Light – Notes for CBSE Class 7 Science

Introduction to Light

Light is a form of energy that enables us to see objects around us. It travels from a source (like the sun, a bulb, or a candle) to our eyes, allowing us to perceive the world. Without light, we cannot see anything, as darkness is simply the absence of light. This chapter explores how light behaves, its properties, and its practical applications.

1. Light Travels in a Straight Line

One of the fundamental properties of light is that it travels in a straight line. This is called rectilinear propagation of light. We can observe this in everyday life:

• When sunlight passes through a small hole, it forms a straight beam.
• Shadows are formed because light cannot bend around objects.

Activity to Prove This:

• Take three cardboard pieces with a hole in the center of each.
• Align them so the holes are in a straight line and shine a torch through them. You’ll see light passing through all holes.
• Now, misalign one card. The light won’t pass through, proving it doesn’t bend.

2. Reflection of Light

When light strikes a surface and bounces back, it is called reflection. This is why we can see ourselves in a mirror. Reflection happens when light hits a smooth, shiny surface like glass, metal, or water.

Types of Reflection:

• Regular Reflection: Occurs on smooth surfaces (e.g., a plane mirror). The reflected light rays are parallel, producing a clear image.
• Diffuse Reflection: Occurs on rough surfaces (e.g., paper or a wall). The reflected rays scatter in different directions, so no clear image is formed.

Laws of Reflection:

1. The angle of incidence (the angle at which light strikes the surface) is equal to the angle of reflection (the angle at which light bounces off).
2. The incident ray, reflected ray, and the normal (an imaginary line perpendicular to the surface) all lie in the same plane.

Mirrors:

• Plane Mirror: A flat mirror that reflects light to form a virtual, upright, and same-sized image. The image appears behind the mirror at the same distance as the object is in front.
• Example: When you stand 1 meter in front of a mirror, your image appears 1 meter behind it.

3. Images Formed by Mirrors

An image is a likeness of an object produced when light reflects or refracts. Mirrors form two types of images:

• Real Image: Can be obtained on a screen (e.g., in a camera or projector). Formed when light rays actually meet after reflection.
• Virtual Image: Cannot be obtained on a screen (e.g., in a plane mirror). Formed when light rays appear to meet but don’t actually converge.

Characteristics of an Image in a Plane Mirror:

• Same size as the object.
• Upright (not inverted).
• Virtual (cannot be projected on a screen).
• Laterally inverted (left appears right and vice versa).

4. Spherical Mirrors

Mirrors that are curved are called spherical mirrors. They are of two types:

• Concave Mirror: Curves inward (like the inside of a spoon). It can form both real and virtual images depending on the object’s position.
  • Uses: Shaving mirrors, headlights of cars, torches (to focus light into a beam).
• Convex Mirror: Curves outward (like the back of a spoon). It forms only virtual, upright, and smaller images.
  • Uses: Rear-view mirrors in vehicles, security mirrors (to see around corners).

Key Terms:

• Pole: The center of the mirror.
• Center of Curvature: The center of the sphere of which the mirror is a part.
• Radius of Curvature: Distance from the pole to the center of curvature.
• Principal Axis: An imaginary line passing through the pole and center of curvature.
• Focus: The point where parallel rays of light meet (or appear to meet) after reflection.

5. Lenses

A lens is a transparent object made of glass or plastic that bends light to form images. Lenses are used in spectacles, cameras, microscopes, and telescopes.

Types of Lenses:

• Convex Lens: Thicker in the middle, thinner at the edges. It converges (brings together) light rays. Also called a converging lens.
  • Uses: Magnifying glass, correcting farsightedness.
  • Image: Can form real (inverted) or virtual (upright) images depending on the object’s position.
• Concave Lens: Thinner in the middle, thicker at the edges. It diverges (spreads out) light rays. Also called a diverging lens.
  • Uses: Correcting nearsightedness.
  • Image: Always virtual, upright, and smaller than the object.

Key Terms:

• Optical Center: The center of the lens through which light passes without bending.
• Focus: The point where light rays converge (convex lens) or appear to diverge from (concave lens).

6. Refraction of Light

When light passes from one medium to another (e.g., air to water), it bends. This bending is called refraction. It happens because light travels at different speeds in different mediums.

Examples:

• A pencil in a glass of water appears bent at the water’s surface.
• A swimming pool appears shallower than it is.

Cause of Refraction:

• Light travels faster in air than in water or glass, causing it to change direction.

7. Dispersion of Light

White light (like sunlight) is made up of seven colors: Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR). When white light passes through a prism, it splits into these colors. This splitting is called dispersion.

Rainbow Formation:

• A rainbow is formed when sunlight refracts, reflects, and disperses inside raindrops in the atmosphere. The colors appear in the order of VIBGYOR.

Newton’s Experiment:

• Sir Isaac Newton passed sunlight through a prism and observed the spectrum of colors. He recombined these colors using another prism to form white light again, proving that white light contains all colors.

8. Applications of Light in Daily Life

• Mirrors: Used in homes, vehicles, and optical instruments.
• Lenses: Used in glasses, cameras, and telescopes.
• Reflection: Helps us see objects and is used in periscopes.
• Refraction: Explains why objects appear distorted in water.

Key Points to Remember

• Light travels in straight lines and can reflect, refract, or disperse.
• Plane mirrors form virtual, upright, and laterally inverted images.
• Spherical mirrors (concave and convex) and lenses (convex and concave) have specific uses based on how they bend light.
• The seven colors of light (VIBGYOR) combine to form white light, as seen in a rainbow or prism experiment.

Important Definitions

• Incident Ray: The ray of light that strikes a surface.
• Reflected Ray: The ray that bounces off the surface.
• Normal: A line perpendicular to the surface at the point of incidence.
• Spectrum: The band of colors formed by dispersion.

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Conclusion

Chapter 11 – Light introduces students to the fascinating properties and behaviors of light. From understanding how we see objects to exploring mirrors, lenses, and rainbows, this chapter lays the foundation for optics in science. Practical examples and simple experiments make it easier to grasp these concepts, which are not only part of everyday life but also essential for advanced studies in physics.

These notes summarize the chapter comprehensively, ensuring all topics—reflection, refraction, dispersion, and the use of mirrors and lenses—are covered in detail. Students can use these notes for revision and to prepare for exams effectively.

Ch 11 Light