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Light Energy
ICSE Class 7 Physics • Chapter 4
Chapter Overview
Light is an invisible form of energy that causes the sensation of vision. In this chapter, we will learn how light travels, how shadows are formed, and explore the precise laws of reflection using plane mirrors.
4.1 Properties of Light
- Rectilinear Propagation: Light always travels in a straight line in a uniform medium. It never bends around corners.
- Speed of Light: Light is the fastest traveling energy in the universe. Its speed in a vacuum (or air) is $3 \times 10^8\text{ m/s}$ (or $300,000\text{ km/s}$).
- It does not require any material medium (like solid, liquid, or gas) to travel. It can easily travel through the empty vacuum of space.
4.2 Shadows and Eclipses
Because light travels in a straight line, it cannot pass through opaque objects. When an opaque object is placed in the path of light, a dark patch forms behind it. This region of darkness is called a shadow.
Parts of a Shadow
- Umbra: The central, darkest part of a shadow where absolutely no light reaches from the source.
- Penumbra: The lighter, outer region of a shadow where only a partial amount of light reaches from the source.
Eclipses
Eclipses are spectacular natural phenomena formed by massive shadows in space.
- Solar Eclipse: Occurs when the Moon comes exactly between the Sun and the Earth, casting its shadow on the Earth. (Order: Sun - Moon - Earth).
- Lunar Eclipse: Occurs when the Earth comes exactly between the Sun and the Moon, casting its shadow on the Moon, making it turn dark or reddish. (Order: Sun - Earth - Moon).
AI Image Prompt: A scientific diagram of a Solar Eclipse in deep space. Show the bright yellow Sun on the left, sending straight white light rays towards the blue Earth on the right. In the middle, place the grey Moon. Highlight the dark cone-shaped Umbra shadow explicitly hitting a small spot on Earth, and the wider lighter Penumbra shadow surrounding it.
4.3 Reflection of Light
When light rays strike a polished surface (like a clean mirror), they bounce back into the same medium. This bouncing back of light is called the reflection of light.
Important Terms:
- Incident Ray: The light ray striking the mirror surface.
- Reflected Ray: The light ray bouncing off the mirror surface.
- Normal: An imaginary perpendicular line drawn at the point of incidence on the mirror.
- Angle of Incidence ($i$): The angle between the incident ray and the normal.
- Angle of Reflection ($r$): The angle between the reflected ray and the normal.
Laws of Reflection:
- The angle of incidence is always exactly equal to the angle of reflection ($i = r$).
- The incident ray, the reflected ray, and the normal at the point of incidence, all lie perfectly in the same flat plane.
AI Image Prompt: A 2D ray diagram showing the laws of reflection. A horizontal flat mirror at the bottom. A red incident ray angled from the left strikes the mirror. A dotted vertical line represents the Normal. A green reflected ray bounces off to the right. Clearly label Angle $i$ and Angle $r$ being perfectly equal.
Images Formed by a Plane Mirror
When you stand in front of a flat mirror in your dressing room, you see your image. The characteristics of this image are:
- Virtual and Erect: The image cannot be projected onto a screen and is standing upright.
- Same Size: The image is exactly the same height and width as the real object.
- Same Distance: The image is formed exactly as far behind the mirror as the object is standing in front of it.
- Laterally Inverted: The left side of the object appears as the right side in the image (which is why the word "AMBULANCE" is written in reverse on emergency vehicles).
Q1. If a light ray strikes a plane mirror at an angle of incidence of $40^\circ$, what is the angle of reflection?
Ans: According to the first law of reflection, angle $i = \text{angle } r$. Therefore, the angle of reflection is also $40^\circ$.
Q2. A boy is standing $2\text{ m}$ away from a plane mirror. What is the total distance between the boy and his image?
Ans: The image is formed $2\text{ m}$ behind the mirror. Thus, total distance = distance from boy to mirror + distance from mirror to image = $2\text{ m} + 2\text{ m} = 4\text{ m}$.