Willer Academy - Light

Willer Academy

Nation Builders Through Education | Founded by Er. Rahul Kumar Dubey Sir

Chapter 10: Light

Subject: Science (Physics)

Class: 7

Series: Base Builder for Moderate to Hard

Learning Objectives

By the end of this chapter, you, the future scientist and nation-builder, will be able to:

Understand the nature and properties of light
Explain reflection and its laws
Differentiate between various types of mirrors
Understand refraction and its applications
Explain the dispersion of light and formation of rainbows
Solve problems related to reflection and refraction
Apply knowledge to real-life optical phenomena

10.1 Introduction: The Nature of Light

Light is a form of energy that enables us to see objects. It travels in straight lines and at an incredible speed of 3 × 10⁸ m/s in vacuum.

Quick Check

Think about how you're able to see objects around you. What role does light play in vision? Why can't we see objects in complete darkness?

10.2 Reflection of Light

Reflection is the bouncing back of light when it strikes a surface. The law of reflection states that the angle of incidence equals the angle of reflection.

Regular Reflection

Occurs on smooth surfaces
Forms clear images
Example: Mirrors
Angle i = Angle r

Diffuse Reflection

Occurs on rough surfaces
Does not form clear images
Example: Paper, walls
Enables us to see objects

Numerical & Application 1: Reflection Angles

Scenario: A light ray strikes a mirror at an angle of 35° to the normal.
Problem: What is the angle of reflection? If the mirror is rotated by 10° away from the light source, what will be the new angle of reflection?
Solution:
- Angle of reflection = Angle of incidence = 35°
- After rotation, new angle of incidence = 35° + 10° = 45°
- New angle of reflection = 45°

This shows how mirror rotation affects reflection angles!

Experiment: Laws of Reflection

Take a plane mirror and place it on a piece of paper. Draw a line along the mirror (mirror line). Shine a flashlight at the mirror at different angles and mark the incident and reflected rays. Measure the angles to verify that angle of incidence equals angle of reflection.

10.3 Types of Mirrors

Mirrors are surfaces that reflect light to form images. Different types of mirrors produce different kinds of images.

Plane Mirror

Flat surface
Virtual, upright image
Image same size as object
Laterally inverted

Concave Mirror

Curved inward
Can form real or virtual images
Magnified or diminished images
Used in telescopes, shaving mirrors

Convex Mirror

Curved outward
Virtual, upright image
Diminished image
Wider field of view
Used in vehicle side mirrors

Task 1: Mirror Applications

Research and list five practical applications of each type of mirror (plane, concave, convex) in daily life and technology. Present your findings with examples.

10.4 Refraction of Light

Refraction is the bending of light when it passes from one medium to another. This occurs due to the change in speed of light in different media.

Refraction Demonstration

When light passes from air to water:

1. Light ray bends towards the normal

2. Speed decreases

3. Wavelength decreases

4. Frequency remains constant

Numerical & Application 2: Refraction Index

Scenario: The speed of light in vacuum is 3 × 10⁸ m/s, and in glass is 2 × 10⁸ m/s.
Problem: What is the refractive index of glass? If light enters glass at an angle of 30° to the normal, what will be the angle of refraction? (Use Snell's Law: n₁sinθ₁ = n₂sinθ₂)
Solution:
- Refractive index = speed in vacuum / speed in medium = 3×10⁸ / 2×10⁸ = 1.5
- n₁sinθ₁ = n₂sinθ₂ → 1 × sin(30°) = 1.5 × sin(θ₂)
- 0.5 = 1.5 × sin(θ₂) → sin(θ₂) = 0.5/1.5 = 1/3
- θ₂ = sin⁻¹(1/3) ≈ 19.47°

This shows how light bends when entering different media!

Experiment: Bending Light

Place a pencil in a glass of water. Observe how the pencil appears bent at the water surface. This demonstrates refraction - light from the submerged part of the pencil bends as it exits water, making the pencil appear bent.

10.5 Dispersion of Light and Rainbows

Dispersion is the splitting of white light into its constituent colors. This occurs because different colors of light bend by different amounts when refracted.

Rainbow Formation

Sunlight enters raindrops
Light refracts, reflects, and refracts again
Different colors bend at different angles
Results in spectrum of colors

Prism

Triangular glass object
Splits white light into colors
Violet bends the most
Red bends the least

Task 2: Create Your Rainbow

On a sunny day, use a garden hose to create a fine mist. Stand with your back to the sun and observe the rainbow formed in the mist. Document the color sequence and explain why the colors appear in that specific order.

