CBSE Class 10 Science : Chapter 8 – Light (Detailed Short Notes)

I. Introduction

• Light is a form of energy that allows us to see. It behaves both as a wave and as a stream of particles (photons).

II. Properties of Light

• Electromagnetic Wave: Light is a part of the electromagnetic spectrum, which includes radio waves, microwaves, infrared waves, ultraviolet waves, X-rays, and gamma rays. All these waves travel at the same speed (3 x 10⁸ m/s) in a vacuum, but differ in wavelength and frequency.
• Rectilinear Propagation: Light travels in straight lines in a vacuum. This is why we see shadows of objects when light is blocked.
• Speed of Light: The speed of light in a vacuum is constant (3 x 10⁸ m/s). However, it slows down when it travels through a denser medium like water or glass.
• Reflection: When light strikes a smooth surface, it bounces back. The angle of incidence (angle between incoming light ray and normal to the surface) is equal to the angle of reflection (angle between reflected light ray and normal).
• Refraction: When light travels from one medium to another (e.g., air to water), it bends due to the change in speed. This phenomenon is called refraction and is responsible for phenomena like twinkling of stars and the apparent bending of a straw in a glass of water.

III. Laws of Reflection

• The incident ray, the reflected ray, and the normal to the point of incidence all lie in the same plane.
• The angle of incidence is equal to the angle of reflection.

IV. Reflection by Spherical Mirrors

• Spherical mirrors have a curved reflecting surface. There are two main types:
  • Concave Mirror: The reflecting surface is curved inwards like a spoon. It can converge light rays to a focal point.
  • Convex Mirror: The reflecting surface is curved outwards like a bowl. It diverges light rays, creating a wider field of view.

V. Image Formation by Spherical Mirrors

• Ray diagrams can be used to predict the location and size of the image formed by a spherical mirror. The position and nature of the image (real or virtual, upright or inverted) depend on the object’s position and the type of mirror.

VI. Refraction of Light

• When light travels from one medium to another with a different density, it bends due to a change in speed. The amount of bending depends on the refractive indices of the two media.
• Refraction is responsible for several phenomena:
  • Twinkling of Stars: As starlight enters the Earth’s atmosphere, it bends slightly due to variations in air density. This causes the starlight to appear to twinkle.
  • Apparent Bending of a Straw in Water: When a straw is inserted into water, light rays entering the water bend, making the straw appear bent at the water surface.

VII. Applications of Reflection and Refraction

• Reflection: Rearview mirrors, telescopes (reflecting telescopes), microscopes (reflecting objectives)
• Refraction: Spectacles, contact lenses, prisms (used to separate white light into its component colors), lenses in cameras and projectors

VIII. Important Points to Remember

• Light travels fastest in a vacuum and slows down when it enters a denser medium.
• The laws of reflection hold true for all types of mirrors.
• The type of spherical mirror (concave or convex) determines whether it converges or diverges light rays.
• Refraction is responsible for many interesting phenomena like twinkling of stars and the apparent bending of a straw in water.
• Light plays a vital role in various technologies like microscopes, telescopes, spectacles, and cameras.

Light (CBSE Class 10th) Questions 

1. What is light?

• Answer: Light is a form of energy that allows us to see. It exhibits properties of both waves and particles (photons).

2. How is light classified scientifically?

• Answer: Light is part of the electromagnetic spectrum, which includes radio waves, microwaves, infrared waves, ultraviolet waves, X-rays, and gamma rays. All these waves travel at the same speed in a vacuum but differ in wavelength and frequency.

3. What is the speed of light in a vacuum?

• Answer: The speed of light in a vacuum is constant at approximately 3 x 10⁸ meters per second (300,000 kilometers per second).

4. How does light travel?

• Answer: Light travels in straight lines in a vacuum. This is why shadows are formed when light is blocked by an object.

5. What happens to the speed of light when it enters a denser medium like water?

• Answer: Light slows down when it enters a denser medium like water or glass compared to its speed in a vacuum.

6. Explain the concept of reflection of light.

• Answer: Reflection occurs when light strikes a smooth surface and bounces back. The angle of incidence (the angle between the incoming light ray and a line perpendicular to the surface) is equal to the angle of reflection (the angle between the reflected light ray and the perpendicular line).

7. What is refraction of light?

• Answer: Refraction happens when light bends as it travels from one medium to another with a different density (e.g., air to water). This bending is caused by the change in speed of light.

8. Describe the laws of reflection.

• Answer: There are two main laws of reflection:
  1. The incident ray, the reflected ray, and the normal to the point of incidence all lie in the same plane.
  2. The angle of incidence is equal to the angle of reflection.

9. Differentiate between concave and convex mirrors.

• Answer: Spherical mirrors have a curved reflecting surface.
  • Concave mirror: The reflecting surface curves inward like a spoon. It can converge light rays to a focal point.
  • Convex mirror: The reflecting surface curves outward like a bowl. It diverges light rays, creating a wider field of view.

10. How does the type of spherical mirror (concave or convex) affect the image formed?

• Answer: The type of mirror determines the location and size of the image formed. Concave mirrors can form real or virtual, upright or inverted images depending on the object’s position. Convex mirrors typically produce virtual, upright, and diminished images.

11. Explain how ray diagrams are used in studying reflection by spherical mirrors.

• Answer: Ray diagrams trace the path of light rays to predict the location and size of the image formed by a spherical mirror. By analyzing how the rays reflect off the mirror, we can determine the image characteristics.

12. What is the cause of twinkling stars?

• Answer: Twinkling of stars is a result of refraction. As starlight enters the Earth’s atmosphere, it bends slightly due to variations in air density. This causes the starlight to appear to flicker or twinkle.

13. Why does a straw appear bent when placed in a glass of water?

• Answer: Refraction is responsible for the apparent bending of a straw in water. Light rays entering the water bend due to the change in medium, making the straw appear offset at the water surface.

14. Give some examples of applications of reflection of light.

• Answer: Reflection has many applications, including rearview mirrors, telescopes (reflecting telescopes), and microscopes (reflecting objectives).

15. How is refraction used in everyday life?

• Answer: Refraction plays a vital role in various technologies:
  • Spectacles and contact lenses: Refraction helps correct vision problems by focusing light rays onto the retina.
  • Prisms: These separate white light into its component colors based on the varying degrees of refraction for different wavelengths.
  • Lenses in cameras and projectors: Lenses use refraction to focus or project light rays for image formation.

16. What factors influence the amount of bending of light during refraction?

• Answer: The amount of bending during refraction depends on the refractive indices of the two media involved. The refractive index is a measure of how much light bends when it enters a particular material.