Chapter 25, Problem 18Q

To determine

Why radio telescopes are so big and why optical telescopes are not equally large.

Answer to Problem 18Q

Solution:

Radio telescopes are large to have better resolution with a large wavelength(the wavelength of radio waves is large).

Explanation of Solution

Given:

The radio telescope in Arecibo, Puerto Rico is the world’s largest radio telescope with 300meter diameter.

The largest optical telescopes have diameters of about 10 meters.

Radio telescopes are large primarily because the wavelength of radio waves is large - on the order of meters to hundreds of meters. To get better resolution with a large wavelength, the Rayleigh criterion says that a large aperture is needed. Making such large optical telescopes is not done for several reasons:

(i)The theoretical resolution obtained by such a large aperture would not be achievable because of atmospheric turbulence that does not affect radio telescopes;

(ii)The much smaller wavelength of light means that the mirror has to be more perfectly shaped than the radio telescope, and that is very difficult to do with huge mirrors. Some large-aperture optical telescopes (such as the “Very Large Telescope” in Chile)are now being made with multiple small mirrors combined to provide some of the capabilities of a single large mirror.