10.6 The Human Eye

The human eye is a complex organ that uses light to form images. It functions much like a camera, with a lens that focuses light onto the retina.

Structure of the Human Eye

1. Cornea: Front transparent part

2. Iris: Colored part that controls pupil size

3. Lens: Focuses light onto retina

4. Retina: Contains light-sensitive cells

5. Optic nerve: Transmits signals to brain

Eye Care Tips

Protection from Sun

Wear sunglasses to protect from UV rays

Reading Habits

Maintain proper distance and lighting while reading

Screen Time

Take regular breaks from screens to reduce eye strain

Multiple Choice Questions (MCQs)

1. Which of the following is a luminous object?

a) Moon

b) Sun

d) Book

2. The law of reflection states that:

a) Angle of incidence = Angle of reflection

b) Angle of incidence > Angle of reflection

c) Angle of incidence < Angle of reflection

d) Angle of incidence ≠ Angle of reflection

3. The image formed by a plane mirror is:

a) Real and inverted

b) Virtual and erect

c) Real and erect

d) Enlarged

4. The device used by a dentist to see teeth is:

a) Plane mirror

b) Concave mirror

c) Convex mirror

d) Lens

5. The mirror used in vehicles as rear-view mirror is:

a) Plane mirror

b) Concave mirror

c) Convex mirror

d) None of these

6. The splitting of white light into seven colours is called:

a) Refraction

b) Dispersion

c) Reflection

d) Scattering

7. Which colour bends the most during dispersion?

a) Red

b) Blue

c) Violet

d) Green

8. Which part of the eye controls the amount of light entering it?

a) Retina

b) Lens

c) Iris

d) Cornea

9. The image formed on the retina is:

a) Real and inverted

b) Virtual and erect

c) Enlarged

d) None of these

10. The defect of eye in which distant objects appear blurred is:

a) Hypermetropia

b) Myopia

c) Cataract

d) Colour blindness

11. Myopia can be corrected by using:

a) Convex lens

b) Concave lens

c) Cylindrical lens

d) Bifocal lens

12. The phenomenon due to which a coin in water appears raised is:

a) Reflection

b) Refraction

c) Dispersion

d) Scattering

13. Rainbow is formed due to:

a) Reflection only

b) Refraction and dispersion

c) Refraction and reflection

d) Refraction, dispersion and reflection

14. The human eye lens is:

a) Concave

b) Convex

c) Plane

d) Cylindrical

15. The unit of power of a lens is:

a) Newton

b) Dioptre

c) Joule

d) Metre

16. A concave mirror always forms:

a) Only real image

b) Only virtual image

c) Both real and virtual images

d) No image

17. The part of eye sensitive to light is:

a) Cornea

b) Pupil

c) Retina

d) Iris

18. Convex lens is also called:

a) Diverging lens

b) Converging lens

c) Plane lens

d) Neutral lens

19. The phenomenon responsible for twinkling of stars is:

a) Reflection

b) Refraction

c) Scattering

d) Dispersion

20. The least deviation colour in a prism is:

a) Violet

b) Blue

c) Red

d) Green

Section C: Answer in Detail (For Answer Writing Practice)

Q1. Explain the laws of reflection with the help of a diagram. How do regular and diffuse reflection differ?

Hint: Discuss incident ray, reflected ray, normal, angles, and surface properties.

Q2. Describe the formation of a rainbow in the sky. What optical phenomena are involved?

Hint: Include refraction, dispersion, and internal reflection in water droplets.

Q3. Compare and contrast the human eye with a camera. How are they similar and different?

Hint: Include components like lens, aperture, and light-sensitive surface.

Section D: Tackle These! (Higher Order Thinking Skills - HOTS)

Q1. Why does a pencil appear bent when placed in a glass of water? Explain with a diagram showing the path of light rays.

Think about: Refraction at the water-air interface and how our brain interprets light rays.

Q2. How do fiber optic cables work? What principle of light do they utilize?

Think about: Total internal reflection and its applications in communication technology.

Q3. Design an experiment to measure the refractive index of water using a laser pointer and a protractor. What measurements would you need to take?

Think about: Snell's Law and how to apply it practically.