Light

Light

Rules for image formation by spherical mirror and ray diagram
The intersection of at least two reflected rays give the position of image of the point object.  Any two of the following rays can be considered for locating the image.
1.A ray parallel to the principal axis, after reflection, will pass through the principal focus n case of a concave mirror or appear to diverge from the principal focus in case of a convex mirror.


2.  A ray passing through the principal focus of a concave mirror or a ray which is directed towards the principal focus of a convex mirror, after reflection, will emerge parallel to the principal axis.

3.    A ray passing through the centre of curvature of a concave mirror or directed in the direction of the centre of curvature of a convex mirror, after reflection, is reflected back along the same path. The light rays come back along the same path because the incident rays fall on the mirror along the normal to the reflecting surface.

Concave mirror: Reflection takes place at concave surface, Focal length of it is negative, It is a converging mirror, It has a real focus
Convex mirror: Reflection takes place at convex surface, Focal length of it is positive.
It is a diverging mirror, It has a virtual focus.
Uses of Concave Mirror:
1)    As reflector in electric torch, head lights of vehicle, search light, etc. The source of light is put at the focus of the reflector, which produces a strong parallel beam of light, which helps in clear visibility.
2)    As shaving mirror to produce larger image of face to facilitate better viewing during shaving.
3)    Concave mirror is used by dentists to see larger image of teeth of the patient. When a tooth is placed between focus and pole, the concave mirror produces a magnified image of the tooth
Uses of Convex Mirror:
1)    Convex mirror is used in rear view mirror of vehicles; so that the driver can see the traffic coming from behind. The field of view is widest in case of a convex mirror, which enables it to show a wider area from behind.
2)    Convex mirror is used on hairpin bends on the road; so that the driver can see the traffic approaching from another side of the bend.
Lenses:    
Spherical lenses are the piece of transparent glass bounded by two spherical mirrors. On the surface of which light rays are refracted. The two types of lenses are concave lens and convex lens.
Convex lens:
    A lens, that is thick at the centre but thinner at the edges.

Concave lens:
    A lens, that is thin in the middle but thicker at the edges.

Power of lens : The reciprocal of the focal length of a lens is called its power or focal power    
    \( P = \frac{1} {f} \) ( if ‘f’ is in metre) 
    =\( \frac{{100}} {f} \)  ( if ‘f’ is in cm) 
    Unit : diopter
    It is positive for convex less and negative for concave lens.
Terms related to lens:
i)  Optic centre (C): The centre point of a lens is known as optic centre.
ii)  Principal axis : It is the line passing through the optic centre of the lens and perpendicular to both the faces of the lens.
iii) Principal Focus (F) : It is a point on its principal axis to which light rays move parallel to the principal axis converge after passing through the convex lens and appear to diverge after passing through the concave lens.
iv) Focal length of a lens(f) : It is the distance between optical centre and the principle focus of a lens.

Position and nature of image fromed by concave lens:
The images formed behind the lens, it is erect, diminished.