Use the radius of the circle and compare it to the distance the object is away from the center of the circle. You probably want to avoid using sqrt, so compare the distance squared to the radius squared. If the distance is <= radius then it is intersecting. To send it in exactly the opposite direction (as opposed to reflecting it) simply negate the entire direction vector.

Use the radius of the circle and compare it to the distance the object is away from the circle. You probably want to avoid using sqrt, so compare the distance squared to the radius squared. If the distance is <= radius then it is intersecting. To send it in exactly the opposite direction (as opposed to reflecting it) simply negate the entire direction vector.