Electromagnetic Spectrum

Infrared Rays

Complete coaching-style notes on Infrared Rays: production, detection, greenhouse effect, remote controls, applications, formulas, diagrams and exam-oriented questions for CBSE, NEET, JEE, IB, IGCSE and A-Level Physics.

Production Detection Greenhouse Effect Remote Controls Applications PYQs

1. Introduction to Infrared Rays

Infrared rays are electromagnetic waves lying between visible red light and microwaves in the electromagnetic spectrum. They have longer wavelength than visible red light but shorter wavelength than microwaves. Infrared radiation is commonly called heat radiation because hot bodies emit a large amount of infrared radiation.

Position: Between visible red light and microwaves.

Nature: Electromagnetic transverse waves.

Main Property: Strong heating effect.

Common Use: Remote controls, thermal imaging, night vision.

c = νλ

E = hν

E = hc / λ

Like all electromagnetic waves, infrared rays travel in vacuum with speed 3 × 10⁸ m/s. They do not require any material medium for propagation. They are emitted by warm objects because thermal motion of atoms and molecules produces electromagnetic radiation.

2. Infrared Rays in Electromagnetic Spectrum

Property	Infrared Rays
Approx Wavelength Range	About 700 nm to 1 mm
Approx Frequency Range	About 3 × 10¹¹ Hz to 4 × 10¹⁴ Hz
Position	Between microwaves and visible red light
Main Effect	Heating effect
Detection	Thermopile, bolometer, infrared sensors, thermal cameras

3. Discovery of Infrared Rays

Infrared radiation was discovered by William Herschel in 1800. He passed sunlight through a prism and measured the temperature in different regions of the spectrum. He found that the temperature was maximum just beyond the red end of the visible spectrum. This invisible radiation beyond red light was called infrared radiation.

Exam Point: Infrared means “below red” in frequency or “beyond red” in wavelength.

4. Production of Infrared Rays

Infrared rays are mainly produced by hot bodies and vibrating molecules. Any object whose temperature is above absolute zero emits thermal radiation. At ordinary temperatures, most of this radiation lies in the infrared region.

Major Sources

Hot Bodies: Every warm object emits infrared radiation.

Sun: A major natural source of infrared radiation.

Electric Heater: Converts electrical energy into infrared heat radiation.

Incandescent Lamp: Emits visible light plus large infrared radiation.

Human Body: Emits infrared radiation due to body temperature.

Molecular Vibrations: Vibrating molecules produce infrared radiation.

Mathematical Idea

For electromagnetic radiation, frequency and wavelength are related by:

c = νλ

If wavelength is large, frequency is smaller. Infrared has wavelength larger than visible light, so its frequency is smaller than visible light. However, it still carries energy and produces heating effect.

5. Thermal Emission and Molecular Vibrations

Atoms and molecules in matter are never perfectly still unless the temperature is absolute zero. Due to thermal energy, molecules vibrate, rotate and collide. These accelerated charges produce electromagnetic radiation. For objects at normal temperature, this radiation is mostly infrared.

Higher Temperature → More Infrared Radiation

This is the basic reason why thermal cameras can detect human beings, animals, electrical faults and overheated machine parts even in darkness. Darkness only means absence of visible light; infrared radiation may still be present.

6. Detection of Infrared Rays

Infrared rays cannot be seen directly by human eyes, but they can be detected by their heating effect or by special sensors. The detector converts infrared energy into an electrical signal or a temperature change.

Thermopile

A thermopile is a collection of thermocouples connected in series. When infrared radiation falls on one junction, it becomes hot. Due to temperature difference between hot and cold junctions, an emf is produced. This emf is measured to detect infrared radiation.

Bolometer

A bolometer detects infrared radiation by measuring change in resistance caused by heating. When infrared radiation falls on the bolometer, its temperature changes. This changes its electrical resistance, which is used as a measure of infrared intensity.

Infrared Sensor

Infrared sensors are used in remote controls, automatic doors, security systems, thermal cameras and night vision devices. They detect infrared radiation and convert it into electrical signals.

