The Use Of Lenses in Optical Dvices

 The Differences between Microscope and Telescope
                  The Compound Microscope

           

  -We use it to view very small objects.

  -Microscope consists of two convex lenses:

      a)The objective 

     -the lens which is nearest to object.

     -a powerful convex lens with a short focal length.

     -focal length of the objective is fo.

      b)Eyepiece

     -the lens which is nearest to the eye.

     -eyepiece has a longer focal length than the objective.

     -focal length of the eyepiece is fe.

                          The Telescope

                                        

 

    -Astronomical telescope consists of two convex lenses:

        a)The objective.

        b)The eyepiece.

    -The objective is a convex lens with a wide aperture and long focal length to  enable the telescope collect as much light as possible. 

    -Eyepiece is a convex lens of short focal length.

    -The picture shows three actual rays coming from the top of a distant object.Since the object is far away,the rays are assumed to be parallel when incident on objective lens.

    -The rays at point I will emerge parallel from the eyepiece and appear to come from the top of final image at infinity.

    -The total distance between the objective and the eyepiece is = fo+fe

    -The magnifying power of the telescope :

                     M=fo/fe

    -If the power of eyepiece is increasing,the power of objective is reducing,it can give a high magnification.

   

Linear Magnification

    Linear magnification is:
        the ratio of the size of the image to the size of the object.
    Linear magnification, m = size of image/size of object

Image Formed By Lenses

                                   Ray diagram

      1)-Virtual                    2)-Virtual
          -Upright                     -Upright
          -Magnified                -Magnified
      3)-Real                        4)-Real
         -Inverted                     -Inverted
         -Magnified                   -Same size 
      5)-Real                         6)-Real
         -Inverted                      -Inverted
         -Diminished                  -Diminished


                           Concave Lens                                                                                                                                               

 

           -Always virtual

           -Upright

           -Diminished

Power Of A Lens

The power of a lens is:
            a measure of the ability of the lans to converge or diverge rays of light
               Formula:   1/Focal length(m)  or  P=1/f                      *f is the local length and measure in metres.          
           --The u nit of the power of lens is dioptres or D.
           --The focal length is inversely propotional to power of lens.
           --If focal length is shorter,power of lens is higher.
           --Thick lens has shorter focal length and higher power.
       a)thick lens

      
     b)thin lens

 

♥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.

♥Understanding total internal reflection of light♥

Explaination of Total Internal Reflection

~If the angle of incidence in the glass is increased further,the light is not refracted any more,but it will be internally reflected.The phenomena is as known as total internal angle.
~The angle of incidence at this limit is called the critical angle,c
~The critical angle is defined as the angle of incidence in the denser medium when the angle of refraction in the less dense medium is 90°
Diagram of Total Internal Reflection and Critical Angle

θ1=θi=incidence angle

θ2=θr=refraction angle

~the green line shows the θi less than θc so the line refracted away from the normal

~the red line shows the θc is the angle of incidence in an optically denser medium when the angle of refraction is 90°

~the blue line shows the θi bigger θc so the ray passes from a denser medium to a less dense medium,this condition is known as total internal reflection

Phenomena of Total Internal Reflection

Mirage occurs due total internal reflection of light.When the sun high in the sky,the sands gets heated first and then the layers of air above it.The rays from the trees travel from an optical denser air layer to a rarer layer and hence bend away from the normal.This bending continues and a stage is reached where the angle of incidence becomes greater than tha critical angle and total internal reflection takes place.

Application of Total Internal Angle

~The cut of the diamond favors total internal reflection.Most rays entering the top of the diamond will internally reflect until they reach the top face of the diamond where they exit.This gives diamonds their bright sparkle.Notice in this animation that a ray entering the top does't exit the diamond until it reaches the top surface.

~A fiber optic is a glass "hair" which is so thin  that once light enters one end,it can never strike the inside walls at less than the critical angle.The light undergoes total internal reflection each time strikes the wall.Only when it reaches the other end is it allowed to exit the fiber.
~Fiber optic cables are used to carry telephone and computer communication.Advantages over electrical wired include:
1.Fiber optics can carry much more information in a much smaller cable.
2.No interference from electromagnet fields result in "clearer" connections.
3.No eletrical resistance.
4.No hazard of electrocution if cable breaks.

Understanding Refraction Of Light

Idea of refraction

Explaination of Refraction

>>Refraction of light is the bending of a light ray.(From one medium to another medium.)

>>Light travels in straight line.When light passes through two different density of medium,its direction changes at the boundary of media.

>>Light ray travel any subtance,the subtansce is called a medium.

>>When a light rat travels two different medium,its speed changes. The change in speed ( light ray) causes it to change its direction.

>>Law of Refraction states that :
    1.The incident ray,the refraction ray and the normal at the point of incident all the three are in the same plane.
    2.The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. ( This is known as SNELL's LAW)

SNELL's LAW :  *Sin I = Vacuum or air @ Sin R= Medium*


Diagram

>>The incident ray is perpendicular ( 90° )to the surface.

>>The ray passes the surface without being refracted.

>>The incident ray hits the boundary.

>>The incident ray slows down after hits the boundary and bends towards the normal.

Phenomenon Due to Refraction of Light

\

>>Through the eye to see the fish,the fish appears closer to the image than actual position of fish.

>>If the fish look to the eye,the eye appears nearer than the actual position of eye.

 

>>The straw look bent when it immesed into the water.

>>It is because the light ray from the straw was refracted away as they normal in air.

>>Rainbow is appear after or during rain.

>> It is because the water droplets in the air was refracted the white sunlight,then the rainbow will be occur in the sky!