Chapter 5 — Light

What is light? Light is the form of energy that makes things visible to our eyes. It comes from different sources and travels out from these sources to reach objects and then our eyes. Our eyes detect light and send signals to the brain so we can see colours, shapes and movement. Light allows us to read, move safely and enjoy the world around us.

Types of sources There are two simple groups of sources: natural and artificial. The Sun is the most important natural source of light. Other natural sources include stars and some animals that glow. Artificial sources are made by people and include electric bulbs, LEDs, torches and candles. Some objects are not sources but can be seen because they reflect light from a source; these are called non-luminous objects, for example, a book or a tree.

Luminous and non-luminous objects A luminous object produces its own light: the Sun, flames and bulbs. A non-luminous object does not make light but becomes visible only when light from a source falls on it and reflects towards our eyes. Understanding this distinction helps to explain many everyday observations, such as why a table is visible during the day or when a lamp is on, but not in complete darkness.

Simple activities A simple way to explore sources is to sit in a dark room and switch on different lights to see how they brighten nearby objects. Note the differences in brightness and colour. Observe safety when using candles: keep them away from flammable items and ask an adult to help.

Light travels in straight lines When light comes from a small source, it moves in straight paths called rays. You can see straight-line travel when sunlight shines through a small hole into a dark room — the light beam appears as a straight band. This straight-line idea helps in drawing simple diagrams to show how light goes from a source to an object and then to our eyes.

Rays and beams A single thin path of light is called a ray. When many rays travel together, we call them a beam. Beams can be narrow, like a torch beam, or wide, like sunlight falling on the ground. The direction of a ray is shown by an arrowhead in diagrams; this tells us which way the light is moving.

Uses of straight-line propagation Knowing that light travels straight allows us to predict shadows, design simple optical devices and understand how images form by mirrors. It also explains why a straight object held partly in water looks bent only if we consider the path of light (later studied under refraction). At this stage, straight rays are used for basic diagrams and experiments.

Simple experiment Make a small hole in a cardboard and let sunlight enter a darkened room; observe the beam on the opposite wall. Use a pencil to mark the edges of the bright patch and see that the shape matches the hole, showing straight paths. Try moving the hole and note how the beam position changes.

What is a shadow? A shadow is a dark area where light from a source is blocked by an opaque object. When light rays travel in straight lines and meet an obstacle, they cannot pass through it. The blocked region behind the object receives less or no light and appears as a shadow.

Umbra and penumbra A shadow can have two parts. The darkest central part where no direct light reaches is called the umbra. Around the umbra there is often a lighter shaded region called the penumbra, where only some of the light rays are blocked. The penumbra forms when the source of light is not a point but has size, like the Sun.

Factors affecting shadow size and sharpness The size of a shadow depends on the distance between the source and the object, and between the object and the screen (where we see the shadow). If the source is far and small, shadows are sharp and clearly outlined. If the source is large or close, shadows have wider penumbra and softer edges. The position of the shadow also changes when the source moves; for example, a tree’s shadow moves and changes length through the day because the Sun appears to change position.

Simple activities Use a torch to cast the shadow of a toy on a wall. Change the distance between the torch and the toy, and then between the toy and the wall. Observe how the shadow size and clarity change. Use a candle and hold a small object to see umbra and penumbra when using a larger light source such as a lamp.

What is an eclipse? An eclipse happens when one celestial body moves into the shadow of another. In our Earth–Moon–Sun system we commonly talk about solar and lunar eclipses. These are special cases of the shadow ideas already studied but on a much larger scale.

Solar eclipse A solar eclipse occurs when the Moon comes between the Sun and the Earth. The Moon blocks sunlight from reaching some parts of Earth and so people in that region see the Sun being partly or fully hidden. During a total solar eclipse the Moon’s umbra falls on Earth and the Sun seems fully covered for a short time at that place. Solar eclipses are rare for any single location because the Moon’s shadow is small on Earth.

Lunar eclipse A lunar eclipse happens when the Earth comes between the Sun and the Moon. The Earth’s shadow falls on the Moon and makes it look darker. Lunar eclipses can be seen by anyone on the night side of Earth when they occur. The idea of umbra and penumbra helps explain partial and total lunar eclipses: if the Moon passes only through the penumbra, the change is small; through the umbra, it becomes a striking dark or red-coloured Moon.

Safety note Never look directly at the Sun. Special solar glasses or pinhole cameras must be used to view solar eclipses safely. Adults should supervise any eclipse viewing.

What is reflection? Reflection is the change in direction of light when it bounces off a surface. Common examples are light bouncing off a mirror or a polished table. When light hits a smooth surface, the rays reflect in predictable ways that can be drawn and measured. Reflection lets us see clear images in mirrors and helps explain why shiny objects look bright.

Regular and diffused reflection If a surface is smooth and even, like a clean mirror or calm water, reflected rays remain parallel and produce a clear reflection; this is called regular reflection. If the surface is rough, light rays scatter in many directions; this is diffused reflection and we still see the object, but not a clear image. Most everyday objects are visible because of diffused reflection: light falls on them and scatters into our eyes.

