Many spells are cast as if they were missiles that burst from the wizard and fly towards their target. Such spells are described as hitting the first target that lies in their path in the same way as a crossbow shot or a bolt thrower, for example.
These spells can be projected from the wizard in any direction, in front, to the side, or even behind him if he wants. The wizard is assumed to be able to turn freely and face whatever direction he wishes before unleashing his spell. Obviously there must be nothing in the way of the spell which would block its path - such as a wall, building or a wood. The restrictions are exactly the same as for an ordinary missile weapon - see p25 of the Warhammer rulebook for a description of how scenery blocks sight.
Not all missile-type spells are projected in a straight line as described above. Some are capable of dodging and weaving to find their target, or they manifest themselves directly upon their victim. If the card does not specify that the spell travels along a straight path then it does not have to do so. Such a spell may be cast even if the wizard cannot see his target, and the spell cannot be blocked by scenery as it will simply move over or around it.
Missile-type spells fall into two broad categories: those that can be directed against a single model chosen by the player, and those that can only be directed against a unit as a whole in the same way as normal bow fire.
Single Model
If the spell specifies that the target is a single model the spell can be cast against any chosen model within the range indicated on the spell card. In the case of some spells a magic projectile shoots forward in a straight path like a pistol shot as described above. For example, in the Bright Magic spell Fire Ball a ball of magic fire shoots out of the wizard's hand and strikes the first model in its path. A Fire Ball spell travels along a straight path and so will be blocked by intervening troops or scenery.
Other spells which can be directed against a single model arc high up into the air and so can be directed at any model in range, an example of this kind of spell is the Gold Magic spell Gleaming Arrow. Spells which specify a single model as the target may be cast against a model regardless of whether it is part of or close to a unit. Obviously these spells are very useful for taking out specific enemy heroes and wizards.
Unit
Some spells specify that they can only be cast against a unit as a whole rather than against a specific model. This is usually because the magic consists of a hail of small missiles which spreads out and hits several individuals amongst a mass of troops. Casualties from these spells are worked out exactly as if the target were shot at by a group of bowmen, crossbowmen, or similar. Normal restrictions regarding shooting at characters apply (see the Warhammer rulebook, p65).
Bear in mind that although the target is described as a unit, a character model is eligible under certain circumstances, as are large monsters, war machines, chariots, and so forth. This is exactly the same as for ordinary missile-armed troops.