Image Formation In Convex Lens Class 10th Science Lab Manual CBSE Solution

Class 10th Science Lab Manual CBSE Solution

Lab Experiment 14a
Question 1.

AIM

Finding the Image distance for varying object distance in case of a convex lens and drawing corresponding ray diagrams to show the nature of Image formed by a convex lens.


Answer:

MATERIALS REQUIRED


A thin convex lens, a lens holder a piece of a semi-transparent sheet act as screen fixed to a stand with the centred mark; a small candle end stand for candle with centred mark and a meter scale (or a ruler)


THEORY


The light ray when refracted through a thin convex lens follow the law of refractions.


The formation of an image by lenses can be described using the new cartesian sign convention.


The new cartesian sign convention can be summarised as below:


(i) The object is always placed to the left of the lens.


(ii) All distances parallel to the principal axis are measured from the optical centre of the lens.


(iii) All distances measured to the right of the origin are taken as positive while those measures to the left of original taken as negative.


(iv) Distance measured perpendicular to and above the principal axis are taken as positive.


(v) Distances measured perpendicular to and below the principal axis are taken as negative.


The position, nature and size of the image of an object formed by s thin convex lens depend on the distance between the object and the optical centre and size of the object.




The nature, position and size of the image can be noted, and distance is measured from the optical centre of the convex lens.


PROCEDURE


1. Place the semi-transparent paper screen in a vertical position fitted to a stand on the right-hand side of the convex lens.


2. Place a thin convex lens vertically on the lens holder.


3. Place the candle in a centred mark stand on the optical bench.


4. Place the lighted candle stand at a distance beyond R(2F1) vertically in the front of a thin convex lens.


5. Adjust the height of the centre of the convex lens to be equal to the height of the flame of the candle as shown below.



6. Obtain a sharp image of the candle flame on the screen


7. Using measuring scale, record the position of lens and screen in the observation table.


8. Repeat the experiment by placing the lighted candle at a distance as given below:


(i) equal to 2F


(ii) less than 2F but more than F.


9. Find the distance between the optical centre of the lens and candle flame (object) to be x and corresponding image distance between the optical centre O of the lens and the screen be y.



10. Draw corresponding ray diagrams in each case to show the nature of the image formed by a convex lens.


OBSERVATIONS AND CALCULATIONS


The approximate focal length of the thin convex lens, F = 10 cm


The height of the candle flame, h = 2 cm.




RESULT


When the object is moved from infinity towards the optical centre of the convex lens.


(a) the image distance increases gradually.


(b) the size of the image also tosses gradually.


PRECAUTIONS


1. The base of the candle stand, convex lens and the screen must be parallel to the measuring scale.


2. The lens should be held vertically inside the lens holder.


3. Flame of the candle must be uniform and continuous throughout the experiment.



Lab Experiment 14b
Question 1.

AIM
To draw the images of an object formed by a convex lens when placed at various positions.


Answer:

MATERIALS REQUIRED


A drawing board, sheets of white paper, measuring scale, protractor and drawing pins or adhesive tape.


THEORY


The light ray when refracted through a convex lens obey the laws of refraction. The formation of images by a convex lens can be studied by drawing ray diagrams. using the new cartesian sign convention as given in the figure.


The new cartesian sign convention can be summarised as below:


(i) The object is always placed to the left of the lens.


(ii) All distances parallel to the principal axis are measured from the optical centre of the lens.


(iii) All distances measured to the right of the origin are taken as positive while those measured to the left of original taken as negative.


(iv) Distance measured perpendicular to and above the principal axis are taken as positive.


(v) Distances measured perpendicular to and below the principal axis are taken as negative.


Daw ray diagrams are forming images by a convex lens for various positions of the object.


The position of the object may be


(a) beyond 2F1



(b) at 2F1



(c) between F1 and 2F1,



(d) at F1,



(e) between focus (F1) and optical centre (O) of the convex lens.



PROCEDURE


1. Place a white sheet of paper on a drawing board using pins and cello tape.


2. Draw a thin line 20 cm in length in the middle of the paper using a sharp pencil and a meter scale. Name it as XX’.


3. Mark a point ‘O’ at the centre of this line. Draw a perpendicular line of equal height at the point O as the optical centre above and below XX’. Name it L1L2.

4. Mark points F1 and F2 on the line ‘XX on either side of the lens L1L2 such that OF1 = OF2 where F1 and F2 are the two principal foci of the lens.


5. Also, mark points R1 and R2 on the line XX’ such that O(R1) = 2(OF1) and O(R2)= 2 (OF2).


6. Draw the object AB of suitable height ‘h1’ at a very far distance from the lens considered to be placed at infinity.


7. Draw thin lines parallel to the principal axis such as CD, GH, PQ and RS incident on the surface of the convex lens.


8. Draw the emergent rays on the other side of the lens such as DF HF QF and SF through the convex lens and intersect at the first focus F.


