Presentation: optical instruments

Look at them. Just look at them. Old school optical instruments: microscopes and telescopes.


Make sure you get a chance to look through them in class--use the pocket microscopes to look at laptop and smartphone screens, and the telescopes to look at the posters across the room. (Focus the microscopes using the ridged wheels, and focus the telescopes by sliding the eyepiece tube in or out.)

First, the similarities between microscopes and telescopes.

A microscope consists of a (short) tube that holds two lenses apart from each other: an objective lens in the front, and the eyepiece in the back.

Similarly, telescope consists of a (long) tube that holds two lenses apart from each other: an objective lens in the front, and the eyepiece in the back.

Let's look at the two-lens model of a microscope, where the objective is lens 1, and the eyepiece is lens 2. The objective takes the light from object, and creates a real image 1 (how do you know that this would be a real image?). This real image 1 then becomes the object 2 for the eyepiece.

Now let's look at the two-lens model of a telescope, where the objective is lens 1, and the eyepiece is lens 2. The objective takes the light from object, and creates a real image 1 (how do you know that this would be a real image?). This real image 1 then becomes the object 2 for the eyepiece.

Second, differences between microscopes and telescopes. (You may have started to notice some of them already.)

For the microscope ray tracing, the object 1 is placed just outside of the focal point of the objective, which makes a greatly enlarged real image 1. (Which ray tracing(s) ((1)-(10)) best match(es) this?)

Then this image 1 becomes the object 2 for the eyepiece, where it is placed on the focal point of the eyepiece to maximize its angular magnification. (Which ray tracing(s) ((1)-(10)) best match(es) this?)

(Strangely enough, the "tube length" for microscopes is defined as the distance measured between the objective and eyepiece focal points. Compare this definition to the "barrel length" for telescopes, below.)

Then for the telescope ray tracing, the object 1 is extremely distant, such that its rays are essentially parallel. The objective lens then focuses these parallel light rays onto an image 1 located at its focal point. (Which ray tracing(s) ((1)-(10)) best match(es) this?)

Then this image 1 becomes the object 2 for the eyepiece, where it is placed on the focal point of the eyepiece to maximize its angular magnification. (Which ray tracing(s) ((1)-(10)) best match(es) this?)

Where are the ray tracings for microscopes and telescopes most similar? Where do they differ?

(Note how the "barrel length" for telescopes is defined as the distance measured between the objective to the eyepiece lenses, which is the same as the sum of their focal points. Compare this definition to the "tube length" for microscopes, above.)

Notice the negative sign in the angular magnification equations for microscopes and telescopes--what does this mean for the orientation of the final image seen through the eyepiece? Did you notice this for both the microscope and telescope?

What type of focal lengths would you want for the objective lens of a microscope? Telescope? What type of focal lengths would you want for the eyepiece lens of a microscope? Telescope?

The telescope angular magnification equation does not explicitly refer to the distance between the objective lens and the eyepiece lens. How is this distance related to the focal lengths f_o and f_e of the objective and eyepiece?