Sources of Light and How It Travels

Class 7 Science · Chapter 11: Light: Shadows and Reflections · NCERT Curiosity section 11.1–11.3

Learn

On a summer trip to a hill village, Neel and Kabir watch hundreds of fireflies flashing in the dark. On the bus home, Neel watches the moonlit hills and the beams of passing headlights. "Does the Moon make its own light?" he wonders. "And why do those beams look so straight?"

Objects that give out their own light are luminous: the Sun, stars, fire, lightning, fireflies, and lamps. Objects that do not are non-luminous. The Moon is non-luminous: moonlight is sunlight reflected off it. LED lamps give bright light using little power and last long; old ones should be recycled, not thrown in the garbage.

Neel lines up three matchboxes with holes at the same height and shines a torch through them: a bright spot appears on a card. Shift one box slightly and the spot vanishes. A candle flame is visible through a straight pipe but not a bent one. Light travels in straight lines.

Materials let light through differently. Transparent ones (clear glass, water) let almost all light pass; translucent ones (tracing paper, frosted glass) let some through; opaque ones (cardboard, wood, metal) block it completely.

Explore: does light pass through?

1. Test each material and sort it.

Remember

• Luminous objects give out their own light; the Moon only reflects sunlight.
• Light travels in straight lines; materials are transparent, translucent or opaque.

Quick check

Why can't you see a candle flame through a bent pipe?

1. light travels in straight lines
2. the pipe absorbs heat
3. the flame goes out
4. pipes are luminous

Think about it

Why are fireflies becoming rarer in many places?

Practice

Practise with fresh questions at three levels (easy, medium, hard), some answered hands-on, with hints and worked answers. Get 4 of your last 5 right to finish the topic.