Question

When viewing the crossed threads with the 40x objective, why are they not all in focus at the same time? Hint: think about what happens when you adjust the focus knob.

Answer 1

Not all threads appear in focus at the same time with a 40x objective due to a reduced depth of field. The microscope should be focused with the coarse knob first for major adjustments, then with the fine knob for precision, using the lower magnifications before high magnifications.

Step-by-step explanation:

When viewing the crossed threads with a 40x objective lens on a microscope, not all of them may appear in focus at the same time due to the limited depth of field at high magnification levels. As magnification increases, the working distance decreases, and thus the depth of field, which is the range of distance within which the specimen appears acceptably sharp, becomes smaller. This means that only a thin slice of the specimen will be in clear focus at any given time.

When focusing a light microscope, it is important to first adjust the focus using the coarse focusing knob before using the fine focusing knob. The coarse focusing knob allows for larger adjustments, typically used with 4x and 10x objectives. Once the image is in approximate focus, the fine focusing knob is used for making smaller adjustments, especially when using the 40x or 100x objectives, to achieve precise focus. The process involves starting with a lower power lens to set the working distance and good focus, then switching to a higher power lens and making minor adjustments with the fine focus, taking advantage of the parfocality feature of the microscope lenses.

Answer 2

Crossed threads may not be all in focus at the same time with a 40x objective because of the shallow depth of field at high magnification, necessitating the use of the coarse and fine focusing knobs to adjust the focus throughout the varying levels of magnification.

Step-by-step explanation:

When viewing the crossed threads with the 40x objective, all of them may not be in focus at the same time because each lens has a specific working distance, which is the distance between the lens and the specimen. A higher magnification lens like the 40x has a shorter working distance and therefore a shallower depth of field. Therefore, only a thin slice of the specimen may be in focus at any given time.

Using the coarse focusing knob helps in adjusting the focus for large-scale movements and is typically used with lower objectives like 4x and 10x. Once a preliminary focus is achieved, the fine focusing knob is used to make small adjustments, especially when higher magnifications, like 40x or 100x, are employed. This process provides a transition from a wider view at lower magnification to a detailed view at higher magnification while maintaining a focused image throughout the magnification changes, thanks to the parfocality of the lenses.

Moreover, the requirement to adjust focus with different objectives is related to the fact that as the magnification increases, the lens aperture decreases in size which can lead to a darker image. Adjusting the illumination and using the fine focus knob for precision are essential steps in achieving the best possible image when switching between different magnification levels.