7. Formula Cards for Infrared Rays

c = νλ

E = hν

E = hc/λ

λ_maxT = constant

P = σAT⁴

Infrared = Heat Radiation

Wien’s displacement law tells us that as temperature increases, the wavelength corresponding to maximum emission decreases. This is why very hot objects begin to glow red and then white; their emission shifts toward shorter wavelengths.

8. Quick Concept Questions

Q1. Why are infrared rays called heat rays?

Infrared rays are strongly absorbed by many objects and increase their internal energy. Therefore, they produce a heating effect and are called heat rays.

Q2. Are infrared rays visible to human eyes?

No. Infrared rays have wavelengths longer than red light, so they are outside the visible range of the human eye.

Q3. Why can thermal cameras work in darkness?

Thermal cameras detect infrared radiation emitted by warm objects. They do not depend on visible light.

Q4. What is the main source of infrared radiation on Earth?

Warm objects, human bodies, heated surfaces and the Sun are major sources of infrared radiation.

Q5. Which detector can be used to detect infrared radiation?

Thermopile, bolometer, infrared sensor, pyroelectric detector and thermal camera can detect infrared radiation.

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9. Greenhouse Effect and Infrared Rays

The greenhouse effect is one of the most important applications of infrared radiation in nature. The Sun sends electromagnetic radiation toward the Earth. A large part of solar radiation is in the visible and near-infrared region. The Earth absorbs this energy and becomes warm. After absorbing sunlight, the Earth re-emits energy mainly in the form of long-wavelength infrared radiation.

Greenhouse gases such as carbon dioxide, methane, nitrous oxide and water vapour absorb a part of this outgoing infrared radiation. After absorption, these gases re-radiate infrared radiation in different directions, including back toward the Earth. Due to this, the lower atmosphere remains warmer than it would be without greenhouse gases.

Incoming Radiation: Mainly visible and short-wavelength radiation from Sun.

Outgoing Radiation: Long-wavelength infrared radiation emitted by Earth.

Greenhouse Gases: CO₂, CH₄, N₂O, water vapour.

Effect: Absorption and re-radiation of infrared radiation.

Important Exam Explanation

Greenhouse effect is not completely harmful. A natural greenhouse effect is necessary because it keeps Earth warm enough for life. The problem begins when excessive greenhouse gases increase infrared trapping, leading to global warming and climate change.

10. Infrared Rays in Remote Controls

TV remotes, AC remotes and many projector remotes use infrared signals. A remote control contains an infrared LED. When a button is pressed, the circuit sends coded pulses to the infrared LED. The LED emits invisible infrared radiation. The receiver in the TV or AC detects this coded infrared signal and performs the required action.

Source: Infrared LED in remote control.

Signal: Coded pulses of infrared radiation.

Receiver: IR sensor in TV, AC or projector.

Limitation: Mostly line-of-sight communication.

The main reason infrared is used in remotes is that it is inexpensive, safe at low power, easy to encode and does not interfere much with radio communication. However, the remote normally needs to be pointed toward the device because infrared does not easily pass through walls.

11. Applications of Infrared Rays

Application	How Infrared Rays are Used
Night Vision	Detects infrared radiation emitted by warm objects in darkness.
Thermal Imaging	Creates heat maps of objects, machines, buildings and human bodies.
Medical Diagnostics	Used to locate abnormal heating, inflammation or blood-flow patterns.
Remote Controls	Used to transmit coded signals to TV, AC, projectors and smart devices.
Security Systems	PIR sensors detect infrared radiation from human bodies.
Astronomy	Infrared telescopes observe cool stars, dust clouds and distant objects.
Weather Forecasting	Satellites use infrared imaging to study clouds and temperature patterns.
Industrial Heating	Infrared heaters dry paint, heat materials and process food.

12. CBSE / NEET / JEE / IB / IGCSE Conceptual Questions

Q1. Why are infrared rays called heat radiation?

Infrared rays are readily absorbed by many materials and increase their internal energy. This produces heating, so they are called heat radiation.

Q2. Why can infrared cameras see objects in darkness?

Infrared cameras detect heat radiation emitted by objects. They do not require visible light, so they can detect warm objects even in darkness.

Q3. Why is infrared radiation important in greenhouse effect?

