origin	The starting point of the ray in world coordinates.
direction	The direction of the ray.
distance	The length of the ray.
layerMask	A Layer mask that is used to selectively ignore colliders when casting a ray.

Returns
bool True when the ray intersects any collider, otherwise false.

Casts a ray against all colliders in the scene.

	function Update () {
		var fwd = transform.TransformDirection (Vector3.forward);
		if (Physics.Raycast (transform.position, fwd, 10)) {
			print ("There is something in front of the object!");
		}
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        Vector3 fwd = transform.TransformDirection(Vector3.forward);
        if (Physics.Raycast(transform.position, fwd, 10))
            print("There is something in front of the object!");
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		fwd as Vector3 = transform.TransformDirection(Vector3.forward)
		if Physics.Raycast(transform.position, fwd, 10):
			print('There is something in front of the object!')

Notes: This function will return false if you cast a ray from inside a sphere to the outside; this in an intended behaviour. If you move colliders from scripting or by animation, there needs to be at least one FixedUpdate executed so that the physics library can update it's data structures, before a Raycast will hit the collider at it's new position.

origin	The starting point of the ray in world coordinates.
direction	The direction of the ray.
distance	The length of the ray.
hitInfo	If true is returned, hitInfo will contain more information about where the collider was hit (See Also: RaycastHit).
layerMask	A Layer mask that is used to selectively ignore colliders when casting a ray.

Returns
bool True when the ray intersects any collider, otherwise false.

Casts a ray against all colliders in the scene and returns detailed information on what was hit.

	function Update () {
		var hit : RaycastHit;
		if (Physics.Raycast (transform.position, -Vector3.up, hit)) {
			var distanceToGround = hit.distance;
		}
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        RaycastHit hit;
        if (Physics.Raycast(transform.position, -Vector3.up, out hit))
            float distanceToGround = hit.distance;
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		hit as RaycastHit
		if Physics.Raycast(transform.position, -Vector3.up, ):
			distanceToGround as float = hit.distance

Another example:

	// Raycast up to 100 meters down
	function Update () {
		var hit : RaycastHit;
		if (Physics.Raycast (transform.position, -Vector3.up, hit, 100.0)) {
			var distanceToGround = hit.distance;
		}
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        RaycastHit hit;
        if (Physics.Raycast(transform.position, -Vector3.up, out hit, 100.0F))
            float distanceToGround = hit.distance;
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		hit as RaycastHit
		if Physics.Raycast(transform.position, -Vector3.up, , 100.0F):
			distanceToGround as float = hit.distance

ray	The starting point and direction of the ray.
distance	The length of the ray.
layerMask	A Layer mask that is used to selectively ignore colliders when casting a ray.

Returns
bool True when the ray intersects any collider, otherwise false.

Same as above using /ray.origin/ and /ray.direction/ instead of origin and direction.

function Update() {
	var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
	if (Physics.Raycast (ray, 100)) {
		print ("Hit something");
	}
}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        if (Physics.Raycast(ray, 100))
            print("Hit something");
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		ray as Ray = Camera.main.ScreenPointToRay(Input.mousePosition)
		if Physics.Raycast(ray, 100):
			print('Hit something')

ray	The starting point and direction of the ray.
distance	The length of the ray.
hitInfo	If true is returned, hitInfo will contain more information about where the collider was hit (See Also: RaycastHit).
layerMask	A Layer mask that is used to selectively ignore colliders when casting a ray.

Returns
bool True when the ray intersects any collider, otherwise false.

Same as above using /ray.origin/ and /ray.direction/ instead of origin and direction.

function Update() {
	var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
	var hit : RaycastHit;
	if (Physics.Raycast (ray, hit, 100)) {
		Debug.DrawLine (ray.origin, hit.point);
	}
}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        if (Physics.Raycast(ray, out hit, 100))
            Debug.DrawLine(ray.origin, hit.point);
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		ray as Ray = Camera.main.ScreenPointToRay(Input.mousePosition)
		hit as RaycastHit
		if Physics.Raycast(ray, , 100):
			Debug.DrawLine(ray.origin, hit.point)