A\)
\(=(1.659-1.641)\times 5\)
\( ={ 0.09 }^{ 0 }\)

As the refractive lens index for red is less than that for blue light and a lens will be bent towards the axis for blue light as compared to red
\( (\frac { 1 }{ f } =(n_{ 21 }-1)\frac { 1 }{ R_{ 1 } } -\frac { 1 }{ R_{ 2 } } ,\ f_{ b }\)
focal length of blue light will be smaller than that of red.