Question

Write two advantages of reflecting telescope over refracting telescope.

Hint:

The two most frequently asked advantages of reflecting telescopes are:
• No chromatic aberration.
• Large aperture can be made easily. These advantages make reflecting telescopes the preferred choice for modern astronomical observations.

Answer 1

Concept: A reflecting telescope uses a large concave mirror as its objective, whereas a refracting telescope uses a convex lens as its objective. Reflecting telescopes are generally preferred for astronomical observations because mirrors overcome several limitations associated with large lenses. Advantage 1: Absence of Chromatic Aberration In a refracting telescope, the objective lens refracts different colours of light by different amounts because the refractive index of the lens material depends on wavelength. As a result:
• Different colours focus at different points.
• Coloured fringes are produced around the image.
• Image quality deteriorates. This defect is called chromatic aberration. A reflecting telescope uses a mirror instead of a lens. Since reflection is independent of wavelength, \[ \text{Angle of Reflection} = \text{Angle of Incidence} \] for all colours. Therefore, all colours are reflected and focused at the same point. Hence, a reflecting telescope is completely free from chromatic aberration. Advantage 2: Larger Aperture Can Be Constructed The objective mirror of a reflecting telescope can be supported from the back. Therefore:
• Very large mirrors can be manufactured.
• The mirror does not sag significantly under its own weight.
• Large apertures become practically possible. In contrast, a lens can only be supported at its edges. Large lenses become heavy and may deform under their own weight, limiting the maximum achievable aperture. A larger aperture provides:
• Greater light-gathering power.
• Better resolving power.
• Brighter images of distant celestial objects. Result: Two important advantages of a reflecting telescope over a refracting telescope are:
• It is free from chromatic aberration.
• Large aperture mirrors can be constructed easily, giving greater light-gathering and resolving power.