General Science Notes Chapter 17 Class 7 Maharashtra Board

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.