♥Understanding Lenses♥

Lenses

A converging lens is called a CONVEX LENS .It is thicker at the centre than at the edges

A diverging lens is called a CONCAVE LENS .It is thinner at the centre than at the edges.

Focal Point and Focal Length of a Lens

For a convex lens light rays parallel and close to the principle axis are refracted inwards and converge to a focus point,F after passing throught the lens.

For a concave lens light rays parallel and close to the principle axis are refracted outwards and appear to diverge from the focal point,F after passing throught the lens.

Principle focus F,focus length f and optical centre C

a) The optical centre c

-This is the point mid-way between the lens surface on the principle axis.

-Rays passing throught the optical centre are not deviated,

b) The principle focus F 

-For the convex lens ,the principle focus F is the point to which all rays parallel and close to the princople axis converge.

-For the concave lens ,the principle focus F is the point from which all rays parallel to the principle axis appear to diverge after refraction by the lens . The focus is virtual.

c) The focus length f 

-the distance between the optical centre c and the focus point F

d)The principle axis 

-the line passing symmetrically throught the optical centre of the lens.

e) The focal plane  

-the plane which passes through F and perpendicular to the principle axis.

An Activity to Determine the Focal Length of a Convex Lens

A. By the Distance object Method

a) Direct a convex lens to a distant illuminated object 

b) Adjust a screen on the other side of the lens,so as toget a well-focus inverted image on it.

c) the image must be as sharp as possible.

d)The distance between the lens and the screen is the focal length of the lens.

e) This activity should be carried out in a dark room so that the image can be seen cearly/

B. By the Self-colimation Method

a) Cut a small hole in a wooden frame and fit a crosswire on it,

b) This, when illuminated acts as the objec.

c) Set up the arrangement as shown in Figure above 

d) Adjust the position of the lens so that the image I of the crosswires is formed just beside the actual crosswire O.

e)The rays of light strike the plane mirror at right angles and being reflected back.

f)The image is formed on the object itself.

g)This happens when the object O is at the principle focus of the lens.

h)Thus the focal length is the distance between the wooden frame and the lens.