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Lens Formula

A student was...

Question

A student was asked by his teacher to find the image distance for various object distance in case of a given convex lens. He performed the experiment with all precautions and noted down his observations in the following table:
$\begin{matrix} S . N o & O j b e c t d i s t a n c e (c m) & I m a g e d i s t a n c e (c m) 1 & 60 & 15 2 & 48 & 16 3 & 36 & 21 4 & 24 & 24 5 & 18 & 36 6 & 16 & 48 \end{matrix}$
After checking the observations table the teacher pointed out that there is a mistake in recording the image distance in one of the observations. Find the serial number of the observations having faulty image distance.

A

2

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B

3

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C

5

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D

6

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Solution

The correct option is B
3

Observation no. 3 is incorrect because the focal length of the lens in all other observations is 12 cm. In observation no. 3, the focal length comes out to be 13.26 cm.
Hence, the correct option is B

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Similar questions

S. No.	Object distance (cm)	Image distance (cm)
1	60	15
2	48	16
3	36	21
4	24	24
5	18	36
6	16	48

A student was asked by his teacher to find the image distance for various object distances in case of a given convex lens. He performed the experiment with all precautions and noted down his observations in the following table :
After checking the observation table the teacher pointed out that there is a mistake in recording the image distance in one of the observations. Find the serial number of the observation having faulty image distance.

Q. A student is using a convex lens of focal length 10 cm to study the image formation by a convex lens for the various positions of the object. In one of his observations, he may observe that when the object is placed at a distance of 20 cm from the lens, its image is formed at (select the correct option)

Q. A student focused the image of a candle flame on a white screen by placing the flame at various distance from a convex lens. He noted his observation as given below:

S.No	Distance of flame from the lens $(c m)$	Distance of screen from the lens $(c m)$
$1.$	$30$	$10$
$2.$	$20$	$12$
$3.$	$15$	$15$
$4.$	$12$	$20$

Find out the length of the given lens.

Q. A student did an experiment with a convex lens. He put an object at different distances 25 cm, 30 cm, 40 cm, 60 cm and 120 cm from the lens. In each case he measured the distance of the image from the lens. His results were 100 cm, 24 cm, 60 cm, 30 cm and 40 cm, respectively. Unfortunately his results are written in wrong order.
(a) Rewrite the image distances in the correct order.
(b) What would be the image distance if the object distance was 90 cm?
(c) Which of the object distances gives the biggest image?
(d) What is the focal length of this lens?

S.No.	Object-Distance u(cm)	Image-Distance v(cm)
1.	-100	+25
2.	-60	+30
3.	-40	+40
4.	-30	+60
5.	-25	+100
6.	-15	+120

Analyse the following observation table showing variation of image-distance (v) with object-distance (u) in case of a convex lens and answer the questions that follow without doing any calculations
What is the focal length of the convex lens? Give reason to justify your answer.

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