1. Introduction to Light

We see the beautiful world around us only because of light. When light from an object enters our eyes, the object becomes visible. Without light, even objects placed very close to us cannot be seen.

In this chapter, you learn how light travels, how it reflects from mirrors and how multiple reflections create patterns and images. These ideas form the base for higher classes (lenses, optics, human eye, etc.), so a clear understanding at Class 8 level is very important.

1.1 Luminous and Non-luminous Objects

Objects that give out their own light are called luminous objects. Examples: Sun, electric bulb (when it glows), candle flame.

Objects that do not emit their own light are called non-luminous objects. Examples: Book, chair, tree, your body, moon (it reflects sunlight).

Non-luminous objects can be seen only when they reflect light coming from a luminous source into our eyes.

1.2 Rectilinear Propagation of Light

Experimental observations (like pin-hole experiments and shadow formation) show that light travels in a straight line. This property is called rectilinear propagation of light.

• Shadows are sharp because light moves in straight paths.
• We cannot see around a corner directly because light does not bend on its own.
• Pinhole camera works on the principle that light travels in straight lines.

2. Reflection of Light

When light falls on a surface and bounces back into the same medium, the phenomenon is called reflection of light. Mirrors are polished surfaces which reflect most of the light falling on them.

2.1 Important Terms

• Incident ray: The ray of light that falls on a reflecting surface.
• Point of incidence: The point where the incident ray strikes the surface.
• Reflected ray: The ray of light that bounces back from the surface.
• Normal: An imaginary line drawn perpendicular (at 90°) to the reflecting surface at the point of incidence.
• Angle of incidence (∠i): The angle between the incident ray and the normal.
• Angle of reflection (∠r): The angle between the reflected ray and the normal.

2.2 Laws of Reflection

For a smooth reflecting surface like a plane mirror, reflection always obeys two laws:

1. First law: The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
2. Second law: The angle of reflection is always equal to the angle of incidence (∠r = ∠i).

These laws are verified using a ray box and a plane mirror on a drawing sheet. The equality of angles makes it possible to trace the path of light accurately in ray diagrams.

3. Regular and Diffused Reflection

Reflection of light depends on how smooth or rough the surface is. Light behaves differently for polished and rough surfaces.

3.1 Regular (Specular) Reflection

• Takes place from a smooth and polished surface (e.g., plane mirror, calm water).
• All parallel incident rays reflect in a definite direction.
• Reflected rays remain parallel to each other.
• Produces a clear and sharp image.

3.2 Diffused (Irregular) Reflection

• Occurs from a rough or uneven surface (e.g., wall, paper, cloth).
• Each part of the surface reflects light in a slightly different direction.
• Reflected rays are scattered in many directions.
• Generally does not produce a clear image but allows us to see objects from many directions.

Note that in diffused reflection the laws of reflection are still obeyed at each tiny point of the surface. The scattering happens only because the surface is irregular.

4. Images in a Plane Mirror

A plane mirror is a flat mirror. It forms images of objects placed in front of it by regular reflection of light.

4.1 Characteristics of the Image in a Plane Mirror

• The image is virtual (cannot be obtained on a screen).
• It is erect (upright) and not inverted vertically.
• Image is of the same size as the object.
• The image is formed at the same distance behind the mirror as the object is in front of it.
• The image is laterally inverted (left-right interchanged).

Lateral inversion explains why words written on an ambulance look reversed in normal view but appear correct in the rear-view mirror of vehicles ahead.

4.2 Lateral Inversion

Lateral inversion is the phenomenon in which the left side of the object appears as the right side in the image and vice versa. It is an important concept tested in both objective and descriptive questions.

5. Multiple Reflection and Interesting Devices

When light reflects more than once, from two or more mirrors, the phenomenon is called multiple reflection. This can produce many images of an object.

5.1 Multiple Images Using Two Mirrors

If an object is placed between two plane mirrors kept at an angle, we see several images. The number of images depends on the angle between the mirrors. When the angle decreases, the number of images increases.

For example, when two mirrors are placed facing each other (angle = 0°), we see a very large number of images as light reflects back and forth repeatedly.

5.2 Kaleidoscope

A kaleidoscope is a device which uses multiple reflections to produce beautiful, symmetrical patterns.

• It consists of three plane mirrors placed in a triangular tube.
• Coloured glass pieces or beads are placed at one end, and the other end is used to look through.
• Multiple reflections between the mirrors create a large number of symmetrical patterns.

Kaleidoscopes are used in designing patterns for wallpapers, fabrics and in decorative work.

5.3 Periscope (Conceptual Overview)

A periscope is an instrument that helps us see objects which are not in the direct line of sight.

• It uses two plane mirrors kept parallel to each other at 45°.
• Light from the object reflects from the upper mirror and then from the lower mirror into our eyes.
• Used in submarines and by soldiers in trenches to see over obstacles.

6. Sunlight – White or Coloured?

Sunlight appears white, but in reality it is made up of many colours. When white light passes through a glass prism, it splits into a spectrum of seven colours – violet, indigo, blue, green, yellow, orange, red (VIBGYOR).

This splitting of white light into its different colours is called dispersion of light. Rainbow in the sky is a natural example of dispersion by water droplets.

7. Uses of Reflection in Daily Life

Reflection of light is used in many practical devices and daily activities:

• Dressing mirrors: Allow us to see our own image.
• Rear-view mirrors in vehicles: Help drivers see vehicles behind them.
• Periscopes and binoculars: Use mirrors and prisms for changing the direction of light.
• Optical instruments: Many scientific instruments (like microscopes and telescopes) use reflection and refraction together.

8. Quick Revision Points – Chapter 16: Light

• Light is essential for vision; objects are visible when light reflected from them reaches our eyes.
• Luminous objects give out light; non-luminous objects can only reflect light.
• Light travels in a straight line – this explains pinhole camera and shadow formation.
• Reflection occurs when light bounces back from a surface.
• Incident ray, reflected ray and normal all lie in the same plane at the point of incidence.
• Angle of incidence equals angle of reflection (∠i = ∠r).
• Regular reflection from a smooth surface gives clear images; diffused reflection from rough surfaces scatters light in many directions.
• Plane mirror forms a virtual, erect image of same size as the object, at same distance behind the mirror.
• Images in a plane mirror are laterally inverted (left-right reversed).
• Multiple reflection can produce many images; used in kaleidoscope and periscope.
• Sunlight is actually a mixture of seven colours; prism separates them to form a spectrum (VIBGYOR).

9. Conclusion – How to Use These Revision Notes

This Study Module on Chapter 16: Light is prepared strictly according to the NCERT Class 8 Science syllabus. To get the maximum benefit:

• Read each section slowly and underline all key terms.
• Draw neat ray diagrams for reflection and plane mirror images in your notebook.
• Prepare a one-page summary sheet from these notes just before exams.
• Practise NCERT in-text and exercise questions using the concepts from these notes.

With regular revision using this module, you will develop a strong grip over the chapter Light and perform confidently in CBSE Class 8 examinations.