Question

Hola Brainlians !!!

Analyse the following observation table showing variation of image distance with object distance in case of a convex lens and answer the question that follow, without doing any calculations :-

a) What is the focal length of the convex lens? Give reason in support of your answer.

b) Write the serial number of that observation which is not correct. How did you arrive at this conclusion?

c) Take an appropriate scale to draw ray diagram for the observation at S. No. 4 and find the approximate value of magnification.

Class - 10th
Chapter - Light, reflection and refraction.

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Answer 1

Explanation:

(a) From serial number 3 we can say that the radius of curvature of the lens is 30 cm because when an object is placed at the centre of curvature of a convex lens its image is formed on the other side of the lens at the same distance from the lens.

Answer

From the table:

When the object is placed at 40 cm in front of the convex lens, the image is formed at 40 cm from the lens on the other side. This means that the image formed is real and inverted. Also, here, the image distance and object distance are same; this happens when the object is placed at the centre of curvature of the lens.

(b) We know that, R=2f, where R= radius of curvature, f= focal length of the lens.

Radius of curvature is defined as the distance from the optical centre to the centre of curvature.

Thus, here R=40cm

$hence \: f = \: \frac{r}{2} = \: \frac{40}{2} = 20cm$

(c) This is the diagram for the observation no 2.

We know that,

Magnification of the object

$m = \frac{v}{u} \\ \\ so \: m = \frac{30}{ - 60} \\ \\ m = - \frac{1}{2} \\ \\ m = - 0.5$

Answer 2

• 1.From the observation 3, we can say that Radius of curvature is 40cm so foal length of the convex lens is 20cm.[f=R/2]

• 2. Observation 6 is incorrect , as the object is in between P and F, the image formed will be virtual image on the same side of object, image distance should be negative.

• 3.Ray diagram for observation 3: Please refer attachment : object beyond C

• 4. magnification=m=v/um=30/-60=-1/2= -0.5so the image is real, inverted and diminished.