How do infrared telescopes detect this invisible light?

In one line

The rule

Infrared telescopes detect electromagnetic waves with wavelengths longer than visible light, using sensors that can convert this invisible energy into signals we can measure and see.

Infrared light has wavelengths from about 700 nanometres (nm) to 1 millimetre (mm), longer than visible red light.

Let's understand it simply

Imagine you are sitting in a dark room. You cannot see anything, but if someone brings a hot roti near you, you can feel its heat. That heat is actually infrared radiation (a type of light we cannot see, but can feel as warmth).

Now, think of an infrared telescope as a special camera. It cannot see like our eyes, but it can "feel" the heat coming from faraway stars, planets, or even dust in space. It turns this heat into pictures, so scientists can see things that are invisible to our eyes.

How it works

1. Collecting infrared light: The telescope has a big mirror or lens to gather as much infrared light as possible from space.
2. Focusing the light: The mirror or lens focuses the infrared rays onto a small spot, just like a magnifying glass focuses sunlight.
3. Detecting with sensors: At the focus point, there is a special sensor (like a camera chip) made from materials such as indium antimonide or mercury cadmium telluride. These materials can "feel" infrared rays.
4. Converting to electrical signals: When infrared rays hit the sensor, they make tiny electrical changes. The sensor converts these changes into electrical signals.
5. Making an image: A computer takes these signals and turns them into a picture. The hotter something is, the brighter it appears in the infrared image.
6. Cooling the telescope: To avoid picking up its own heat, the telescope and sensors are often cooled using liquid nitrogen or helium.

The story behind it

Infrared light was discovered by William Herschel in 1800. He was looking for which color of sunlight warmed things the most. He found that just beyond the red part of a rainbow, there was invisible light that made his thermometer rise. This was the first proof of infrared light.

The first infrared telescopes were built in the 1960s. Scientists wanted to see things hidden by dust or too cool to shine in visible light. Early infrared telescopes were simple, but now we have advanced ones like the Spitzer Space Telescope and the James Webb Space Telescope.

Where you see it in real life

• Remote controls: Your TV remote uses infrared light to send signals to the TV. The TV has a sensor to detect this invisible light.
• Night vision cameras: Security cameras can "see" in the dark using infrared light, showing warm objects like people or animals.
• Thermal scanners at airports: These detect body heat (infrared) to check for fever.
• Astronomy: Infrared telescopes see stars hidden behind dust clouds, or planets that are not hot enough to shine in visible light.
• Checking leaks in buildings: Infrared cameras find places where heat escapes, helping to save energy.
• Medical imaging: Doctors use infrared to see blood flow or inflammation in the body.

Why it matters

• Sees through dust: Infrared telescopes can look through dust clouds in space, revealing new stars or galaxies.
• Finds cool objects: They can spot planets, brown dwarfs, or comets that are too cold to shine in visible light.
• Studies the early universe: Infrared light travels far, so we can see very old galaxies and learn about the universe's history.
• Helps in technology: The same sensors are used in cameras, remotes, and even medical tools.

Worked example

Let's say an infrared telescope detects a planet that is not visible to our eyes. How does it "see" it?

Step 1: The planet gives off heat (infrared rays), even if it is not shining in visible light.
Step 2: The telescope's mirror collects these rays and focuses them onto the sensor.
Step 3: The sensor absorbs the infrared rays and produces tiny electrical signals.
Step 4: The computer reads these signals and creates an image, showing the planet as a bright spot.

Common mistakes

• Thinking infrared is only heat: Infrared is a type of light, not just heat. All warm things give off infrared, but it is still part of the light spectrum.
• Believing our eyes can see infrared: We cannot see infrared, but we can feel it as warmth. Special sensors are needed to "see" it.
• Forgetting about cooling: If the telescope is not cooled, its own heat will confuse the sensor. Cooling is very important.
• Assuming all telescopes are the same: Infrared telescopes need special sensors and cooling, unlike normal optical telescopes.

Remember it

Infrared telescopes "see" heat, not light—by using special sensors that turn invisible rays into pictures.