Rays from the Camera

In the section Understanding the View Frustum, it was explained that any point in the cameraA component which creates an image of a particular viewpoint in your scene. The output is either drawn to the screen or captured as a texture. More info
See in Glossary’s view corresponds to a line in world space. It is sometimes useful to have a mathematical representation of that line and Unity can provide this in the form of a Ray object. The Ray always corresponds to a point in the view, so the Camera class provides the ScreenPointToRay and ViewportPointToRay functions. The difference between the two is that ScreenPointToRay expects the point to be provided as a pixelThe smallest unit in a computer image. Pixel size depends on your screen resolution. Pixel lighting is calculated at every screen pixel. More info
See in Glossary coordinate, while ViewportPointToRay takes normalized coordinates in the range 0..1 (where 0 represents the bottom or left and 1 represents the top or right of the view). Each of these functions returns a Ray which consists of a point of origin and a vector which shows the direction of the line from that origin. The Ray originates from the near clipping planeA plane that limits how far or close a camera can see from its current position. A camera’s viewable range is between the far and near clipping planes. See far clipping plane and near clipping plane. More info
See in Glossary rather than the Camera’s transform.position point.

Raycasting

The most common use of a Ray from the camera is to perform a raycast out into the sceneA Scene contains the environments and menus of your game. Think of each unique Scene file as a unique level. In each Scene, you place your environments, obstacles, and decorations, essentially designing and building your game in pieces. More info
See in Glossary. A raycast sends an imaginary “laser beam” along the ray from its origin until it hits a colliderAn invisible shape that is used to handle physical collisions for an object. A collider doesn’t need to be exactly the same shape as the object’s mesh - a rough approximation is often more efficient and indistinguishable in gameplay. More info
See in Glossary in the scene. Information is then returned about the object and the point that was hit in a RaycastHit object. This is a very useful way to locate an object based on its onscreen image. For example, the object at the mouse position can be determined with the following code:

using UnityEngine;
using System.Collections;

public class ExampleScript : MonoBehaviour {
    public Camera camera;

    void Start(){
        RaycastHit hit;
        Ray ray = camera.ScreenPointToRay(Input.mousePosition);
        
        if (Physics.Raycast(ray, out hit)) {
            Transform objectHit = hit.transform;
            
            // Do something with the object that was hit by the raycast.
        }
    }
}

Moving the Camera Along a Ray

It is sometimes useful to get a ray corresponding to a screen position and then move the camera along that ray. For example, you may want to allow the user to select an object with the mouse and then zoom in on it while keeping it “pinned” to the same screen position under the mouse (this might be useful when the camera is looking at a tactical map, for example). The code to do this is fairly straightforward:

using UnityEngine;
using System.Collections;

public class ExampleScript : MonoBehaviour {
    public bool zooming;
    public float zoomSpeed;
    public Camera camera;

    void Update() {
        if (zooming) {
            Ray ray = camera.ScreenPointToRay(Input.mousePosition);
            float zoomDistance = zoomSpeed * Input.GetAxis("Vertical") * Time.deltaTime;
            camera.transform.Translate(ray.direction * zoomDistance, Space.World);
        }
    }
}