This MCQ module is based on: Radiation — Heat from the Sun and Its Uses
TOPIC 25 OF 46
Radiation — Heat from the Sun and Its Uses
🎓 Class 7
Science
CBSE
Theory
Ch 7 — Heat Transfer in Nature
⏱ ~14 min
🌐 Language: [gtranslate]
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7.4 Radiation — Heat That Needs No Medium
Grandfather leads the twins out to the veranda, where the morning Sun is warming the wooden railing. "Put your palm near the railing without touching it," he says. Pema feels a gentle warmth on her skin. "Now look up," he continues, "that warmth has travelled 150 million kilometres through empty space — from the Sun — to reach your hand. There is no rope of metal, no flowing air between the Sun and us. How on earth does the heat get here?"
The answer is the third mode of heat transfer — radiation. Radiation is how heat hops across space without any need for a solid, a liquid or even a gas to carry it. Every hot body — the Sun, a bonfire, a red-hot iron, even your own skin — constantly sends out invisible heat waves called infrared radiation.
Radiation in one line: heat travels as invisible waves that can cross a vacuum; no material medium is required.
Activity 7.4 — Feel the Candle Flame L2 Understand
You will need: a candle, matches, and your hand.
Steps:
- Light the candle and place it safely on a table.
- Hold your palm about 10 cm beside the flame (not above it).
- Slowly move your palm closer and farther from the flame.
- Now hold your palm about 10 cm above the flame for just a second.
Predict: Will your palm feel warmer when held beside the flame or above it? Why?
Beside the flame, you feel gentle heat at a distance — that heat reaches you by radiation (neither air flow nor contact). Above the flame you feel a much stronger heat because hot air rises up by convection, adding to the radiation. The activity shows radiation is real but generally weaker than a convection current from the same source.
Where Else Do We Meet Radiation?
- Sitting in front of a wood fireplace — your face feels warm though no wire or current touches you.
- A red-hot electric heater — warmth reaches you even when the room air is still.
- The warmth on your arms under sunlight on a cold winter day in Gangtok.
Dive Deeper — Light Colours or Dark Colours?
Why do people in Rajasthan usually wear white kurtas in summer, while Pema and Palden in Gangtok reach for dark-coloured sweaters in winter? The answer lies in how different surfaces absorb radiation.
- Dark surfaces (black, navy, dark brown) absorb most of the radiation that falls on them. They get hot quickly — ideal when you want warmth.
- Light surfaces (white, cream, pastel shades) reflect most of the radiation back. They stay cooler — ideal when you want to avoid getting hot.
So the choice is simple: wear white or light cotton in an Indian summer, and dark woollens in a Himalayan winter. In fact roofs in hot towns are often painted white, while solar water-heaters are painted black to soak up every ray.
Applications of Radiation
1. The Thermos Flask — Blocking All Three Modes
When Palden's mother goes trekking, she carries tea in a thermos that keeps it hot for hours. How does one clever flask defeat all three modes of heat transfer at once?
- Silvered inner walls reflect infrared radiation back into the liquid (blocking radiation).
- A vacuum between the double walls means there are almost no particles to either carry heat by conduction or swirl as a convection current.
- The plastic/cork stopper at the top is a poor conductor, so very little heat escapes upward through the cap.
The same flask keeps cold lemonade cold — for the simple reason that it blocks heat from getting in just as well as it blocks heat from getting out.
2. Solar Cookers and Solar Heaters
A solar cooker is a shallow box lined with black-painted metal and topped with a clear glass lid. Radiation from the Sun streams through the glass, gets soaked up by the black interior, and heats food placed inside — all without a single stick of firewood. Rooftop solar water heaters work the same way: black panels catch sunlight and warm the water running through them.
3. Greenhouse Effect (A First Glimpse)
A glass greenhouse lets sunlight enter and warm the soil and plants inside. The warm surfaces give off their own heat as longer-wavelength radiation, which the glass does not let escape as easily. So the inside stays warmer than the outside — perfect for tomatoes in a cold country. The atmosphere of the Earth does something similar on a planetary scale.
4. Geysers, Room Heaters and Infrared Lamps
Infrared lamps used in bathrooms and in newborn-care units radiate gentle warmth directly to the skin without having to heat all the surrounding air.
