Why do new born baby exposed to sunlight ?

No. under 6 months of age, new born should not be exposed to the sun at all and that older babies should be fully protected with sunscreen.

 To get adequate Vitamin D people expose the baby to sunlight. But, exposing a new born baby to sunlight is dangerous. Because,  newborn's skin is very sensitive and can burn easily. Direct sunlight should be avoided and the baby should wear appropriate clothing to cover his/her skin, including hats.

Sunlight through window for 20 mts/day (before 10 pm or after 4 pm) is enough for the child to make vitamin D.