Effects of Light | Class 7 | Science | Chapter 17 | Maharashtra State Board
Explore the captivating world of light in this lesson, unravelling the mysteries of umbra and penumbra in shadows, experiencing the awe-inspiring phenomena of solar and lunar eclipses. Delve into the science of light scattering, and witness the enchanting zero-shadow condition. Join us on a journey through the Effects of Light, where celestial events and atmospheric phenomena come to life.
Questions & Answers
1. Fill in the blanks.
(a) When the beams from the headlights of a car fall on an object in the 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.
2. Give reasons.
(a) Space beyond the earth’s atmosphere appears dark.
Ans. Space beyond Earth's atmosphere appears dark because there is no atmosphere to scatter and diffuse sunlight, causing the absence of direct sunlight and creating a perception of darkness.
(b) We are able to read while sitting in the shade.
Ans. We can read in the shade because light is scattered in all directions by the atmosphere, objects, and surfaces. This scattered light, known as diffuse light, illuminates the shaded areas, allowing us to see and read.
(c) We should not observe the solar eclipse with naked eyes.
Ans. Observing a solar eclipse with naked eyes can cause eye damage because the intense sunlight during an eclipse can harm the retina, leading to permanent or temporary vision impairment. Using proper eye protection, such as solar viewing glasses, is essential to safeguard your eyes during a solar eclipse.
3. Give some examples of scattering of light that we come across in day-to-day life.
Ans. Some examples of scattering of light that we come across in day-to-day life are:
1. The blue colour of the sky and the red colour of the sun at sunrise and sunset. This is due to Raylight scattering, which occurs when small particles in the atmosphere scatter shorter wavelengths (blue and violet) more than longer wavelengths (red and orange).
2. The white colour of the clouds and the fog. This is due to Mie scattering, which occurs when larger particles in the atmosphere scatter light in a more directional manner, causing all wavelengths to be reflected equally.
3. The rainbow and the prism. This is due to refraction and dispersion, which occur when light passes through a medium with a different refractive index, such as water or glass, and splits into its component colours.
4. The mirage and the twinkling of stars. This is due to refraction and scattering, which occur when light travels through layers of air with different temperatures and densities, causing it to bend and flicker.
4. Why is the shadow of a bird flying high not seen on the earth?
Ans. The shadow of a bird flying high is not seen on the Earth because it's too far away. The shadow becomes faint and spread out, making it hard to see against the bright ground. Also, light from the sun gets scattered in all directions, which softens shadows. So, the shadow is there, but it's too faint for us to see.
5. Why is a penumbra not obtained from a point source?
Ans. A penumbra is not obtained from a point source because a point source of light emits light rays in all directions that are parallel to each other. This means that any object blocking the light will create a sharp, well-defined shadow with no penumbra, which is the blurry or less dark region at the edges of a shadow. The penumbra is created when light comes from a source that's spread out or "diffuse", like a light bulb, because the rays of light are coming from different points and are not all parallel. So, with a point source, there's no penumbra, just a sharp shadow.
6. Answer the following questions in your own words.
(a) What is meant by scattering of light?
Ans. Scattering of light refers to the process where light bounces off in different directions when it encounters particles, molecules, or imperfections. This is why we see a blue sky, as the short-wavelength blue light from the sun is scattered more in all directions by the molecules in Earth's atmosphere. It's a fundamental process that influences how we perceive the world around us.
(b) Does the shadow really vanish in the zero-shadow condition?
Ans. In the zero-shadow condition, the shadow doesn't actually vanish. Instead, it aligns directly underneath the object. This happens when the sun is directly overhead, and the shadow of the object falls directly below it, making it appear as if there's no shadow. So, the shadow is still there, but it's hidden beneath the object. This is why we call it the "zero-shadow" moment.
(c) Will the laser beam be seen if it passes through a glass box which contains a lighted incense stick?
Ans. Yes, the laser beam will be visible because the smoke from the incense stick scatters the light, making the path of the beam visible. This is known as the Tyndall effect.
