Chapter Overview
We see when light from a source reaches our eyes directly or after reflecting from objects. Straight-line travel, blocking and reflection explain shadows and mirror images.
The chapter follows an enquiry-based approach: observe carefully, distinguish evidence from explanation, use appropriate units and communicate conclusions honestly.
Learning Objectives
After studying this chapter, you should be able to:
- explain rectilinear propagation in clear scientific language;
- explain shadows in clear scientific language;
- explain reflection in clear scientific language;
- explain plane-mirror images in clear scientific language;
- connect scientific ideas with familiar situations;
- plan or evaluate a safe and fair investigation.
Key Concepts
| Concept | Explanation | |---|---| | Rectilinear propagation | In a uniform medium, light travels approximately in straight lines. | | Shadows | An opaque object blocks light and forms a shadow on a surface. Shadow size depends on relative positions. | | Reflection | A smooth reflecting surface redirects light. The angle of reflection equals the angle of incidence. | | Plane-mirror images | A plane mirror forms an upright virtual image of the same size, apparently as far behind the mirror as the object is in front. |
Detailed Explanation
Rectilinear propagation
In a uniform medium, light travels approximately in straight lines.
Shadows
An opaque object blocks light and forms a shadow on a surface. Shadow size depends on relative positions.
Reflection
A smooth reflecting surface redirects light. The angle of reflection equals the angle of incidence.
Plane-mirror images
A plane mirror forms an upright virtual image of the same size, apparently as far behind the mirror as the object is in front.
A scientific statement should be supported by observations, measurements or a tested explanation. Record unexpected results instead of hiding them.
Investigation
Use a torch, opaque object and screen in a darkened room. Change one distance at a time and observe shadow size. Never direct bright light into anyone’s eyes.
For a fair test, identify what is changed, what is measured, and which important conditions are kept the same. Record results before interpreting them.
Important Definitions
- Rectilinear propagation: In a uniform medium, light travels approximately in straight lines.
- Shadows: An opaque object blocks light and forms a shadow on a surface. Shadow size depends on relative positions.
- Reflection: A smooth reflecting surface redirects light. The angle of reflection equals the angle of incidence.
- Plane-mirror images: A plane mirror forms an upright virtual image of the same size, apparently as far behind the mirror as the object is in front.
Use precise terms, labelled diagrams where relevant, and complete cause-and-effect statements. A correct scientific reason is more useful than a one-word answer.
Common Mistakes
- Calling a shadow a reflection.
- Saying a plane-mirror image is formed on the mirror’s surface.
- Ignoring the need for a screen or surface on which a shadow is observed.
Quick Revision
- Rectilinear propagation: In a uniform medium, light travels approximately in straight lines.
- Shadows: An opaque object blocks light and forms a shadow on a surface.
- Reflection: A smooth reflecting surface redirects light.
- Plane-mirror images: A plane mirror forms an upright virtual image of the same size, apparently as far behind the mirror as the object is in front.
Practice Questions
- What three things are needed to observe a shadow?
- What is lateral inversion?
- How does a plane-mirror image compare in size with the object?
- Why can we see a non-luminous book?
Answers and Explanations
- A light source, an opaque object and a screen or surface.
- The apparent left-right reversal in a plane-mirror image.
- It is the same size.
- Light reflects from the book into our eyes.
Apply Your Learning
Find one example of this chapter’s science in your home, school or neighbourhood. Describe what you observe, propose an explanation, and state what additional evidence would test that explanation.