give me a full answer of finding the image distance for varying object distances in case of a convex lens and drawing corresponding ray diagram to show the nature of image formed
Answers
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Explanation:
Aim
To find the image distance for varying object distances in case of a convex lens and draw corresponding ray diagrams to show the nature of image formed.
Theory
Lens: It is a piece of a transparent medium. It may be curved at both the surfaces or may be plane at one surface and curved at the other surface. Two main types of lenses are: (a) convex lens or converging lens and (b) concave lens or diverging lens.
Convex lens and its types: It is also called converging lens because it converges a beam of light incident on it. Double convex lens is bulging in the centre, i.e., it is thicker in the middle and thinner at the edges.
NCERT Class 10 Science Lab Manual Image Formation by a Convex Lens 3
Convex lens: It is a converging lens with real focus. These lenses are thick in the middle and thin at the edges.
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The distance of object from the centre of lens is represented by letter ‘u’. The distance of image from the centre of lens is represented by letter V.
Lens formula: This formula gives the relationship between object distance (u), image-distance (v) and the focal length (ƒ). The lens formula is expressed as
\frac { 1 }{ f } =\frac { 1 }{ v } -\frac { 1 }{ u }
New Cartesian sign conventions for u, v and ƒ
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The object is always placed to the left of the lens.
Incident rays are drawn from left towards the right of the lens.
All distances are measured from the optical centre as origin.
Distances measured in the direction of the incident rays are taken as positive.
Distances measured opposite to the direction of the incident rays are taken as negative.
Distances measured perpendicular to and above the principal axis are taken as positive.
Distances measured perpendicular to and below the principal axis are taken a negative.
Focal length of a convex lens is taken as positive while that of concave lens is taken as negative.
For the sake of clarity of ray diagrams, only two rays are considered.
The intersection of atleast two refracted rays gives the position of image of the point object. Any two of the following rays can be considered for locating the image:
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Materials Required
A convex lens of a short focal length (12-20 cm), measuring scale, optical bench and a needle or a candle.
Procedure
Fix a thin convex lens on a lens holder and place the screen on the other side of the lens.
Focus a sharp, clear and inverted image of the distant object on the screen. This is the rough focal length, measure it with the help of a metre scale.
Mark the position of lens on optical bench or on a table. Fix the lens at this point, label it as ‘O’.
Mark a point ‘F’ at both the sides of the lens as focus of the lens by knowing the focal length as calculated in first step.
Mark a point 2F at both the sides of the lens, the distance of 2F from the lens is double the focal length of the lens.NCERT Class 10 Science Lab Manual Image Formation by a Convex Lens 7
Place a candle on the table or needle on optical bench at distance beyond 2F and adjust the height of the centre of lens nearly equal to the height of the flame of the candle.
To locate a sharp image of the candle flame in the convex lens from the other side of the lens, adjust the position of the screen and record your observations.
Now, place the object, e., the lighted candle or the needle at 2F and record your observations.
Now, shift the object between F and 2F and record the observations.
Now, place the object at F and record the observations.
Place the object between O and F of the lens and record your observations.
Draw ray diagrams for all the positions of the object.
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Observation Table
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Mathematical Calculation of ƒ:
NCERT Class 10 Science Lab Manual Image Formation by a Convex Lens 10
> Hence, the focal length of the given lens is 10 cm.
Ray Diagrams
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Result
NCERT Class 10 Science Lab Manual Image Formation by a Convex Lens 12The focal length of the given lens is 10 cm.
Precautions
The focal length of the convex lens must be between 15 to 20 cm.
Use thin convex lens of small aperture.
Perform this experiment in calm air to avoid the flickering of the candle flame.
To obtain distinct and sharp image of the candle flame, perform this experiment in a dark room.
The optical bench or the bench holding the lens, object and image screen should not be shaky.