7. Discuss and write:
(a) Write a science-based paragraph on ‘What if the sun did not rise’?
Ans. If the sun did not rise one day, it would mean that the Earth stopped rotating on its axis. This would have drastic consequences for the planet and its life forms. One side of the Earth would be plunged into eternal darkness, while the other side would face constant daylight. The temperature difference between the two sides would create extreme winds and storms. The oceans would redistribute themselves, flooding the poles and leaving the equator dry. Most plants and animals would die, unable to adapt to the new conditions. Only some microorganisms and deep-sea creatures might survive in this scenario.
(b) What efforts will you make to remove the misconceptions about eclipses?
Ans. I would use methods such as creating and sharing informative content, organizing and participating in public outreach events, encouraging and supporting scientific literacy and curiosity, and collaborating and communicating with other educators, scientists, media, and influencers.
(c) Various eclipses and the conditions during that period.
Ans. There are different types of eclipses, primarily solar and lunar, each occurring under specific conditions:
1. Solar Eclipse: Occurs when the Moon passes between the Earth and the Sun, blocking all or part of the Sun's light. There are three main types:
- Total Solar Eclipse: The Moon completely covers the Sun.
- Partial Solar Eclipse: The Moon partially covers the Sun.
- Annular Solar Eclipse: The Moon covers the central part of the Sun, leaving a ring-like appearance.
2. Lunar Eclipse: Occurs when the Earth passes between the Sun and the Moon, causing the Earth's shadow to fall on the Moon. There are three main types:
- Total Lunar Eclipse: The Moon is completely in the Earth's shadow.
- Partial Lunar Eclipse: Only a part of the Moon enters the Earth's shadow.
- Penumbral Lunar Eclipse: The Moon enters the Earth's penumbral shadow, resulting in a subtle shading.
The conditions during these eclipses can vary, affecting visibility and the overall experience. The clarity of the sky, geographic location, and the Moon's position in its orbit contribute to the specific conditions during each eclipse.
8. Explain the difference:
(a) Point sources and Extended sources.
Point sources | Extended sources |
Emit energy from a single, concentrated point. | Emission distributed over a significant area. |
Localized emission. | Spread out over an extended region. |
Size is negligible compared to the observer's distance. | Size comparable to or larger than the observer's distance. |
Examples: Stars, light bulbs. | Examples: Nebulae, galaxies, lighted billboards. |
(b) Umbra and Penumbra.
Umbra | Penumbra |
Darkest part of a shadow. | Lighter shading around shadow edges. |
Light is completely blocked. | Partial blocking of light. |
Occurs behind an object. | Partial blocking of light. |
Total eclipse during astronomical events. | Experienced during partial eclipses and everyday shadows. |
E X T R A
1. Fill in the blanks:
1. During solar eclipse moon comes between the sun and earth.
2. A solar eclipse is seen only on a new moon day.
3. During lunar eclipse earth comes between the sun and the moon.
4. A lunar eclipse is seen only on a full moon night.
5. The day on which the sun reaches exactly overhead is called the zero shadow day.
6. As seen from the earth, when a planet or star passes behind the moon, that state is called a occultation.
2. Give scientific reasons:
1. Sky appears blue to us.
Ans. The sky appears blue because of Raylight scattering, where shorter-wavelength blue light is scattered more by the gases and particles in Earth's atmosphere, causing it to be more prevalent in the sky and reaching our eyes.
2. Solar eclipse is either partial or total.
Ans. A solar eclipse is either partial or total based on the observer's location relative to the umbra and penumbra of the Moon's shadow. In the umbra, the Sun is completely blocked, leading to a total solar eclipse, while in the penumbra, only a part of the Sun is obscured, resulting in a partial solar eclipse.
3. Answer the following.
1. What is Occultation?
Ans. Occultation is an event that occurs when one object is hidden from view by another object that passes between them. This term is often used in astronomy, for instance, when a celestial body like the moon or a planet passes in front of a star, hiding it from view.
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