Full Chapter Summary & Detailed Notes – Light: Shadows and Reflections
Introduction
Keshav and Jatin enjoy watching fireflies at night in rural Maharashtra. Fireflies produce their own light. The moon, however, only reflects sunlight. This leads to the central questions of the chapter:
- What objects produce their own light?
- How does light travel?
- Why do we see shadows and reflections?
11.1 Sources of Light
Natural Sources: Sun, stars, lightning, fireflies, glow-worms, some deep-sea creatures (bioluminescence).
Artificial Sources:
- Ancient → fire (wood, animal fat, oil lamps, candles)
- Modern → incandescent bulbs, tube lights, CFL, LED
Luminous objects → emit their own light (Sun, bulb, firefly)
Non-luminous objects → do not emit light; visible only when light falls on them and reflects (Moon, book, table, human body)
Science & Society Box
LED lamps → consume very less electricity, brighter, last 10–15 years, environment-friendly. Indian government promotes them heavily. At end-of-life → must be recycled, not thrown in garbage.
11.2 Does Light Travel in a Straight Line? (Rectilinear Propagation of Light)
This is the most important concept of the chapter.
Activity 11.1 → Three matchboxes with holes
| Condition | Observation |
|---|
| Holes perfectly aligned | Bright spot visible on screen |
| One box moved slightly | No spot visible |
Inference: Light travels only in a straight line.
Activity 11.2 → Candle flame through pipe
| Pipe type | Observation |
|---|
| Straight pipe | Flame clearly visible |
| Bent pipe | Flame not visible |
Inference: Light cannot bend around corners on its own → travels in straight line (rectilinear propagation).
11.3 Shadows
When an opaque object blocks light → dark patch formed behind it = shadow.
- Shadow forms only when light travels in straight line.
- Size of shadow depends on distance between object, light source and screen.
- During solar eclipse → Moon blocks sunlight → shadow falls on Earth.
11.4 Reflection of Light
When light falls on a smooth/shiny surface, it bounces back → reflection.
- Regular reflection: Smooth surface (mirror) → clear image
- Diffused reflection: Rough surface (wall) → no clear image, but object visible
11.5 Images formed by Plane Mirror
- Virtual, erect, same size, laterally inverted, same distance behind mirror as object is in front.
- Lateral inversion → left appears right and vice-versa (AMBULANCE written reversed on ambulances).
Key Takeaways & Exam Points
| Concept | Key Fact |
|---|
| Rectilinear propagation | Light always travels in straight line – proved by both activities |
| Luminous objects | Sun, bulb, firefly, candle flame |
| Non-luminous | Moon, table, book, human body |
| Shadow formation | Only when light is blocked by opaque object |
| Plane mirror image | Virtual, erect, laterally inverted, same size & distance |
| LED advantage | Less electricity, brighter, long life, eco-friendly |
One-Page Revision Map
Sources → Luminous (own light) | Non-luminous (reflect)
Travel → Straight line (Activities 11.1 & 11.2)
Shadow → Opaque object blocks light blocked
Reflection → Light bounces back
Plane Mirror → Virtual, erect, laterally inverted
LED → Energy saver, promoted by government
Key Terms & Definitions – Complete Glossary (Chapter 11)
All important terms with detailed explanation, examples, historical notes, exam tips & common mistakes.
Luminous Object
Definition: An object that emits its own light.
Examples: Sun, stars, lightning, fireflies, glowing insects, burning candle, electric bulb, LED, tube light.
Historical Note: Fireflies use bioluminescence (chemical reaction inside body) – same principle used in glow sticks.
Exam Tip: Always remember – Sun, bulb, candle flame, firefly = luminous.
Non-Luminous Object
Objects that do not produce light but become visible when light falls on them and reflects back.
Examples: Moon, planets, table, chair, book, human body, tree, mirror, wall.
Most Important Example: Moon – reflects sunlight → we see moonlight.
Exam Tip: Question “Why can we see the Moon?” → Answer: It reflects sunlight (non-luminous).
Rectilinear Propagation of Light
Definition: Light always travels in a perfectly straight line in a uniform medium.
Proof: Activity 11.1 (three matchboxes) & Activity 11.2 (straight vs bent pipe).
Real-life Proof: Laser beam, beam from torch in dark room, sunlight rays through gaps in leaves.
