The correct depth for sowing seeds depends upon the seed size.
Most vegetable seeds are sown in 'drills' outdoors. Whether indoors or outdoors sowings, there is a simple rule to follow for success if in doubt.
Almost without exception, all seeds can/should be planted at twice their depth in the ground or in seed boxes. Fine seed can literally just be sprinkled on the surface - where it will find its way down into the minute crevices in the soil's surface. There is no need to sow them deep. Larger seeds - such as peas and beans - can be planted in drills that are two to three times deeper than the size of the seed.
In the wild habitats, most seed simply falls from the parent plant onto the surrounding soil This is not as haphazard as it may sound, for there are inbuilt 'extras' to ensure the seed's viability and success. For instance, some plants only grow on 'rough' ground, and their resulting seeds can easily be covered in the crevices on the ground. Larger seeds are invariably covered by falling leaves to give then the requires depth.
It is often the case with some types of seed, that they will need two years before germination. Successive leaf falls ensure the safety of the seed - hiding it from ravaging seed eaters.
Clever things are plants!
See this also -
Seed Sowing Depths