Question Video: Comparing a Radio Telescope to a Reflecting Telescope Physics • 9th Grade

The diagram shows a radio telescope and a reflecting telescope. Radio telescopes use radio waves rather than visible light. Which part of a radio telescope has an equivalent function to the primary mirror of a reflecting telescope?

03:06

Video Transcript

The diagram shows a radio telescope and a reflecting telescope. Radio telescopes use radio waves rather than visible light. Which part of a radio telescope has an equivalent function to the primary mirror of a reflecting telescope?

Okay, as we consider this question, let’s look at our diagram which shows, on the bottom, a reflecting telescope and, on the top, a radio telescope. If we start with a reflecting telescope, this takes in visible light here, and the light is then reflected off of this primary mirror at the back of the telescope. That reflected light is gathered and collected on the secondary mirror and then reflected once more to this eyepiece lens where an image is observed. So, visible light comes in, is reflected off one mirror, is reflected off a second mirror, and then is sent along to a viewing apparatus, the eyepiece lens.

Knowing this, let’s now consider how a radio telescope works. This is a telescope that accepts radio waves from space rather than visible light. These radio waves are collected on this big parabolic dish. And then, they’re reflected and collected on this reflector called a sub reflector, which then focuses these radio waves on the radio receiver. So, radio waves come in, bounce off the parabolic dish, then bounce off the sub reflector to the radio receiver, where an image can be formed. So, we see there are similarities between a radio telescope and a reflecting telescope. They use different kinds of light; one uses visible light and the other radio waves. But the way they’re constructed is very alike.

Our first question asks, what part of a radio telescope has an equivalent function to the primary mirror of a reflecting telescope? Let’s recall that the primary mirror is the first mirror that receives the incoming radiation. And looking up at our radio telescope, we see that incoming radio waves first encounter this parabolic dish. Just like the primary mirror is the first reflector of visible light in a reflecting telescope, so the parabolic dish is the first reflector of radio waves in a radio telescope. So then, it’s the parabolic dish that has an equivalent function to the primary mirror.

Now let’s look at a second question about these two types of telescopes.

What part of a radio telescope has an equivalent function to the secondary mirror of a reflecting telescope?

We saw that the job of the secondary mirror is to collect the light reflected from the primary mirror and then reflect and focus that light even more tightly towards the eyepiece lens. In our radio telescope, it’s the sub reflector that receives the radio waves from the parabolic dish and then reflects and focuses them even more toward the radio receiver. The sub reflector then has an equivalent function to the secondary mirror.

Let’s now consider one last question about these two types of telescopes.

Which part of a radio telescope has an equivalent function to the eyepiece lens of a reflecting telescope?

The eyepiece lens is what finally forms an image of the light incident on the telescope. Similarly, for our radio telescope, after the incoming radio waves have been reflected from the parabolic dish in the sub reflector, they’re sent to the radio receiver where an image of these waves will be formed. So then, it’s the radio receiver on a radio telescope that has an equivalent function to the eyepiece lens of a reflecting telescope.