Exam Tip: This is the reason for formation of shadows and eclipses.
Shadow
Definition: Dark region formed when an opaque object blocks light rays.
Conditions for shadow: (1) Light source (2) Opaque object (3) Screen behind.
Types: Umbra (complete dark), Penumbra (partial dark) – though only umbra is discussed in Class 7.
Exam Tip: Shadow forms only because light travels in straight line.
Reflection of Light
Definition: Bouncing back of light rays when they strike a surface.
Regular reflection: On smooth surfaces (mirror) → clear image formed.
Diffused/Irregular reflection: On rough surfaces (wall, paper) → no clear image, but object visible from all sides.
Exam Tip: We can see most objects due to diffused reflection.
Plane Mirror
Properties of image:
1. Virtual (behind mirror)
2. Erect (upright)
3. Same size as object
4. Laterally inverted (left-right reversed)
5. Distance of image = distance of object from mirror
Real-life Example: AMBULANCE written reversed on ambulances so driver in front sees it correctly in rear-view mirror.
Lateral Inversion
Definition: Left side of object appears on right side in mirror and vice-versa.
Examples: Your right hand appears as left hand in mirror; “AMBULANCE” written reversed.
Exam Tip: Most frequently asked property of plane mirror image.
LED (Light Emitting Diode)
Advantages:
• Consumes 70–80 % less electricity
• Brighter light
• Life 50,000–1,00,000 hours
• No mercury – environment friendly
Government Initiative: UJALA scheme – distributing LED bulbs at low cost.
Exam Tip: Always mention “recycle, don’t throw in garbage”.
Bioluminescence
Definition: Production of light by living organisms through chemical reaction.
Examples: Fireflies, glow-worms, some jellyfish, deep-sea angler fish.
Purpose: Attract mate, prey, or defence.
Pinhole Camera (mentioned in related chapters)
Works on rectilinear propagation – light from object passes through tiny hole in straight line to form inverted real image on screen.
Exam Quick Tips & Mnemonics
- Luminous objects: Sun, Bulb, Candle, Firefly (SBCF)
- Non-luminous: Moon, Book, Table, Mirror (MBTM)
- Plane mirror image: “VELSD” → Virtual, Erect, Laterally inverted, Same size, same Distance
- Activity 11.1 & 11.2 prove: Light travels in straight line
- Shadow forms because: Light travels in straight line
- LED advantages: Low power, bright, long life, eco-friendly
Activities & 25+ Questions and Answers - Light: Shadows and Reflections
Activity 11.1: Light Through Holes
- Observation: Spot only when holes aligned; no spot if misaligned.
- Reason: Light travels straight.
Activity 11.2: Candle and Pipe
- Observation: Flame visible in straight pipe, not bent.
- Reason: Light travels straight, can't bend around corners.
Let Us Enhance Our Learning – Fully Solved
1. Choose the correct option.
(i) Green plants are called (a) autotrophs
(ii) The process by which plants make their food is (c) photosynthesis
(iii) The tiny pores present on the surface of leaves are called (b) stomata
(iv) The tissue that transports food in plants is (c) phloem
(v) Loss of water in the form of vapour from aerial parts of the plant is known as (a) transpiration
2. Fill in the blanks.
(i) Plants take carbon dioxide from air through stomata.
(ii) The green pigment in leaves is chlorophyll.
(iii) Water is transported in plants through xylem.
(iv) Cuscuta is an example of a parasite.
(v) Pitcher plant is an insectivorous plant.
3. Give one word for the following.
(i) Food-making process in plants → photosynthesis
(ii) Green pigment that traps sunlight → chlorophyll
(iii) Loss of water vapour from leaves → transpiration
(iv) Plant that traps insects → insectivorous
(v) Relationship where both organisms benefit → symbiosis
4. State whether the following statements are True or False.
(i) Plants need sunlight and water for growth. → True
(ii) Photosynthesis takes place only at night. → False (It takes place in the presence of sunlight during the day.)
(iii) Xylem transports food from leaves to other parts. → False (Xylem transports water and minerals; phloem transports food.)
(iv) Transpiration helps in upward movement of water. → True
(v) Mushroom is an autotrophic plant. → False (Mushroom is saprophytic.)
5. Short Answer Questions
(a) What is photosynthesis?
