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# Lens' watch

1. Hi guys!

I am a bit confused about the topic of lens' and telescopes.

I've been learning about how 2 converging lens' can be used as a telescope -the objective lens and eyepiece lens.

I understand that the light rays from a distant object travels parallel to the principal axis, and once it enters the objective lens, the parallel rays are refracted (focused) onto a focus - a real, inverted (upside-down) image is produced.
Then the light rays, from the focal point, travel to the eyepiece lens and refract once again, but this time they are refracted parallel to the principal axis.

Is this correct? and what happens to the image produced on the other side of the eyepiece lens. Is the right-way up?

. I
2. I am a bit confused about the topic of lens' and telescopes.

I've been learning about how 2 converging lens' can be used as a telescope -the objective lens and eyepiece lens.

I understand that the light rays from a distant object travels parallel to the principal axis, and once it enters the objective lens, the parallel rays are refracted (focused) onto a focus - a real, inverted (upside-down) image is produced.
Then the light rays, from the focal point, travel to the eyepiece lens and refract once again, but this time they are refracted parallel to the principal axis.

Is this correct? and what happens to the image produced on the other side of the eyepiece lens. Does it flip again and become the right-way up?
3. In astronomical telescopes the image is reversed (upside down). You can get a upside up image by using more lenses, but each extra lens reduces the amount of light passing, which is unacceptable to astronomers.
4. (Original post by Joinedup)
In astronomical telescopes the image is reversed (upside down). You can get a upside up image by using more lenses, but each extra lens reduces the amount of light passing, which is unacceptable to astronomers.
So using an eyepiece lens gets the image the right-way up?
5. (Original post by park1996)
So using an eyepiece lens gets the image the right-way up?
you can get a special eyepiece called an 'erector' to do that. The simple arrangement you described in the op will give a reversed image.
6. The image is upside down for the viewer. The objective lens refracts the light towards the focal point for that lens... It converges to that point and then diverges past it. The eye piece lens takes this diverged light and refracts it back to the normal axis.

As you can see from the picture, the light "at the top" is refracted and levels out "at the bottom" of the eye. The opposite is true for light entering the bottom of the objective lens.

The lenses by themselves will just create an inverted image to the viewer. It can be corrected by adding other optics, but this is generally not suitable. More glass in the way means more light is being absorbed and the image is dimmer. Many astronomers replace the person looking down the eye piece with a camera instead. It's easy to flip a digital image and it's more responsive to the light, especially for the "invisible" wavelengths.
7. (Original post by SillyEddy)
The image is upside down for the viewer. The objective lens refracts the light towards the focal point for that lens... It converges to that point and then diverges past it. The eye piece lens takes this diverged light and refracts it back to the normal axis.

As you can see from the picture, the light "at the top" is refracted and levels out "at the bottom" of the eye. The opposite is true for light entering the bottom of the objective lens.

The lenses by themselves will just create an inverted image to the viewer. It can be corrected by adding other optics, but this is generally not suitable. More glass in the way means more light is being absorbed and the image is dimmer. Many astronomers replace the person looking down the eye piece with a camera instead. It's easy to flip a digital image and it's more responsive to the light, especially for the "invisible" wavelengths.

Thanks man!

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Updated: March 27, 2013
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