Earth emits long-wavelength infrared radiation. Greenhouse gases absorb and re-radiate this infrared radiation, warming the lower atmosphere.

Q4. Why do TV remotes use infrared rays?

Infrared LEDs are cheap, compact, low-power and safe for short-distance coded communication.

Q5. Why is infrared used in thermal imaging?

All warm bodies emit infrared radiation depending on their temperature. Thermal cameras detect this radiation and convert it into an image.

Q6. Which has higher frequency: infrared or microwaves?

Infrared rays have higher frequency than microwaves because infrared lies after microwaves toward the visible region in the EM spectrum.

Q7. Which has longer wavelength: infrared or visible red light?

Infrared rays have longer wavelength than visible red light.

Q8. What is the role of CO₂ in greenhouse effect?

CO₂ absorbs outgoing infrared radiation from Earth and re-radiates part of it back, contributing to atmospheric warming.

Q9. What is a PIR sensor?

A PIR sensor is a passive infrared sensor. It detects infrared radiation emitted by warm bodies, commonly used in security systems.

Q10. Why is infrared astronomy useful?

Infrared astronomy helps observe cool stars, dust clouds and objects hidden behind interstellar dust because infrared can pass through dust better than visible light.

13. NEET-Style MCQs

1. Infrared rays lie between: (A) UV and X-rays (B) visible red and microwaves (C) radio and microwaves (D) X-rays and gamma rays

Answer: B. Infrared rays lie between visible red light and microwaves.

2. Infrared radiation is mainly detected by: (A) Geiger counter (B) Thermopile (C) Gold leaf electroscope (D) Cloud chamber

Answer: B. Thermopile detects infrared radiation by heating effect.

3. Greenhouse gases mainly absorb: (A) Gamma rays (B) X-rays (C) Infrared radiation (D) Radio waves

Answer: C. Greenhouse gases absorb outgoing infrared radiation from Earth.

4. TV remote control generally uses: (A) X-rays (B) Infrared rays (C) Gamma rays (D) Sound waves

Answer: B. TV remotes commonly use infrared LEDs.

5. Infrared rays are also called: (A) Chemical rays (B) Heat rays (C) Nuclear rays (D) Cathode rays

Answer: B. Infrared rays produce strong heating effect.

6. The wavelength of infrared rays is generally: (A) shorter than visible light (B) longer than visible red light (C) shorter than X-rays (D) equal to gamma rays

Answer: B. Infrared has longer wavelength than visible red light.

14. JEE Main and JEE Advanced Style Questions

JEE Q1. An infrared wave has wavelength 1000 nm. Find its frequency.

Given: λ = 1000 nm = 1000 × 10^-9 m = 10^-6 m
Formula: c = νλ
ν = c/λ = 3 × 10⁸ / 10^-6
ν = 3 × 10¹⁴ Hz
Answer: 3 × 10¹⁴ Hz

JEE Q2. Compare photon energy of infrared radiation and visible radiation.

Photon energy is E = hν = hc/λ. Infrared radiation has longer wavelength than visible radiation, so its frequency and photon energy are lower.

Advanced Q1. Why does increasing temperature shift peak emission toward shorter wavelength?

According to Wien's displacement law, λ_maxT = constant. If temperature T increases, λ_max decreases. Therefore, hotter bodies emit maximum energy at shorter wavelengths.

Advanced Q2. A body emits maximum radiation at 10 μm. If temperature doubles, what is new peak wavelength?

Using Wien's law: λ_maxT = constant.
If T becomes 2T, λ becomes λ/2.
New wavelength = 10 μm / 2 = 5 μm.

15. IB, IGCSE and A-Level Physics Questions

IB Q1. Explain how infrared radiation is involved in global warming.

The Earth emits infrared radiation after absorbing sunlight. Greenhouse gases absorb some of this infrared radiation and re-emit it, reducing heat loss to space. Increased greenhouse gas concentration increases this effect, causing global warming.

IGCSE Q1. State one use of infrared radiation in daily life.

Infrared radiation is used in TV remote controls, thermal cameras and night vision devices.

A-Level Q1. Describe the working principle of a bolometer.

