How does extinction affect observations in infrared or radio wavelengths?

In one line

The rule

Extinction is the dimming or loss of light from an object as it passes through dust or gas, affecting how we observe it in different wavelengths like infrared or radio.

Let's understand it simply

Imagine you are watching a Diwali rocket in the night sky. If there is thick fog or smoke, you may not see the rocket’s light clearly. The smoke blocks or scatters the light, making it look dim or even hiding it completely. In space, there is a lot of dust and gas between stars, planets, and us. When we use telescopes to look at these objects, the dust and gas can block or absorb some of the light coming from them. This blocking is called extinction.

Now, not all light is blocked the same way. Light comes in many types, called wavelengths. Some are visible (what our eyes see), some are infrared (heat rays), and some are radio waves (used in radios and TVs). Dust and gas block visible light the most, but they also affect infrared and radio waves, just in different ways.

How it works

1. Light from a star or galaxy travels through space.
2. It passes through clouds of dust and gas (like fog on a road).
3. Dust grains absorb or scatter some of the light.
4. Shorter wavelengths (like blue light) are blocked more than longer ones (like red or infrared).
5. Infrared and radio waves can pass through dust better, but thick clouds can still block or weaken them.
6. The light that reaches us is dimmer or changed, so we see less than what is really there.

The story behind it

Astronomers noticed that some stars looked redder and dimmer than expected. In the early 20th century, they realized this was because dust in space was blocking some of the starlight. This effect was first studied in detail by Robert Trumpler in 1930, who showed that dust between stars was causing this extinction. Later, scientists found that different wavelengths are affected differently, which is why we use infrared and radio telescopes to see through dust clouds in space.

Where you see it in real life

• Star-forming regions: New stars are born inside thick clouds of dust. Visible light is blocked, but infrared telescopes (like the James Webb Space Telescope) can see through the dust.
• The centre of our galaxy: The middle of the Milky Way is hidden by dust in visible light, but radio telescopes can detect signals from there.
• Radio astronomy: Some radio waves can pass through dust, letting us study things like pulsars (spinning neutron stars) even when they are hidden in dust clouds.
• Infrared images of Orion Nebula: Infrared cameras show stars inside the nebula that are invisible in normal photos, because dust blocks visible light but not all infrared.
• Monsoon clouds and mobile signals: On Earth, heavy rain or clouds can block radio signals, just like dust blocks radio waves in space.
• Kitchen steam and heat: If you try to see a candle through thick steam, you may not see it, but you can still feel its heat (infrared), just like some infrared gets through space dust.

Why it matters

• Astronomy research: Helps scientists know which telescopes (visible, infrared, or radio) to use for different objects.
• Finding hidden stars and galaxies: Infrared and radio observations reveal things we cannot see in visible light.
• Understanding the universe: Knowing about extinction helps us correct our measurements and get the true brightness and distance of stars.
• Space missions: Helps plan what instruments to send on satellites and where to look for new discoveries.

Worked example

Suppose a star emits light, but there is a dust cloud between us and the star. The dust blocks 90% of visible light, 50% of infrared, and only 10% of radio waves.

Given: Visible light blocked = 90%, Infrared blocked = 50%, Radio blocked = 10%
Step 1: Star emits 100 units of light in each wavelength.
Step 2: After passing through dust:
    Visible: 100 - 90 = 10 units reach us
    Infrared: 100 - 50 = 50 units reach us
    Radio: 100 - 10 = 90 units reach us
Step 3: We see the star as very dim in visible, brighter in infrared, and brightest in radio.

Common mistakes

• Thinking extinction only affects visible light: It also affects infrared and radio, just less strongly.
• Believing infrared and radio can pass through anything: Thick enough dust or gas can block even these wavelengths.
• Ignoring extinction when measuring star brightness: This leads to wrong estimates of distance and size.
• Forgetting Earth's atmosphere: Our own air can also absorb some infrared and radio waves, so some telescopes are put in space.

Remember it

Extinction is like fog in space: it blocks some light, but infrared and radio waves can often see through it better than visible light.