NCERT Class 7 Science Curiosity Chapter 11 Light Shadows and Reflections PDF Download

Chapter 11: Light: Shadows and Reflections

In the Western Ghat region of Maharashtra, Keshav spends part of his summer vacation at his friend Jatin's grandparents' village. Having lived in a big city, he finds the forests, fresh air, sounds of gushing streams, and chirping birds a novel experience.

However, for Keshav, the most fascinating sight is the dance of hundreds of fireflies at night, flashing their lights in a wonderful performance. Jatin's grandparents explain that fireflies are seasonal insects and they use light to communicate. Unfortunately, Keshav also learns the number of fireflies is decreasing due to light pollution, reduced forest cover, and excessive tourism.

At the end of their vacation, Keshav and Jatin board an evening bus back to their city. As the bus winds through the hilly roads, Keshav watches the moonlit landscape and the beams from the headlights of passing vehicles flashing by. He is reminded of the many poems and songs about moonlight, and wonders - does the Moon actually produce its own light? Did we not learn in the chapter 'Beyond Earth' in the Grade 6 Science textbook Curiosity that all other objects in our solar system shine only by reflecting the light of the Sun? Is moonlight just reflected sunlight? Which objects give off their own light? While thinking, he notices something strange - light seems to move in a straight line!

11.1 Sources of Light

The Sun gives out or emits its own light and is the main source of natural light on the Earth. Stars, lightning, natural fire, and certain animals also emit their own light (Fig. 11.1).

In ancient times, humans learnt to create fire - the earliest form of artificial lighting. With time, they learnt to create fire using different fuels, such as animal fat, oil, wax, and gas (Fig. 11.2). With the invention of electricity and different kinds of electric light sources, most of the lighting needs of humans are now met by electric lighting (Fig. 11.3).

[Figure 11.1: Some natural sources of light, See in your textbook]

[Figure 11.2: Fire as a source of artificial light, See in your textbook]

[Figure 11.3: Some sources of electric light, See in your textbook]

Objects that emit their own light are called luminous objects. Objects that do not emit their own light are called non-luminous objects. The Moon is a non-luminous object. It does not emit its own light. It just reflects the light emitted by the Sun that falls on it.

Science and Society

Light Emitting Diode (LED) lamps are modern light sources that consume much less power, are brighter and last longer than traditional lamps. This not only reduces electricity bills but is also better for environment. Recognising their advantages, the Indian government has made substantial efforts to promote the use of LED lamps nationwide. At their end of life, LED lamps must be appropriately disposed or recycled, and not thrown in the garbage.

Teacher's Note

Remember the difference between luminous and non-luminous: luminous objects make their own light (the Sun, a torch, a candle), while non-luminous objects only shine because light bounces off them (the Moon, your desk, a mirror). In exams, when asked why the Moon glows, always say it reflects the Sun's light - it does not produce light itself.

11.2 Does Light Travel in a Straight Line?

Let us do an activity to try to find out.

Activity 11.1: Let us investigate

• Take three matchboxes and make a hole in the inner tray of each matchbox, exactly at the same position.
• Arrange these three matchboxes in a straight line. Make sure that all three holes are exactly at the same height and are in a line as shown in Fig. 11.4.
• Place a torch light on one side of the matchboxes, ensuring that its lamp is at the height of the holes.
• Place a cardboard (screen) on the other side of the matchboxes and obtain a bright spot on it. (You may need to slightly adjust the heights of boxes.)
• Move one of the matchboxes slightly to a side or up and down. Are you able to obtain the light spot on the screen now?

When all the three holes are not in the same line, we could not obtain the light spot on the screen. These observations suggest that light travels in a straight line.

[Figure 11.4: Matchbox activity - light through holes, See in your textbook]

Activity 11.2: Let us explore

Caution - Use a lighted candle under adult supervision only.

• Take a long hollow pipe of some flexible material and align it so that you can see the candle flame as shown in Fig. 11.5a.
• Now, bend the pipe and try to see the candle flame again (Fig. 11.5b). Can you still see it?

You could see the candle flame through a straight pipe but not through a bent pipe. This shows that light travels in a straight line.

[Figure 11.5: Viewing candle flame through (a) a straight pipe (b) a bent pipe, See in your textbook]

Teacher's Note

The key idea in both activities is that light only travels along a straight path. If you block the straight path (by moving a matchbox or bending the pipe), the light cannot go around the obstacle - it stops. This is why shadows are sharp with straight edges: light cannot bend around objects to fill in the dark space behind them.

Key Points

• Objects that make their own light, such as the Sun, stars, and fire, are called luminous objects. Objects that do not make their own light, like the Moon, are non-luminous and only shine by reflecting light from luminous sources.
• Light always travels in straight lines. You can see this when light passes through holes in a line, or when you cannot see an object through a bent pipe even though you can through a straight one.
• Humans have used different sources of light through history: first natural fire, then lamps with oil and wax, and now mostly electric lights including LED lamps which use much less power.

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