Study Module — Light: Shadows and Reflections

These revision notes follow the NCERT Class 7 syllabus closely and are designed to help you revise concepts clearly and quickly. Read actively — draw diagrams, perform simple observations at home, and practise labelling diagrams as you go.

1. What is Light?

Light is a form of energy that makes objects visible and enables us to see the world. In everyday situations we often treat light as moving along straight lines called rays. When we draw diagrams, we represent light as straight arrows to show the direction of travel. This simple model is called the ray model of light and is sufficient to explain the ideas in this chapter.

2. Rectilinear Propagation of Light

Rectilinear propagation means light travels in straight lines. You can observe this yourself: in a dark room, make a small hole (pinhole) in paper and hold another sheet behind it — a bright spot of light appears on the second sheet because light travels straight through the hole. This behaviour explains why shadows have sharp edges when the light source is small and close.

3. Shadows — Formation and Types

A shadow is formed when an opaque object blocks the path of light falling on a surface behind it. Key points:

• Umbra — the fully shaded inner region where all direct light is blocked (completely dark).
• Penumbra — the partially shaded outer region where only part of the light is blocked (appears lighter).

The size and sharpness of a shadow depend on:

• Size of the light source: A small, point-like source makes sharp shadows (little penumbra). A large extended source creates fuzzy shadows with larger penumbra.
• Distance between object and screen: Moving the object farther from the screen increases shadow size and may reduce sharpness.
• Distance between light source and object: Bringing the source closer makes the shadow larger and often fuzzier.

4. Multiple Shadows

If more than one light source illuminates an object, it can cast multiple shadows — one for each source. Each light produces its own umbra and penumbra; overlapping shadows create complex patterns. This often happens under stage lighting or on a sunny day when sunlight mixes with artificial lights.

5. Reflection of Light — Basics

Reflection is the bouncing back of light from a surface. There are two main types:

• Regular (specular) reflection: Occurs on smooth surfaces like mirrors; reflected rays are orderly and form clear images.
• Diffuse reflection: Occurs on rough surfaces like paper or cloth; reflected rays scatter in many directions and no clear image is formed.

In this chapter we learn the basic laws of reflection which govern how light behaves at a reflecting surface:

1. Incident ray, reflected ray and normal (a line perpendicular to the surface at the point of incidence) all lie in the same plane.
2. The angle of incidence (between incident ray and normal) equals the angle of reflection (between reflected ray and normal).

Use a ray diagram to practise — draw the normal, incident ray, and reflect it with equal angles on the other side. This simple rule explains why plane mirrors produce predictable images.

6. Plane Mirrors and Image Formation

A plane mirror is a flat, smooth surface that reflects light regularly. Important characteristics of images formed by plane mirrors:

• The image is virtual — it cannot be projected on a screen because light only appears to come from the image point behind the mirror.
• The image is laterally inverted — left and right appear swapped (your left hand looks like the image's right hand).
• The image is same size as the object and located the same distance behind the mirror as the object is in front.
• The image is erect (not upside down).

Draw multiple ray diagrams to see these features: pick two rays from the top of the object — one parallel to the mirror (reflects through the focal behaviour? not for plane) and one toward the mirror at some angle; trace the reflected rays backward — their intersection gives the virtual image location.

7. Uses of Plane Mirrors — Practical Examples

• Everyday mirrors: In bathrooms and dressing tables for seeing one’s appearance.
• Periscope: Two plane mirrors placed at 45° in a tube allow viewers to see over obstacles (used in submarines and by soldiers).
• Kaleidoscope: Uses multiple mirrors arranged at specific angles to create repeated reflective patterns.

8. Periscope — How it Works

A periscope consists of two plane mirrors mounted parallel in a vertical tube. Each mirror is tilted at 45°. Light from an object strikes the top mirror, reflects down the tube to the second mirror, and then reflects into the viewer’s eye. The periscope preserves the direction of view while allowing observation from a concealed position. Try making a simple cardboard periscope in class to visualise ray paths.

9. Regular vs Diffuse Reflection — Why we see objects

We see most objects because they reflect light diffusely into our eyes. Smooth shiny surfaces give clear images because of regular reflection — that's why mirrors are polished. Rough surfaces scatter light in various directions, so we see the object itself rather than its mirror-like reflection.

10. Practical Activities & Experiments (NCERT Suggested)

Hands-on observation strengthens understanding. Suggested activities:

• Pinhole experiment to demonstrate straight-line propagation of light.
• Form shadows using a lamp and measure how shadow size changes with distances.
• Use a plane mirror and trace ray diagrams to locate virtual images.
• Build a simple periscope from a cardboard tube and two small mirrors to see how light changes direction by reflection.

11. Common Misconceptions & Clarifications

• Light does not ‘bend’ around small objects noticeably; any apparent bending is due to diffraction (not part of this chapter) — use straight-line rays for these topics.
• Virtual images are not ‘behind’ the mirror in space — they are points where reflected rays appear to come from; you cannot catch them on a screen.
• Left-right inversion in mirrors is not a physical flip of left and right — it is due to how we face the mirror; mirrors swap front-back direction, which we interpret as left-right inversion.

12. Key Terms — Remember These

13. Quick Revision Checklist

1. Understand and be able to demonstrate rectilinear propagation of light.
2. Explain formation of umbra and penumbra and how to change shadow size.
3. State and apply the two laws of reflection in ray diagrams.
4. Describe image properties formed by plane mirrors and explain lateral inversion.
5. Know at least two practical applications (periscope, kaleidoscope) and one classroom experiment.

14. Model Questions for Practice

• Explain, with a diagram, why the size of a shadow changes when the object is moved closer to the light source.
• Draw ray diagrams to show image formation in a plane mirror when an object is placed at different distances.
• Describe how a periscope helps a submarine see above the water surface (include ray path diagram).

Conclusion

Chapter 11 helps students build a practical understanding of the behaviour of light. Focus on clear diagrams, hands-on experiments and remembering the laws of reflection. These simple but fundamental ideas form the basis for later optics topics and are directly examinable in CBSE Class 7 assessments.