Question

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Depth of focus refers to how much of the thickness of the specimen is in focus a particular magnification (ie, is the top, middle and bottom of the specimen in focus at the same time or as you focus up or down only the top middle, or bottom of the specimen is in focus Long depth of focus Short depth of focus Thickness of specimen 1. Obtain a prepared crossed colored thread slide and view using scanning (40X-total) power. Center the slide where the threads cross. Slowly focus up and down using the fine focus adjustment. Repeat at low power (100X-total) and high power (400X total). Notice that when one thread is in focus, the others seem to blur. This is most evident at 400x. How does the depth of focus change as the power increases? 2. Now determine the order of the threads and complete the table. Hint: Notice which way the stage is moving (up or down) as each thread comes into focus. Depth Thread Color Top Middle Bottom

Answer 1

1-In a microscope, the field of view is greatest at low power. This means it is possible to distinguish between two different objects ( resolution power is greatest at low power). As the power of the lens increases , the resolution power decreases. This results in reduced differentiating ability. And the magnification power of the lens is increased. Hence the depth of focus changes. Likewise if object A is in the top, object B is in the middle and object C is at the bottom, then object A will be visible at low power and as the magnification power increases subsequent layers come under focus and the previous layer about it blurs off.

2- thread colours are not mentioned. As the magnification power changes to higher power , stage moves upward.