Real-World Heat Transfer — Usually All Three at Once
In most real kitchens and living rooms, heat travels by more than one mode at the same time. A few familiar examples:
| Situation | Modes at work |
|---|---|
| Water boiling in a kettle on the stove | Conduction (through the metal base) + Convection (hot water rising within the kettle) |
| Making a roti on a tawa | Conduction (from tawa to dough) + Radiation (from the red-hot pan surface) |
| The Sun warming the Earth | Radiation only (through the vacuum of space) |
| Clothes drying on a line | Evaporation + Radiation from the Sun |
| Sitting by a winter fire | Radiation (front of body) + Convection (warm air rising) |
Match the Weather to the Clothing
Click one item from the left column and then the matching clothing on the right. Correct matches turn green.
Weather
Hot, sunny Chennai afternoon (40 °C)
Freezing Gangtok morning (2 °C)
Humid Kolkata monsoon (32 °C)
Cold desert night in Ladakh (-5 °C)
Best clothing
Thick dark woollen jacket + thermals
Multiple layers of fur-lined clothing
Loose white cotton kurta
Light cotton with breathable weave
7.5 Heat Transfer in Daily Life
The ideas of conduction, convection and radiation come together every time we choose clothes, build a house, or cook a meal.
Clothes for Every Season
- Summer: loose, light-coloured cotton — cotton absorbs sweat, the light colour reflects the Sun's radiation, and the loose fit lets a convection breeze flow past the skin.
- Winter: thick woollen sweaters, caps and blankets — the wool traps still air, a poor conductor, so body heat stays inside.
Houses for Every Climate
- Hot regions (Jaisalmer, Chennai): thick walls of mud or stone, light-coloured roofs, small windows facing away from the Sun, and courtyards that allow convection breezes.
- Cold regions (Gangtok, Ladakh): double walls with insulating material, dark-coloured roofs that absorb solar radiation during the day, and small windows to stop warm air from escaping.
Blankets and sweaters trap warmth not by producing heat themselves but by trapping the still air between their fibres — one of nature's finest insulators.
Competency-Based Questions
Pema's mother packs hot soup in a thermos flask for a trek. Six hours later on top of the hill, the soup is still steaming hot even though the outside air is very cold. Palden is amazed: "How can one small flask hold back all the heat?"
1. The vacuum between the two walls of a thermos flask prevents ___________ and ___________. L1
conduction and convection — both need particles, which are mostly absent in a vacuum.
2. The inside walls of a thermos flask are silvered because: L2
(b) Silver coatings reflect infrared radiation, stopping heat from leaving (or entering) through radiation.
3. Why do people in coastal Kerala usually wear white or light-coloured cotton clothes? L2
Light colours reflect most of the Sun's radiation, so the body absorbs less heat. Cotton also absorbs sweat and allows air movement, keeping the wearer cool in a hot, humid climate.
4. True or False: Heat from the Sun reaches Earth by convection. L1
False. The space between the Sun and Earth is a vacuum, and convection cannot occur in a vacuum. Heat from the Sun reaches us by radiation.
5. A village woman roasts peanuts on a hot iron tawa. Identify all the modes of heat transfer at work. L4
Two modes are at work: (i) conduction — from the hot tawa directly to the peanuts resting on it; (ii) radiation — infrared heat given off by the glowing hot pan also reaches the upper surface of the peanuts. A small amount of convection also occurs as warm air rises from the pan.
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R does not explain A. (C) A true, R false. (D) A false, R true.
A: Radiation can transfer heat through a vacuum.
R: Radiation is in the form of electromagnetic waves that do not need any material medium.
(A) — Electromagnetic (infrared) waves travel through empty space, which is why sunlight reaches us at all.
A: Solar cookers are painted black on the inside.
R: Black surfaces are very good absorbers of heat radiation.
(A) — The black lining soaks up the Sun's rays efficiently, heating the food inside.
A: We feel cooler wearing white clothes in summer.
R: White clothes produce a cooling chemical on contact with sweat.
(C) — A is true but R is false. White clothes feel cooler because they reflect most of the Sun's radiation, not because of any chemical reaction with sweat.
