Question

In a telescope the objective has much larger aperture than the eyepiece. Why?

Hint:

Remember: \[ \text{Large Objective Aperture} \Rightarrow \text{More Light Collection} \Rightarrow \text{Brighter Image} \] The aperture of a telescope determines its light-gathering and resolving power.

Answer 1

Concept: The aperture of a lens is the effective diameter through which light enters the optical instrument. In astronomical telescopes, celestial objects are extremely far away and hence appear very faint. To observe them clearly, the telescope must collect as much light as possible. The objective lens is responsible for gathering light from distant objects and forming a real image. Therefore, it is made with a much larger aperture than the eyepiece.

Step 1: Role of the objective lens. The objective lens is the first lens that receives light coming from distant objects. Its main functions are:
• To collect light from distant objects.
• To form a real image of the object.
• To increase the brightness of the image. The amount of light collected by a lens depends upon its aperture.

Step 2: Effect of a large aperture. The light-gathering power of a lens is proportional to the area of its aperture. Since, \[ \text{Area} \propto D^2 \] where \(D\) is the diameter of the aperture. Therefore, a larger aperture collects much more light from distant celestial objects. As a result:
• Faint stars become visible.
• The image appears brighter.
• The resolving power of the telescope increases.

Step 3: Why is the eyepiece aperture smaller? The eyepiece does not collect light directly from distant objects. Its purpose is only to magnify the image already formed by the objective lens. Hence, a large aperture is not required for the eyepiece. Result: The objective lens of a telescope is provided with a much larger aperture so that it can collect a large amount of light from distant celestial objects, producing a brighter image and improving the resolving power of the telescope.