Microwaves heat food but infrared is called 'heat waves'. Which one actually heats? Plus how to remember the spectrum order
Mehak AroraDropper Sample
13 Aug 2026 · 1 min read
can't sleep so doing physics lol. EM spectrum chapter. I can list radio, microwave, infrared, visible, ultraviolet, X-ray, gamma, but I get confused about the uses and ideas.
NCERT says microwave ovens work because the microwave frequency matches the resonant frequency of water molecules, so food heats up. But NCERT also calls infrared waves "heat waves". If microwaves are heating the food, why are IR the "heat waves"? Which one actually heats?
Some more doubts:
- X-rays and gamma rays overlap in wavelength, so how do we tell them apart?
- all EM waves travel at the same speed in vacuum, so the only difference is frequency/wavelength, right?
- is it true that the Sun's UV is mostly absorbed by ozone?
And I always forget the order. Any mnemonic that isn't silly? My brain refuses to stay awake long enough to learn.
7 answers & replies
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[+][–]Kavya NaikDropperSample· 2mo ago
Any EM wave heats whatever absorbs it. Heating = the molecules absorb energy and their thermal motion increases. Water molecules (and many others like CO₂, NH₃) absorb infrared strongly, so IR makes things warm and we call it "heat waves". Hot objects also emit IR, which is why you feel warmth from a heater or the Sun from far away.
Microwave ovens use a frequency that water molecules absorb very efficiently, so all the energy goes into the food's water. So both heat, both through absorption. The "heat wave" name just says IR is the part of the spectrum that makes warmth in most everyday objects.
[+][–]Mehak AroraDropperSample· 2mo agoAuthor
ahh absorption is the key word, thank you. so IR isn't magic, water just absorbs it a lot. ok I can sleep now 😴
[+][–]Kavya NaikDropperSample· 2mo ago
sleep first, physics tomorrow 🙂 amma says same
[+][–]Farhan ShaikhDropperSample· 2mo ago
Order of increasing frequency: Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, Gamma. My silly one: "Rice Mixed In Vegetable Upma, eXtra Ghee". Silly but sticks 😄
Wavelength goes the other way round (radio is longest). NCERT's rough ranges: radio above ~0.1 m, microwaves 0.1 m to 1 mm, IR 1 mm to 700 nm, visible 700 nm to 400 nm, UV 400 nm to 1 nm, X-rays 1 nm to nm, gamma below nm.
[+][–]Dev Agarwal12thSample· 2mo ago
Uses, the quick version, since that's what MCQs ask:
- Radio waves: broadcasting and mobile communication
- Microwaves: radar, microwave ovens, and some communication links
- Infrared: remote controls, night vision, physiotherapy lamps, and the greenhouse effect
- Visible: seeing, photosynthesis
- UV: killing germs (water purifiers), and it's why we get tanned
- X-rays: medical imaging and checking for cracks in metals
- Gamma: radiotherapy and sterilising medical equipment
[+][–]Rahul DubeyDropperSample· 2mo ago
On X-ray vs gamma, since you asked: they overlap in wavelength so you can't separate them by wavelength alone, but they are not just two names. The difference is how they're produced: X-rays come from the atom's electrons (high-speed electrons hitting a metal target, or transitions of inner electrons), while gamma rays come from the nucleus (radioactive decay, nuclear reactions). So the same photon energy could in principle be called either one depending on the source.
This was a PYQ-type MCQ in some form: "which is produced in nuclear process?" Gamma.
[+][–]Aman YadavDropperSample· 2mo ago
Yes to both. All EM waves travel at m/s in vacuum. They differ in frequency/wavelength (so photon energy), and that decides how they interact with matter. UV is absorbed by ozone in the stratosphere, that's why the ozone layer protects us. UV has higher frequency than visible but lower than X-rays.
