What is the equation for the tube length of a telescope?

What is the equation for the tube length of a telescope?

• A. d = fobj + foc
• B. d = fobj - foc
• C. d = foc - fobj
• D. d = fobj x foc

Answer: (A)

In a simple refracting telescope designed to view distant objects, the tube length is determined by the sum of the objective’s focal length and the eyepiece’s focal length. The objective forms a real image at its focal plane, and the eyepiece acts as a magnifier whose focal plane must coincide with that image. Placing the eyepiece so that its focal plane lines up with the objective’s focal plane means the distance between the two lenses equals the sum of their focal lengths. That makes the tube length equal to fobj plus foc. So the correct relationship is d = fobj + foc. The other forms (difference, product, or a negative value) wouldn’t describe this standard arrangement.

In a simple refracting telescope designed to view distant objects, the tube length is determined by the sum of the objective’s focal length and the eyepiece’s focal length. The objective forms a real image at its focal plane, and the eyepiece acts as a magnifier whose focal plane must coincide with that image. Placing the eyepiece so that its focal plane lines up with the objective’s focal plane means the distance between the two lenses equals the sum of their focal lengths. That makes the tube length equal to fobj plus foc.

So the correct relationship is d = fobj + foc. The other forms (difference, product, or a negative value) wouldn’t describe this standard arrangement.