Advertisements
Advertisements
Question
Draw a ray diagram for the formation of image of an object by an astronomical telescope, in normal adjustment. Obtain the expression for its magnifying power.
Advertisements
Solution
The illustration displays an astronomy telescope used to provide angular magnification of distant objects. It sports a big diameter objective lens with a long focal length. The diameter and focal length of the eyepiece are both smaller than those of the objective lens. When light from a distant object enters the objective, an actual image is formed in the tube at its second focal point. The eyepiece enlarges this image, resulting in the final inverted image. The "tube length" or distance between the objective and the eyepiece, is the sum of the focal lengths of the two lenses when their innermost points meet.
The angle “β” that the picture subtends at the eye compared to the angle “α” that the object subends at the lens is the magnifying power or m.
`m = (beta/alpha)`
`beta = tanbeta = h/f_e`
Where h is image height and fe is focal length of eyepiece.
`alpha = tanalpha = h/f_o`
Where fo is focal length of objective lens.
Hence, we get magnifying power m as,
`m = (h"/"f_e)/(h"/"f_o) = f_o/f_e`
APPEARS IN
RELATED QUESTIONS
A small telescope has an objective lens of focal length 140 cm and an eyepiece of focal length 5.0 cm. What is the magnifying power of the telescope for viewing distant objects when
- the telescope is in normal adjustment (i.e., when the final image is at infinity)?
- the final image is formed at the least distance of distinct vision (25 cm)?
You are given three lenses of power 0.5 D, 4 D, and 10 D to design a telescope.
1) Which lenses should be used as objective and eyepiece? Justify your answer.
2) Why is the aperture of the objective preferred to be large?
With regard to an astronomical telescope of refracting type~ state how you will increase its:
1) magnifying power
2) resolving power
Write the two important factors considered to increase the magnifying power?
"A telescope resolves whereas a microscope magnifies." Justify this statement ?
Define magnifying power of a telescope. Write its expression.
A lady cannot see objects closer than 40 cm from the left eye and closer than 100 cm from the right eye. While on a mountaineering trip, she is lost from her team. She tries to make an astronomical telescope from her reading glasses to look for her teammates. (a) Which glass should she use as the eyepiece? (b) What magnification can she get with relaxed eye?
An astronomical telescope uses two lenses of powers 10 dioptres and 1 dioptre. If the final image of a distant object is formed at infinity, calculate the length of the telescope
A small telescope has an objective lens of focal length 140 cm and an eyepiece of focal length 5.0 cm. Find the magnifying power of the telescope for viewing distant objects when
- the telescope is in normal adjustment,
- the final image is formed at the least distance of distinct vision.
Assertion: An astronomical telescope has an objective lens having large focal length.
Reason: Magnifying power of an astronomical telescope varies directly with focal length of the objective lens.
