Observing Across the Electromagnetic Spectrum

The sky in every kind of light

Visible light is only a thin slice of the electromagnetic spectrum. Astronomers now observe the sky in radio waves, infrared, ultraviolet, X-rays and gamma rays too, because different objects shine brightest at different wavelengths. This module looks at the telescopes that capture each kind of light, and why some must be launched into space.

The language of observing

Four ideas run through the topic:

Order the spectrum

Put these parts of the electromagnetic spectrum in order, from the longest wavelength to the shortest.

• Radio waves, the longest wavelength
• Infrared
• Visible light
• Ultraviolet
• X-rays
• Gamma rays, the shortest wavelength

Match each waveband to what it reveals

On the ground or in space?

Where a telescope is placed depends on whether its light can reach the ground.

Through the atmosphere

Which two kinds of light pass most easily through the Earth's atmosphere?

• Visible light and radio waves
• X-rays and gamma rays
• Ultraviolet and X-rays
• Gamma rays and ultraviolet

Which need a space telescope?

Select the TWO kinds of light that can only be observed well from SPACE.

• X-rays from hot, energetic sources
• Gamma rays from violent events
• Radio waves from cold gas clouds
• Visible light from bright stars

Bigger dishes, finer detail

Radio telescopes use huge dish apertures, because a bigger aperture gives better resolution, letting them pick out finer detail. To sharpen the image further, many separate dishes can be linked to act as one giant telescope, a technique called aperture synthesis. Modern telescopes record their images with digital sensors, such as CCDs, rather than by eye or on film.

What the atmosphere lets through

The atmosphere only lets _____ light and radio waves through easily, so optical and radio telescopes work on the ground. Ultraviolet, X-ray and gamma rays are _____ by the atmosphere, so those telescopes must be placed in _____. A larger _____ gathers more light and gives better resolution. Linking many radio dishes to act as one is called aperture _____.

Choose the right telescope

An astronomer wants to study three different objects. Help choose the right telescope for each.

• To map cold hydrogen gas clouds in the galaxy, which telescope is best?
• To detect X-rays from hot gas around a black hole, where must the telescope be?
• To see finer detail with a radio telescope, what helps most?

Why go to space?

Besides reaching blocked wavelengths, what is another advantage of a space telescope?

• It avoids atmospheric turbulence and skyglow, giving sharper images
• It is much cheaper and easier to repair
• It can only be used in the daytime
• It makes stars twinkle more strongly

Which telescope, and where

Tap the TWO statements that are correct about observing the sky.

• The atmosphere lets visible light and radio waves through most easily.
• X-ray telescopes work best at sea level.
• A larger aperture gives a telescope better resolution.
• Radio waves from space can only be detected from orbit.
• Space telescopes make images blurrier than ground telescopes.

Across the whole spectrum

Explain why astronomers observe the sky across the whole electromagnetic spectrum, and why some telescopes must be placed in space.

• Explain why different objects are studied at different wavelengths
• State which wavelengths pass through the atmosphere and which are blocked
• Explain why ultraviolet, X-ray and gamma telescopes must be in space
• Give one other advantage of observing from space