Photosynthesis is the process by which green plants make their own food using carbon dioxide, water, sunlight, and chlorophyll, releasing oxygen as a by-product.
(b) Write the balanced equation of photosynthesis.
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of sunlight and chlorophyll).
(c) Why was the plant in Pot C yellow in Activity 10.1?
The plant in Pot C was kept in the dark, so it lacked sunlight. Without sunlight, chlorophyll (green pigment) is not formed or breaks down, making the leaves yellow (chlorosis).
(d) Name two special modes of nutrition in plants.
Parasitic nutrition (e.g., Cuscuta) and insectivorous nutrition (e.g., pitcher plant).
6. Long Answer Questions
(a) Explain the role of sunlight and water in plant growth with reference to Activity 10.1.
In Activity 10.1, three pots with similar saplings were used:
- Pot A (direct sunlight + water): Showed maximum growth, healthy green leaves, increased height and number of leaves because sunlight is needed for photosynthesis (food making) and chlorophyll formation, while water is essential for photosynthesis, nutrient transport, and maintaining turgidity.
- Pot B (direct sunlight + no water): Poor growth, wilted or dried leaves because lack of water stops photosynthesis and causes loss of turgidity.
- Pot C (dark + water): Yellow leaves, elongated weak stem because no sunlight means no photosynthesis and no chlorophyll, leading to starvation and etiolation.
Conclusion: Both sunlight and water are crucial for healthy plant growth.
(b) Differentiate between xylem and phloem.
Xylem: Transports water and minerals from roots to leaves (upward only); made of dead cells; unidirectional.
Phloem: Transports food (glucose) from leaves to all parts (both directions); made of living cells; bidirectional.
(c) Describe any two special types of nutrition in plants with examples.
1. Parasitic nutrition: Plants without chlorophyll depend on host for food. Example: Cuscuta (amarbel) – twines around host, absorbs nutrients via haustoria.
2. Insectivorous nutrition: Plants trap and digest insects for nitrogen (in nitrogen-poor soil). Example: Pitcher plant – leaf modified into pitcher with lid and digestive juices; insects slip in and are digested.
7. Give reasons.
(i) Leaves are green in colour.
Due to the presence of chlorophyll, a green pigment that absorbs sunlight for photosynthesis.
(ii) Excessive transpiration causes wilting.
Loss of too much water makes plant cells lose turgidity (stiffness), causing leaves and stems to droop.
(iii) Plants kept in dark become yellow.
No sunlight prevents formation of chlorophyll, the green pigment breaks down, leading to yellowing (chlorosis).
8. Draw a labelled diagram of a leaf showing stomata.
(Draw a leaf structure with lower epidermis showing stomata – tiny pore guarded by two bean-shaped guard cells. Label: Stoma, Guard cells, Epidermal cells.)
25+ Solved Questions
1. What are luminous objects?
Objects emitting own light (Sun, fireflies).
2. Why Moon non-luminous?
Reflects Sun's light, no own light.
3. How light travels?
In straight lines (rectilinear).
Extra Practice Questions (Exam Booster)
4. Natural light sources?
Sun, stars, lightning, fireflies.
5. Artificial light sources?
Fire, bulbs, LED.
6. LED advantages?
Less power, brighter, longer life.
7. Why no light spot if matchbox moved?
Holes misaligned; light straight.
8. Why flame not visible in bent pipe?
Light can't bend.
9. Rectilinear propagation?
Straight line travel of light.
10. Firefly luminous?
Yes, bioluminescence.
11. Moonlight source?
Reflected sunlight.
12. Government LED promotion?
Energy saving, environment friendly.
13. Dispose LED how?
Recycle, not garbage.
14. Light travel proof?
Activities 11.1, 11.2.
15. Gnomon in sundial?
Rod casting shadow.
16. Why fireflies glow?
Chemical reaction.
17. Ancient lighting?
Fire with fat/oil/wax.
18. Electric light sources?
Bulbs, tubes, LED.
19. Non-luminous example?
Moon, planets.
20. Why light straight important?
Explains shadows, reflections.
21. Activity 11.1 inference?
Light straight path.
22. Activity 11.2 inference?
Light can't bend.
23. LED environment benefit?
Less energy, less pollution.
24. Lightning luminous?
Yes, natural discharge.
25. Stars luminous?
Yes, nuclear fusion.
