A microscope is focused on an ink mark on the top of a table. If we place a glass slab 3cm thick on it, how should the microscope be moved to focus the ink spot again? The refractive index of glass slab is 1.5 cm.a)2 cm downwardb)1 cm upwardsc)1 cm downwardsd)2 cm upwardsCorrect answer is option 'B'. Can you explain this answer?

Question

A microscope is focused on an ink mark on the top of a table. If we place a glass slab 3cm thick on it, how should the microscope be moved to focus the ink spot again? The refractive index of glass slab is 1.5 cm.&#8203;a)2 cm downwardb)1 cm upwardsc)1 cm downwardsd)2 cm upwardsCorrect answer is option 'B'. Can you explain this answer?

Answer

The image of the ink spot moves up by 1cm.

Refractive index = Real depth/ apparent depth

1.5 = 3/ apparent depth

Apparent depth = 3/1.5 = 2 cm

Distance through which the microscope is moved is 3 cm -2 cm = 1 cm upwards

Answer

Concept: Relation between real depth,apparent depth and refrective index.Refractive index = Real depth/ apparent depth1.5 = 3/ apparent depthApparent depth = 3/1.5 = 2 cm.Distance through which the microscope is moved is=> 3 cm -2 cm = 1 cm upwards.Hence, correct answer is (b).

Answer

D

Answer

C

Answer

To understand why the correct answer is option 'B', let's break down the concept and the steps involved in refocusing the microscope after placing the glass slab on the ink mark.

1. Understanding Refraction:
Refraction is the bending of light as it passes from one medium to another with a different refractive index. The refractive index is a measure of how much the light bends when passing through a medium. In this case, the glass slab has a refractive index of 1.5, which means it can significantly alter the path of light passing through it.

2. Initial Focusing:
When the microscope is initially focused on the ink mark, it is adjusted to form a clear image by aligning the ink mark with the focal point or the plane of the eyepiece. At this point, the microscope is correctly focused for the air medium.

3. Introduction of the Glass Slab:
When the glass slab is placed on the ink mark, the light rays passing through the glass slab will experience refraction. The glass slab is thicker than the ink mark, so the light rays passing through it will bend further downwards due to the higher refractive index of the glass.

4. Effect on Image Formation:
Due to refraction, the light rays coming from the ink mark will bend downwards as they pass through the glass slab. This will shift the apparent position of the ink mark when viewed through the microscope. To bring the ink mark back into focus, the microscope needs to be adjusted accordingly.

5. Adjusting the Microscope:
To refocus the microscope after placing the glass slab, it needs to be moved in the opposite direction of the apparent shift caused by refraction. In this case, since the image appears to have shifted downwards, the microscope should be moved upwards to compensate for the downward shift.

6. The Correct Answer:
Based on the explanation above, option 'B' is the correct answer as it suggests moving the microscope 1 cm upwards. This adjustment will counteract the downward shift caused by the refraction in the glass slab and bring the ink mark back into focus.

To summarize, when a glass slab is placed on an ink mark viewed through a microscope, the light passing through the glass slab bends downwards, causing the image to shift downwards. To refocus the microscope, it needs to be moved in the opposite direction of the apparent shift, which in this case is upwards. Therefore, the correct answer is option 'B' - 1 cm upwards.

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