Correct statement about telescopes

[songoku]

Homework Statement

Why are we able to see details of a far object more clearly with telescope?
a. the image formed by the telescope is always virtual and inverted
b. the image formed by the telescope is larger than the object
c. both the objective and eyepiece form real images
d. the intermediate image formed by the objective lens is greatly magnified
e. the image formed by the telescope subtends a larger angle at the eye than the object does

Relevant Equations

None

My answer would be (e) but is it just the same as (b)? So there are 2 correct answers?

Thanks

 

[pbuk]

My answer would be (e) but is it just the same as (b)? So there are 2 correct answers?

The moon is pretty big; how big is the image it forms on your retina?

 

[songoku]

The moon is pretty big; how big is the image it forms on your retina?

The image formed on retina will be real, inverted and smaller

The characteristics of image formed by, let say, Galilean telescope would be virtual, upright and bigger?
By "bigger" here, it refers to the comparison of image seen by eye without using telescope and with using telescope, so not comparing the size of image with the size of actual object?

Thanks

 

[pbuk]

This is what b. might say if it were also a correct answer:
b. the image formed on your retina by the telescope is larger than the image formed without the telescope

This is what b. actually says:
b. the image formed by the telescope is larger than the object

b. is not correct.

 

[haruspex]

The moon is pretty big; how big is the image it forms on your retina?

b. refers to the image formed by the telescope, not the image formed on the retina. So it could be true that the image is larger than the object, but that is not what makes it clearer.

 

[songoku]

For the case of lens or mirror or magnifying glass or microscope, bigger image means that the size of image is compared to the actual size of the object.

But I think it will be different for telescope. In my notes, the characteristics of image formed by telescope (I take the type of telescope as Galilean telescope) is virtual, upright, bigger.

What does it mean by bigger in that context? Is it also the same as lens / mirror, that the size of image formed by telescope is bigger than the size of actual object? Or maybe it has different meaning, such as bigger in that context means that the image is bigger when seen using telescope compared to without using telescope (not comparing the size of image to actual size of object)?

b. refers to the image formed by the telescope, not the image formed on the retina. So it could be true that the image is larger than the object, but that is not what makes it clearer.

What makes it clearer? I thought bigger means clearer

Thanks

 

[hutchphd]

Both bigger and clearer are comparative syntax. Bigger than what? Clearer than what?
There is semantics as well as physics here
One issue is whether the definition of the telescope includes the eyepiece. The Galilean scope produces no real image providing instead a large virtual image at infinity. A Keplerian or Newtonian scope produce a real image but an eyepiece must be used to examine that image.
The only definition among those provided to you that is universal is the "correct" one (e) and the rest are semantically challenged. Learn the physics (pictures, equations) and try to avoid the words. You will be happier!

 

[haruspex]

I thought bigger means clearer

If you make the image twice as wide but four times further away it will look smaller, and certainly not clearer.

 

[Delta2]

I don't know much about telescopes (for example I don't know what the term intermediate image means or objective lens) however my common logic tells me that it is because the image is magnified, so why d) isn't the correct answer, why common logic fails here?

 

[haruspex]

I don't know much about telescopes (for example I don't know what the term intermediate image means or objective lens) however my common logic tells me that it is because the image is magnified, so why d) isn't the correct answer, why common logic fails here?

For the reason I gave in post #8.
Don't confuse the size of the image (a length) with its angular size.

 

[Delta2]

Don't confuse the size of the image (a length) with its angular size.

What is the angular size of an image. AND I still don't understand if the telescope doesn't magnify the image of a distant object then what the heck (sorry for the expression) it does?

 

[haruspex]

What is the angular size of an image. AND I still don't understand if the telescope doesn't magnify the image of a distant object then what the heck (sorry for the expression) it does?

What matters is the size of the image on the retina, and that depends on the angle the object (or if using a telescope or microscope, the image created by the final lens) subtends at the eye.

 

[Delta2]

So the size of the image on the retina is larger when seeing through telescope, compared to the image on the retina with the naked eye?

 

[haruspex]

So the size of the image on the retina is larger when seeing through telescope, compared to the image on the retina with the naked eye?

Yes.

 

[Delta2]

So d) would be correct if it was saying something along the lines of "the size of the object's image on eye is magnified compared to the naked eye image" instead of saying that the intermediate image of the objective lens is the one that gets magnified.

 

[haruspex]

So d) would be correct if it was saying something along the lines of "the size of the object's image on eye is magnified compared to the naked eye image" instead of saying that the intermediate image of the objective lens is the one that gets magnified.

Yes.

 

[kuruman]

So d) would be correct if it was saying something along the lines of "the size of the object's image on eye is magnified compared to the naked eye image" instead of saying that the intermediate image of the objective lens is the one that gets magnified.

Of course when you say "size", you mean the angular size of the image formed on the retina. You may find this link informative. The two main functions of optical telescopes are to gather more light than the pupil of the naked eye and to magnify the angular separation between two distant stars.

 

[collinsmark]

Of course when you say "size", you mean the angular size of the image formed on the retina. You may find this link informative. The two main functions of optical telescopes are to gather more light than the pupil of the naked eye and to magnify the angular separation between two distant stars.

Indeed. Except I would add a third function being the system's resolving power. All three functions can be related to how a "far object" can be seen more "clearly," depending, of course, on the details of the object (is the object dim when viewed with the naked eye? Does the object subtend a small angle with the naked eye in the first place?)

So of the given choices (a through e), one is better than the rest. But I would argue that none of the given choices fully address the big picture.

 

[songoku]

Thank you very much for all the help and explanation pbuk, haruspex, hutchphd, Delta2, kuruman, collinsmark