Frequently Asked Questions — Radiation — Heat from the Sun and Its Uses
What does the topic 'Radiation — Heat from the Sun and Its Uses' cover in Class 7 Science?
The topic 'Radiation — Heat from the Sun and Its Uses' is part of NCERT Class 7 Science Chapter 7 — Heat Transfer in Nature. It covers the key ideas of radiation, heat transfer, Sun, solar cooker, dark surfaces, absorption, emission, explained through everyday examples, labelled diagrams and hands-on activities drawn from the NCERT Curiosity textbook. Students learn not just definitions but also the reasoning behind each concept so they can answer competency-based questions and assertion–reason items. The lesson helps Class 7 students build a strong base for higher classes by linking each idea to real observations at home, school and in nature, and by preparing them for CBSE school assessments and Olympiads.
Why is 'Radiation — Heat from the Sun and Its Uses' important for Class 7 NCERT Science?
'Radiation — Heat from the Sun and Its Uses' is important because it builds core scientific thinking that Class 7 students will use throughout middle and secondary school. NCERT Chapter 7 — Heat Transfer in Nature — introduces radiation and related ideas that appear again in Class 8, 9 and 10 Science. Mastering this subtopic helps students read labels and safety signs, understand news about science and technology, and perform better in CBSE school exams. The chapter also encourages curiosity and evidence-based thinking — skills that support the National Education Policy (NEP) 2020 focus on conceptual understanding and competency-based learning.
What are the key concepts students should remember from Radiation — Heat from the Sun and Its Uses?
The key concepts in 'Radiation — Heat from the Sun and Its Uses' for Class 7 Science are: radiation, heat transfer, Sun, solar cooker, dark surfaces, absorption, emission. Students should be able to define each term in their own words, give at least one everyday example, and explain how the concept connects to other chapters in NCERT Class 7 Science. For example, linking the idea to daily life — in the kitchen, classroom or outdoors — makes revision easier. Writing short notes, drawing labelled diagrams and solving the NCERT in-text and exercise questions for Chapter 7 will help students retain these concepts for unit tests and the annual CBSE examination.
How is Radiation — Heat from the Sun and Its Uses taught using activities in NCERT Curiosity Class 7?
NCERT Curiosity Class 7 Science teaches 'Radiation — Heat from the Sun and Its Uses' using an inquiry-based approach with Predict–Observe–Explain activities. Students are asked to make a guess first, then perform a simple experiment with safe, easily available materials, and finally explain what they observed. This matches the NEP 2020 focus on learning by doing. For Chapter 7 — Heat Transfer in Nature — the textbook includes hands-on tasks, labelled diagrams and questions that build Bloom's Taxonomy skills from Remember (L1) to Create (L6). Teachers use these activities, along with competency-based questions (CBQs) and assertion–reason items, to check real understanding rather than rote memorisation.
What real-life examples of radiation can Class 7 students observe at home?
Class 7 students can observe radiation at home in many simple ways linked to 'Radiation — Heat from the Sun and Its Uses'. Kitchens, school bags, playgrounds and the night sky are full of examples that connect to NCERT Chapter 7 — Heat Transfer in Nature. For instance, students can check labels on food and cleaning products, watch changes while cooking, or observe the Sun and Moon across a week. Keeping a small science diary — noting the date, what was observed and a quick sketch — turns everyday life into a science lab. These real-life connections make concepts stick and prepare students well for competency-based questions in CBSE Class 7 Science.
How does 'Radiation — Heat from the Sun and Its Uses' connect to other chapters of Class 7 Science?
'Radiation — Heat from the Sun and Its Uses' connects to many other chapters in NCERT Class 7 Science Curiosity. The ideas of radiation appear again when students study related topics like heat, light, changes, life processes and Earth-Sun-Moon. For example, understanding this subtopic helps in building mental models for later chapters and for Class 8, 9 and 10 Science. Teachers often use cross-chapter questions in CBSE examinations to test whether students can apply what they learned in Chapter 7 — Heat Transfer in Nature — to new situations. This integrated approach matches the NEP 2020 and NCF 2023 focus on holistic, competency-based learning.
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