An astronomical refractive telescope has an objective of focal length 20m and an eyepiece of focal length 2cm.
A. The length of the telescope tube is 20.02m.
B. The magnification is 1000.
C. The image formed is inverted.
D. An objective of a larger aperture will increase the brightness and reduce chromatic aberration of the image.
The ray diagram of a refracting telescope is shown below.
Source: Wikimedia commons
Note that feand foare the focal lengths of the objective and the eyepiece respectively. Also, note that the lens on the left is the objective and the one of the right is the eyepiece.
We can see from the diagram that the length of the tube is given by:
Hence, option (a) is correct.
The magnification of a refracting telescope is given by:
Hence, option (b) is correct.
A refracting telescopes forms an inverted image as it can be seen from the ray diagram. Light emanating from far away objects (e.g. stars) pass through the objective and converge at the second focus of the objective which is also the first focus of the eyepiece. The image formed is inverted and real. Now taking this real image as the object, we get a virtual image far away formed by the eyepiece. As virtual images have the same vertical orientation as the object, the image formed is inverted. Hence, option (c) is correct.
Changing the aperture of the lens has no effect on chromatic aberration. Hence, (d) is incorrect.
Rate this question :
Define the magnifying power of a compound microscope when the final image is formed at infinite. Why both the objective and the eyepiece of a compound microscope must has short focal lengths? Explain.Physics - Board Papers
There are two sources of light, each emitting with a power of 100 W. One emits X-rays of wavelength 1nm and the other visible light at 500 nm. Find the ratio of number of photons of X-rays to the photons of visible light of the given wavelength?Physics - Exemplar
Draw a labelled ray diagram of an astronomical telescope in the near point adjustment position. A giant refracting telescope at an observatory has an objective lens of focal length 15 m and an eyepiece of focal length 1.0 cm. If this telescope is used to view the Moon, find the diameter of the image of the Moon formed by the objective lens. The diameter of the Moon is 3.48 × 106 m, and the radius of lunar orbit is 3.8 × 108 m.Physics - Board Papers
Define power of a lens. Show that it is inversely proportional to the focal length of the lens.Physics - Exemplar
A magnifying glass is used, as the object to be viewed can be brought closer to the eye than the normal near point. This results inPhysics - Exemplar
An astronomical refractive telescope has an objective of focal length 20m and an eyepiece of focal length 2cm.Physics - Exemplar
A myopic adult has a far point at 0.1 m. His power of accommodation is 4 diopters. (i) What power lenses are required to see distant objects? (ii) What is his near point without glasses? (iii) What is his near point with glasses? (Take the image distance from the lens of the eye to the retina to be 2 cm.)Physics - Exemplar