A bolometer absorbs infrared radiation and becomes warmer. This changes its electrical resistance. The change in resistance is measured and used to determine the intensity of infrared radiation.

A-Level Q2. Why is infrared radiation useful in astronomy?

Many cool astronomical objects emit strongly in infrared. Infrared radiation can also pass through interstellar dust better than visible light, helping astronomers observe hidden regions.

16. Case Studies

Case Study 1: Greenhouse Effect and Global Warming

The Earth receives solar radiation and emits energy back mainly as infrared radiation. Greenhouse gases such as carbon dioxide and methane absorb part of this infrared radiation.

Q1: Which radiation is emitted by Earth?
Answer: Infrared radiation.

Q2: Name two greenhouse gases.
Answer: CO₂ and CH₄.

Q3: Why does temperature increase?
Answer: Because outgoing infrared radiation is trapped and re-radiated.

Q4: Is natural greenhouse effect useful?
Answer: Yes, it keeps Earth warm enough for life.

Case Study 2: Thermal Camera in Hospital

A hospital uses thermal cameras to screen patients. The camera detects infrared radiation from the body surface.

Q1: What does the camera detect?
Answer: Infrared radiation.

Q2: Why can it work without visible light?
Answer: Human body emits infrared radiation naturally.

Q3: What is the main principle?
Answer: Hotter bodies emit more infrared radiation.

Case Study 3: TV Remote Control

A TV remote sends coded infrared pulses to the receiver. When a button is pressed, the TV sensor receives the code and performs the function.

Q1: Which source is inside the remote?
Answer: Infrared LED.

Q2: Why should remote face the TV?
Answer: Infrared communication is mostly line-of-sight.

Q3: Is infrared visible?
Answer: No.

Case Study 4: Night Vision

Security cameras use infrared LEDs and infrared sensors to see in darkness.

Q1: Which radiation is used?
Answer: Infrared rays.

Q2: Why is it useful at night?
Answer: It does not require visible light.

Q3: Give one military application.
Answer: Night vision goggles.

17. One-Page Revision Sheet

Infrared = Heat Radiation

c = νλ

E = hν

E = hc/λ

λ_maxT = constant

P = σAT⁴

Infrared lies between visible red light and microwaves.
Infrared rays are emitted strongly by hot bodies.
Thermopile and bolometer can detect infrared radiation.
Infrared radiation is important in greenhouse effect.
Remote controls commonly use infrared LEDs.
Thermal cameras detect infrared radiation from warm objects.

18. Common Mistakes

Confusing infrared rays with ultraviolet rays.
Writing infrared wavelength shorter than visible light.
Forgetting that infrared rays are invisible to human eyes.
Thinking remote controls use radio waves instead of infrared.
Not understanding that greenhouse gases absorb infrared radiation.
Confusing thermal imaging with ordinary photography.
Forgetting that all warm bodies emit infrared radiation.
Using wrong relation between frequency and wavelength.

19. Need Personal Help?

Still confused about Infrared Rays?

If Infrared Rays, Greenhouse Effect, Remote Controls, Thermal Imaging, EM Spectrum or NEET/JEE numerical questions are not clear, you can take one-to-one online Physics guidance from Kumar Sir.

Kumar Physics Classes

Phone / WhatsApp: +91-9958461445

Email: kumarsirphysics@gmail.com

Website: https://kumarphysicsclasses.com

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20. CBSE Board Important Questions

Q1. Why are infrared rays called heat rays?

Infrared rays are strongly absorbed by most materials and increase their internal energy. Therefore they produce a heating effect and are called heat rays.

Q2. Name two applications of infrared radiation.

1. Remote controls
2. Thermal imaging cameras

Q3. Why are thermal cameras able to work in darkness?

Because thermal cameras detect infrared radiation emitted by warm bodies rather than visible light.

Q4. Explain greenhouse effect using infrared radiation.

Earth emits infrared radiation after absorbing sunlight. Greenhouse gases absorb part of this infrared radiation and re-radiate it, causing warming of the atmosphere.

Q5. State one medical application of infrared rays.

Infrared thermography is used to detect abnormal temperature distribution in the human body.

