Lab Manual: Image Formation by a Convex Lens | Lab Manuals for Class 10 PDF Download

Objective

Materials Required
A thin convex lens, a lens holder fitted in a stand, a piece of rice paper screen fixed to a stand, a small candle with stand, a match box and a measuring scale.

Theory/Principle

Types of Images
There are two types of images:

• Real images: These are formed by the light rays which after refraction actually meet or intersect at a point or when light rays actually converge at a point. It can be obtained on the screen.
• Virtual images: These images are formed by the light rays which after refraction appear to meet at a point or appear to diverge from a point. It cannot be obtained on the screen.

Formation of Image by a Convex Lens
The table given below illustrates the ray diagrams alongwith the position and nature of image, formed by convex lens for various positions of the object.

Formation of Image by Convex Lens for Different Positions of Object

Cartesian Sign Convention
In this convention, the optical centre 0 of a convex lens LL’ is considered as the origin and its principal axis as the X-axis (X’X) of the coordinate system. The principal axis X’X is an imaginary straight line passing through two centres of curvature C₁ and C₂ of two spherical surfaces of the convex lens LL’ (as shown in Fig.1).

The optical centre 0 is a point associated with the convex lens such that a ray of light passing

through O does not suffer any deviation. Here, we consider the convex lens as a thin lens having

a small aperture, much less than its radius of curvature.

The New Cartesian sign convention can be summarised as below:

• The object is always placed to the left of the lens. This implies that the light from the object falls on the lens from the left hand side.
• All distances parallel to the principal axis are measured from the optical centre of the lens.
• All distances measured to the right of the origin along +X-axis, are taken as positive while those measured to the left of the origin along -X-axis, are taken as negative.
• Distances measured perpendicular and above the principal axis along +Y-axis, are taken as positive.
• Distances measured perpendicular and below the principal axis along -Y-axis, are taken as negative.

Thus, the nature, position and size of the image can be noted and measured from the optical centre O of a thin convex lens.

Procedure

• Hold a thin convex lens in hand and determine its approximate focal length F by obtaining a sharp and bright image of a distant object (such as Sun or distant tree) on a wall and measuring the distance between the image and lens.
• Fix the thin convex lens on a lens holder and place it vertically near the middle of the tabie.
• Note down the position (l) of the thin convex lens in the observation table.
• Place the semi-transparent rice paper screen, fitted to a stand on the other side of the lens.
• Mount a small candle in a centred mark stand vertically and light it.
• Adjust the height of the centre of lens nearly equal to the height of the flame of the candle. Here the candle flame is considered as the object AB. Measure and record the height h of the candle flame
  
• Place the lighted candle flame in front of convex lens beyond the twice of the aproximate focal length, i.e. beyond the centre of curvature (2F) of convex lens as shown in Fig. 2.
• Note down the position of lighted candle (c). Find the distance, x (say) between the optical centre O of the lens and candle flame (object).
• Locate a sharp image A’B’ of the candle flame AB in the convex lens from the other side of the lens, adjusting the position of the screen.
• Note down the position of screen, s. Find the distance between optical centre O of the lens and the screen, y (say). Also, measure and record the height h’ of the image of the candle flame obtained on the screen.
• Now shift the lighted candle nearby but beyond, 2F of convex lens. Note down and record the position of lighted candle AB (c) and lens (l).
• Adjust the position of the screen by moving it away from the previous position to get a real and sharp image A’B’ of the candle flame AB. Note down the position of screen, s. Find the distance y and record the height h’ of image of candle flame obtained on the screen.
• Repeat the experiment by shifting the candle in front of convex lens at a distance:
  • Equal to 2F
  • Less than 2F but more than F
    Obtain the real and sharp image of the candle flame on the screen in each case by moving it further away from lens and record your observations.
• Now place the lighted candle at F and try to locate the image on screen by moving it gradually away from the lens. You may not be able to obtain the image because it will be formed at infinity and is blurred due to high magnification.
• Record all the observations in the corresponding observation table.

Observations and Calculations

• Approximate focal length of thin convex lens, f = …………… cm
• Height of candle flame, h = ………….. cm
  

Result
The image distances for varying object distances in case of convex lens is given below:

Precautions
It is clearly visible from the above table as the object is movqd from infinity towards the optical centre of thin convex lens, the image distance increases gradually and height of image also increases gradually.

• A thin convex lens, having small aperture should be used to perform the experiment.
• The base of the stands of the convex lens and screen should be parallel to the measuring scaie.
• For obtaining distinct and sharp images of candle flame, perform this experiment in a dark room.
• The flame of candle should be uniform throughout the experiment. To avoid its flickering, perform the experiment in calm air and switch OFF the fan.
• While finding out the approximate value of focal length F of convex lens by using Sun as an object, do not look at the image directly with the naked eyes, otherwise it may damage-the eyes.’
• The focal length of thin convex lens should be preferably between 15 to 20 cm.

Sources of Error

• The convex lens may be thick and may have scratches.
• The centre of lens, centre of stand of candle flame and centre of screen, may not be in same straight line and parallel to measuring scale.
• The experiment may not be performed in a dark room.
• Flame may not be uniform throughout the experiment.