9. Record the result in an observation table.


10. Fix second white sheet of paper on the drawing board.


11. Repeat the steps (2) to (6).


12. Draw an object AB of suitable height beyond 2F1 as shown.


13. Draw a ray of light AE parallel to the principal axis F1OF2 incident on the surface of the convex lens at point E.


14. Draw another ray AO through the optical centre O of a convex lens and extend it to another side of the lens as OA.’


15. On the other side of the convex lens, draw the ray EF2 passing through the focus F2 and intersecting the ray OA’ at A as shown in the figure, forming an image B’A.


16. Measure the height of the image A’B’(h2).


17. We observe from the figure that the image formed is real, inverted, smaller in size and in between F2 and 2F2.


18. Record the reading in the table


19. Repeat the above steps similar to the previous case. Draw ray diagrams for other positions of the object as shown in the



20. Measure the height of the object AB (h1) and height of image A’B’(h2) respective in all cases (c) to (f) as listed in step 20. Record them in the observation table.


21. Note down nature. Relative size and position of the image formed by the convex lens for the various positions of the object.


22. Tabulate your observations in the observation table.



OBSERVATION AND CALCULATION



RESULT

1. As the object is moved from infinity towards the optical centre of the convex lens.
(i) The image distance increases gradually
(ii) Size of the image also increases gradually
2. When the object is placed At infinity, real and highly diminished image is formed at the focus.
3. When the object is placed At Beyond 2F1, real, inverted and smaller image is formed at a point between Fand F2.
4. When the object is placed At the 2F2 real, inverted and same size of the image of the object is formed at 2F2.
5. When the object lies between F1 and the 2F2 real, inverted and enlarged image is formed beyond 2F2.
6. When the position of th object lies at F1, the image formed is at infinity, It is real, inverted and highly enlarged.

PRECAUTIONS


1. The convex lens should be thin and have a small aperture and should be without any scratches to get the distinct image of the object.


2. The lens should be placed completely vertically inside the lens holder.


3. The ray diagram should be drawn with a sharp pencil.



Viva Questions
Question 1.

What is the difference between concave and convex lens?


Answer:

A convex lens is used for focusing of light rays at a specific point whereas a concave lens is a diverging lens diverges the light rays




Question 2.

What happens to the size of the image when the object lies in moved from infinity towards the optical center of the lens ?


Answer:

The size of the image increases gradually when it is brought close to the optical center. Example, when a candle is placed between focus “F” and optical centre “O”, then a very enlarged image is formed as shown below:



Question 3.

What would be the nature of image when the object lies in between the infinity and “F”?


Answer:

Real and the inverted image are formed. The image formation diagram is shown here:



Question 4.

What will happen when the experiment is performed with scratches on the lens?


Answer:

Sharp and distant images cannot be obtained accurately due the scratches on the lens.



Question 5.

Is virtual image erect or inverted ?


Answer:

The virtual image is always erect and enlarged.



Question 6.

Where an object should be placed in order to use a convex lens as a magnifying lens?


Answer:

Between focus and optical center of the lens.



Question 7.

What factors determine the size of the image formed?


Answer:

It depends on the position of an object from the lens.



Question 8.

Why you draw the equiconvex lens?


Answer:

An equiconvex lens has the same radius of curvature on both surfaces. If the drawn lens is not equiconvex, then the condition OF1 = OF2 will not hold.



Question 9.

Why we draw only two rays in a ray diagram?


Answer:

We draw two rays because:

1. For the sake of clarity of ray diagram.


2. To know their direction easily after refraction from the lens.



Question 10.

What is the quantity which remains constant when light undergo refraction? Why?


Answer:

The frequency of the ray of light remains constant during refraction. When light changes it medium, then there is no loss in energy of the wave. Since, there is no loss in energy hence, no change in frequency as well. The frequency remains constant.



Question 11.

What is the nature of an image formed by a thin convex lens for a distant object?


Answer:

Nature of the image formed by a thin convex lens for a distant object is real, inverted and highly diminished. It is formed at the focus of a convex lens.



Question 12.

How will you distinguish between a convex lens and a concave lens by holding in hand and looking on the printed page?


Answer:

If the letter of the printed page appears enlarged then it is a convex lens , but if it appears diminished than it is a concave lens.



Question 13.

Why would we require a calm atmosphere to perform this experiment?


Answer:

With the flickering flame we can not get a sharp and bright image of our object.



Question 14.

Why it is preferred to perform this experiment in the dark or in the light?


Answer:

To get the sharp and distinct image of the candle flame, it should be better this experiment in the dark.



Question 15.

Where should the object be placed in front of a convex lens to get inverted, magnified image formed beyond its center of curvature?


Answer:

The object should be placed between F1 and 2F1 in front of the convex lens so that the obtained image is real, inverted, magnified and formed beyond the center of curvature.

The ray diagram is shown below:



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