21. NEET PYQ Style MCQs

1. Infrared radiation lies between:

A. UV and X-rays
B. Visible red and microwaves
C. Radio waves and microwaves
D. X-rays and Gamma rays

Answer: B

2. Which detector is commonly used for infrared radiation?

A. GM Counter
B. Cloud Chamber
C. Thermopile
D. Electroscope

Answer: C

3. Greenhouse gases mainly absorb:

A. X-rays
B. UV rays
C. Infrared radiation
D. Gamma rays

Answer: C

4. TV remote controls generally use:

A. Radio waves
B. Microwaves
C. Infrared rays
D. UV rays

Answer: C

5. The wavelength of infrared rays is:

A. Greater than visible light
B. Less than visible light
C. Equal to visible light
D. Equal to radio waves

Answer: A

22. JEE Main Numerical Problems

Numerical 1

An infrared wave has wavelength 2 μm. Find frequency.

Given: λ = 2 × 10⁻⁶ m

ν = c/λ
ν = (3 × 10⁸)/(2 × 10⁻⁶)
ν = 1.5 × 10¹⁴ Hz

Answer: 1.5 × 10¹⁴ Hz

Numerical 2

An infrared photon has frequency 3 × 10¹⁴ Hz. Find energy.

E = hν
E = 6.63 × 10⁻³⁴ × 3 × 10¹⁴
E = 1.99 × 10⁻¹⁹ J

Answer: 1.99 × 10⁻¹⁹ J

23. JEE Advanced Conceptual Questions

Advanced Q1

Why does increasing greenhouse gas concentration increase Earth's average temperature?

Because greenhouse gases absorb outgoing infrared radiation and reduce heat loss to outer space.

Advanced Q2

Why are infrared telescopes often placed at high altitudes?

Water vapour in the atmosphere absorbs infrared radiation. High-altitude observatories reduce atmospheric absorption.

Advanced Q3

Explain why stars invisible in visible light can be detected in infrared astronomy.

Dust clouds block visible light but allow infrared radiation to pass through. Therefore hidden stars can be detected.

24. IB Physics Questions

IB Question 1

Explain how thermal imaging works.

All objects above absolute zero emit infrared radiation. Thermal cameras detect intensity of infrared radiation and convert it into temperature maps.

IB Question 2

State one advantage and one limitation of infrared communication.

Advantage: Secure short-range communication.

Limitation: Requires line-of-sight.

25. IGCSE Physics Questions

IGCSE Q1

Which region of EM spectrum is mainly responsible for heat transfer from the Sun?

Answer: Infrared radiation.

IGCSE Q2

Why do dark surfaces absorb infrared radiation more effectively?

Dark surfaces have higher absorptivity and lower reflectivity.

26. A-Level Physics Questions

A-Level Q1

Explain the role of infrared radiation in Earth's energy balance.

Earth absorbs solar radiation and emits infrared radiation. The balance between incoming solar energy and outgoing infrared radiation determines Earth's temperature.

A-Level Q2

Why does a thermal camera detect temperature differences?

Because hotter objects emit greater infrared intensity than cooler objects.

27. Memory Tricks

IR = Invisible Radiation

Remote = Infrared

Heat = Infrared

Greenhouse = Infrared Trapping

Thermal Camera = Infrared Detector

Night Vision = Infrared Imaging

28. Exam Revision Sheet

c = νλ

E = hν

E = hc/λ

Infrared = Heat Radiation

Remote Controls → IR

Greenhouse Gases absorb IR

Infrared lies between visible red and microwaves.
Infrared rays are invisible.
Infrared radiation causes heating.
Thermopile and bolometer detect infrared radiation.
Greenhouse gases absorb infrared radiation.
Remote controls use infrared LEDs.
Thermal cameras detect infrared radiation.
Night vision works using infrared radiation.

Still Confused About Infrared Rays?

If Infrared Rays, Greenhouse Effect, Thermal Imaging, Remote Controls or Electromagnetic Spectrum concepts are not clear, contact Kumar Sir for one-to-one Physics guidance.

Kumar Physics Classes

📞 +91-9958461445

📧 kumarsirphysics@gmail.com

🌐 https://kumarphysicsclasses.com