Question

Geometric optics

What are some benefits of using larger mirrors or lens? What are some disadvantages?

Answer 1

1] Larger the mirror or lens, more the collecting area for light and hence more light can be captured from the same object. This also means that dimmer objects can be imaged effectively with a larger mirror/lens since more light is gathered.

2] Large mirror or lens gives a higher resolution image. This is apparent from Rayleigh's diffraction limit formula:

where D is the distance between the object and the lens.

so, larger the diameter of the lens/mirror d, smaller will be the resolvable distance x and hence higher the resolving power of the optical element.

Disadvantages of using large mirrors or lens are:

1] Cost: Designing and building a large mirror or lens requires more material and other resources and hence expensive.

2] Resolution: An equally good resolution can also be achieved by decreasing the wavelength in which the object is imaged while keeping the size of the mirrors/lens the same. This is apparent from the above equation for diffraction limit.

3] Handling: Using large optical elements is not advised all the time since it compromises the portability of the device. This is particularly true for telescopes.