Chapter 17 – Effects of Light

Maharashtra Board Textbook Solutions – Standard Seven

📝 1️⃣ Fill in the Blanks

(a) When the beams from the headlights of a car fall on an object at night, the shadows called umbra and penumbra can be seen.

(b) During a lunar eclipse, the shadow of the earth falls on the moon.

(c) During a solar eclipse, the shadow of the moon falls on the earth.

(d) Various shades of colour are seen in the sky at sunrise and sunset due to scattering of light.

💡 2️⃣ Give Reasons

(a) Space beyond the earth’s atmosphere appears dark.

  1. Sunlight gets scattered by particles in the atmosphere, making the sky appear bright.
  2. In space, there is no atmosphere.
  3. Without air particles, light cannot scatter.
  4. Therefore, space beyond the earth’s atmosphere appears dark.

(b) We are able to read while sitting in the shade.

  1. The sun is an extended source of light.
  2. Shadows formed in sunlight usually contain a penumbra (partial shadow).
  3. The shade still receives some scattered light.
  4. Hence, there is enough light available to read in the shade.

(c) We should not observe a solar eclipse with naked eyes.

  1. The sun emits harmful ultraviolet rays.
  2. Even during a solar eclipse, these rays reach the earth.
  3. Watching the eclipse without protection can damage our eyes.
  4. Therefore, a solar eclipse should not be viewed with naked eyes.

🌈 3️⃣ Examples of Scattering of Light in Daily Life

  1. A beam of light from a cinema projector.
  2. Car headlights visible in fog.
  3. The sun appearing red at sunset.
  4. The blue colour of the sky.

🐦 4️⃣ Why is the Shadow of a Bird Flying High Not Seen on the Earth?

  1. The sun is a very large (extended) source of light.
  2. As the bird flies higher, its umbra becomes smaller.
  3. The penumbra becomes faint and eventually disappears.
  4. Therefore, the shadow of a bird flying high is not visible on the ground.

🔦 5️⃣ Why is a Penumbra Not Obtained from a Point Source?

A point source of light produces only a dark shadow (umbra). Since the light comes from a single point, partial shadow (penumbra) is not formed.

📘 6️⃣ Answer the Following in Your Own Words

(a) What is meant by scattering of light?

When light rays strike dust particles, molecules, and tiny particles in the atmosphere, they spread in different directions. This spreading of light is called scattering of light. It explains why the sky appears blue and the sun looks red at sunset.

(b) Does the shadow really vanish in the zero shadow condition?

At noon, when the sun is directly overhead, the shadow forms exactly under our feet. It is very small and not visible clearly. The shadow does not vanish; it just appears invisible.

(c) Will a laser beam be seen if it passes through a glass box containing a lighted incense stick?

Yes. The smoke particles from the incense stick scatter the laser light. The scattered light enters our eyes, making the laser beam visible.

✍️ 7️⃣ Discuss and Write

(a) What if the Sun Did Not Rise?

If the sun did not rise, the earth would remain dark. Plants would not be able to prepare food due to lack of sunlight. This would disturb the food chain and affect all living organisms. Solar energy production would also stop. Since the sun is the primary source of energy for life on earth, survival would become impossible.

(b) Efforts to Remove Misconceptions About Eclipses

  1. Spread scientific information through media.
  2. Create awareness before an eclipse occurs.
  3. Provide safe viewing through special goggles and telescopes.
  4. Demonstrate eclipses using models of the sun, earth, and moon.
  5. Explain that eating food during an eclipse does not cause harm.

(c) Types of Eclipses and Their Conditions

🌑 Solar Eclipse

Occurs when the moon comes between the sun and the earth.

Types:

  1. Total Solar Eclipse – The sun is completely covered; visible in the umbra region.
  2. Partial Solar Eclipse – Only part of the sun is covered; visible in the penumbra region.
  3. Annular Solar Eclipse – The centre of the sun is covered, leaving a bright ring visible.

🌕 Lunar Eclipse

Occurs when the earth comes between the sun and the moon.

Types:

  1. Total Lunar Eclipse – The entire moon comes under the earth’s shadow.
  2. Partial Lunar Eclipse – Only part of the moon comes under the earth’s shadow.

🔍 8️⃣ Explain the Difference

(a) Point Source and Extended Source of Light

Point Source of LightExtended Source of Light
Forms only the umbra.Forms both umbra and penumbra.
Example: A small pinhole letting sunlight in.Example: The Sun.

(b) Umbra and Penumbra

UmbraPenumbra
Region where light is completely blocked.Region where light is partially blocked.
Darker part of the shadow.Lighter outer part of the shadow.
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