Notes For All Chapters – General Science Class 7
Effects of Light
1. Introduction
- Light is a form of energy that helps us see objects.
- It travels in a straight line and can be reflected, scattered, and bent (refracted).
- Light helps in forming shadows, eclipses, and various natural effects like the blue sky and colorful sunsets.
2. Shadows and Their Formation
✔ A shadow is formed when an opaque object blocks the path of light.
✔ Three Conditions for a Shadow to Form:
- There must be a source of light (e.g., sun, bulb, torch).
- An opaque object should block the light.
- A surface (ground or wall) is needed to form the shadow.
✔ Types of Shadows:
| Type | Description | Example |
|---|---|---|
| Umbra | Completely dark region of a shadow. | Seen in a total solar eclipse. |
| Penumbra | Partially shaded region of a shadow. | Seen in a partial solar eclipse. |
✔ Why does a bird flying high not cast a shadow on the ground?
- The bird is far from the ground, so its shadow spreads out and disappears before reaching the surface.
✔ Why do we not see shadows at noon on a zero shadow day?
- On a zero shadow day, the sun is exactly overhead, so the shadow falls directly below the object and becomes invisible.
3. Eclipses – Shadows in Space
✔ What is an eclipse?
- An eclipse happens when one celestial body (earth, moon, or sun) blocks the light of another.
✔ Types of Eclipses:
| Type | Description | When Does It Occur? |
|---|---|---|
| Solar Eclipse | The moon comes between the sun and the earth, blocking sunlight. | During the day (New Moon phase). |
| Lunar Eclipse | The earth comes between the sun and the moon, casting a shadow on the moon. | During the night (Full Moon phase). |
✔ Why should we not watch a solar eclipse with bare eyes?
- The sun’s rays contain harmful ultraviolet (UV) radiation, which can damage our eyes permanently.
✔ How can we safely observe an eclipse?
- Use special eclipse glasses, a pinhole projector, or watch live streams.
4. Scattering of Light – Why the Sky Looks Blue
✔ What is scattering of light?
- Scattering occurs when light rays hit tiny particles in the air and spread in different directions.
✔ Examples of Scattering in Nature:
| Effect | Cause |
|---|---|
| Blue sky | The atmosphere scatters blue light more than other colors. |
| Red and orange sunsets | During sunrise and sunset, light travels a longer distance, scattering blue light and leaving red light. |
| Dust visible in sunlight | Particles in the air scatter light, making them visible. |
✔ Why is space dark?
- There is no air in space to scatter light, so space appears completely black.
✔ Why do we see the sun even after it sets?
- The earth’s atmosphere bends (refracts) light, allowing us to see the sun for a few minutes after it has set.
5. Point Source vs. Extended Source of Light
✔ A light source can be a single point or spread over a larger area.
| Point Source of Light | Extended Source of Light |
|---|---|
| Gives light from one small point. | Spreads light from a large area. |
| Produces a sharp shadow. | Produces both umbra and penumbra. |
| Example: Torch, Candle. | Example: Sun, Tube Light. |
✔ Why does a point source not create a penumbra?
- A point source emits light from a single location, so the shadow is sharp (umbra only).
6. Importance of Light in Daily Life
✔ Light plays a vital role in our daily activities and natural processes.
✔ Uses of Light:
| Use | Example |
|---|---|
| Seeing Objects | Light reflects off objects and enters our eyes. |
| Photosynthesis | Plants make food using sunlight. |
| Solar Energy | Solar panels use sunlight to generate electricity. |
| Medical Uses | Lasers in eye surgeries and treatments. |
7. Safety Measures for Observing Light Effects
✔ How to observe an eclipse safely?
- Use special eclipse glasses or a pinhole projector.
- Never look directly at the sun, as it can cause permanent eye damage.
✔ Protecting eyes from bright light:
- Do not stare at bright sources like welding flames or the sun.
- Wear sunglasses in extremely bright environments.
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