Willer Academy - Light

Willer Academy

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

Chapter 11: 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 the laws of reflection and their applications
Differentiate between reflection and refraction
Understand how lenses work and form images
Explain the dispersion of light and formation of rainbows
Solve problems related to light behavior
Apply knowledge to real-life optical phenomena

11.1 Introduction: The Nature of Light

Light is a form of energy that enables us to see the world around us. It travels in straight lines and at an incredible speed of approximately 300,000 km/s in vacuum.

Quick Check

Think about how light helps you perform daily activities. What would the world be like without light?

11.2 Reflection of Light

Reflection occurs when light bounces off 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 Angle

Scenario: A light ray strikes a mirror at an angle of 30° to the normal.
Problem: What is the angle of reflection? If the mirror is rotated by 10°, what will be the new angle of reflection?
Solution:
- Angle of reflection = Angle of incidence = 30°
- When mirror rotates by 10°, angle of incidence becomes 40°
- New angle of reflection = 40°

This shows how reflection angles work according to the law of reflection!

Experiment: Law of Reflection

Take a plane mirror and place it vertically on a sheet of paper. 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.

11.3 Refraction of Light

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

Towards Normal

When light travels from rarer to denser medium
Speed decreases
Bends towards the normal
Example: Air to water

Away from Normal

When light travels from denser to rarer medium
Speed increases
Bends away from the normal
Example: Water to air

Task 1: Real-life Refraction

Observe a straw in a glass of water. Why does it appear bent? Take photographs and explain the phenomenon using the concept of refraction.

11.4 Lenses and Image Formation

Lenses are transparent materials with curved surfaces that refract light in specific ways to form images.

Convex Lens (Converging)

1. Thicker at center than edges

2. Converges light rays

3. Can form real and virtual images

4. Used in magnifying glasses, cameras

Concave Lens (Diverging)

1. Thinner at center than edges

2. Diverges light rays

3. Always forms virtual, erect, diminished images

4. Used in correcting myopia

Numerical & Application 2: Focal Length

Scenario: An object is placed 30 cm from a convex lens with focal length 15 cm.
Problem: Where will the image be formed? What is the nature of the image?
Solution:
- Using lens formula: 1/f = 1/v - 1/u
- 1/15 = 1/v - 1/(-30) [object distance is negative]
- 1/v = 1/15 - 1/30 = 1/30
- v = 30 cm (real image, inverted, same size)

This shows how to calculate image position using the lens formula!

11.5 The Human Eye

The human eye is a complex organ that works like a camera, using a lens to focus light onto the retina.

Eye Structure and Function

Cornea

Transparent front part that refracts light

Iris and Pupil

Controls amount of light entering the eye

Retina

Light-sensitive layer where images form

Task 2: Vision Defects

Research common vision defects like myopia and hypermetropia. Create a presentation explaining their causes and how they are corrected using lenses.

11.6 Dispersion of Light

Dispersion is the splitting of white light into its constituent colors (VIBGYOR) when it passes through a prism.

Rainbow Formation

Sunlight refracts in raindrops
Disperses into colors
Internal reflection occurs
Forms circular arc of colors

Experiment: Creating a Rainbow

On a sunny day, use a garden hose to spray a fine mist of water in the air. Stand with your back to the sun and observe the rainbow formed in the mist. Alternatively, use a prism to split sunlight into its spectrum.

Willer Academy - Light MCQ Test

Class 7 Science – Light (MCQ Test)

Willer Academy | 20 Multiple Choice Questions

Enter Your Name:

Test Results

Name:

Score: /20

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

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

Hint: Include definitions, diagrams, and examples of both types of reflection.

Q2. Describe the process of image formation by a convex lens when the object is placed beyond 2F. Draw a ray diagram to illustrate.

Hint: Include the lens formula, nature of image, and practical applications.

Q3. What is dispersion of light? Explain how a rainbow is formed in nature.

Hint: Include the role of water droplets, refraction, and internal reflection.

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

Q1. Why does a pencil appear bent when partially immersed in water? Explain using the concept of refraction with a labeled diagram.

Think about: Change in medium, light bending, and apparent position.

Q2. Compare and contrast the working of the human eye with a camera. What are the similarities and differences?

Think about: Lens system, focusing mechanism, and image capture.

Q3. Design an experiment to measure the focal length of a convex lens using a distant object and a screen. What precautions would you take?

Think about: Setup, measurements, and sources of error.