Plane surface reflection A flat reflecting surface such as a piece of polished metal or a plane mirror reflects light in a way that allows us to predict where the image will appear. In plane reflection the direction of each incident ray and its reflected ray follow simple rules that can be represented with straight-line rays and angles. These simple rules form the starting point for drawing ray diagrams to find images.

Activity Use a torch and a small mirror on a table. Shine the torch beam at different angles and watch the reflected beam on the wall. Mark the incoming and outgoing directions to see the pattern of reflection. Try comparing a polished spoon and a wooden board to see diffused versus regular reflection.

Statement of the laws The behaviour of reflection from a smooth plane surface follows two simple laws. First, the incident ray, the reflected ray and the normal (an imaginary line perpendicular to the surface at the point of contact) all lie in the same plane. Second, the angle of incidence is equal to the angle of reflection. These laws allow us to predict how rays will reflect and to draw correct ray diagrams.

Measuring angles The angle of incidence is measured between the incident ray and the normal; the angle of reflection is between the reflected ray and the normal. In exams and practical work it is common to draw a straight line for the mirror, then the normal as a dotted or faint line, and mark these angles with a protractor. Accuracy in drawing helps in showing that both angles are equal.

Why these laws matter The laws explain many practical devices: how periscopes reflect images, why rear-view mirrors show objects, and how light is directed in simple optical instruments. Knowing the laws also helps to understand multiple reflections between two mirrors, where repeated equal-angle reflections guide rays back and forth.

Simple experiment Place a plane mirror on a table and use a ray box or a torch with a slit to form a thin ray that hits the mirror at an angle. Put a sheet of paper and mark the incident and reflected rays and the normal, then measure both angles. You will find the angles are equal as predicted by the law.

Plane mirror image A plane mirror produces an image that appears to be as far behind the mirror as the object is in front. This image is virtual — light does not actually come from behind the mirror; the reflected rays only seem to come from that point. The image is upright and of the same size as the object, but left and right are reversed (laterally inverted).

How to draw the image To find the image location, draw two or more rays from a point on the object to the mirror, reflect them according to the laws of reflection, and extend the reflected rays backward behind the mirror with dashed lines. Their intersection gives the apparent position of the image. Repeating for another point on the object gives the full image position and size.

Characteristics of the image Images in plane mirrors have specific properties: they are virtual (cannot be projected onto a screen), laterally inverted (left-right swapped), same size as the object, and at equal distance behind the mirror as the object is in front. These properties hold for small objects and simple mirrors used in homes and shops.

Everyday examples Bathroom mirrors, dressing mirrors and shop mirrors use plane mirrors so we can see ourselves. Understanding the virtual nature explains why you cannot put a screen behind a plane mirror to catch the image — the light never comes from behind the glass.

Why we see our image When you stand before a plane mirror, many rays from your face hit the mirror and reflect into your eyes. Your brain traces these rays back in straight lines and thinks they came from behind the mirror. That is why you see a picture of yourself in the mirror even though nothing is behind it.

Left-right reversal explained A mirror does not actually swap left and right in a physical sense; it swaps front and back. When you raise your right hand, the person in the mirror raises what appears to be their left hand because they face you and your right hand is on their left side. This lateral inversion is a result of reversing the front-back direction and how we interpret orientation.

Virtual images and no projection Since the reflected rays only appear to come from behind the mirror, the image is virtual. You cannot catch it on a screen placed behind the mirror. Photographers and students should note this difference between real and virtual images when planning experiments.

Practical uses Make-up mirrors, shaving mirrors and dressing mirrors use plane mirrors because the image is upright and of the same size. Understanding lateral inversion helps explain why left-right labels on clothing may look reversed in a mirror and why some signs are intentionally printed in mirror image for ambulance fronts so drivers see normal text when viewed in a mirror.

Multiple reflections When light reflects more than once, we call it multiple reflection. This happens when two or more mirrors face each other, creating repeated images. For example, two mirrors placed parallel produce many images due to rays bouncing back and forth. The number and arrangement of images depend on the angle between mirrors and the observer’s position.

Periscope A simple practical device using two plane mirrors is the periscope. It lets a person see over an obstacle by reflecting light down from a higher view to the viewer’s eye. In a school periscope, two mirrors are placed parallel at 45° to the tube so that light from the scene at the top reflects down and then into the eye. Periscopes are used in submarines, lookout posts and by students for experiments.

How to build and test Make a cardboard periscope by cutting two slits and placing small rectangular plane mirrors at 45° inside each end. Look through one end while pointing the other end towards an object you cannot see directly. Adjust the mirrors slightly if the image is tilted. Notice how the image appears upright and how each reflection follows the laws of reflection.

Applications and safety Periscopes and multiple-reflection setups teach about directing light and forming images. They are safe to use if mirrors are handled carefully. Avoid glass breakage and adult help is recommended when cutting materials.

Curved mirrors at a basic level Besides flat mirrors, mirrors can be curved: concave (curving inward like a bowl) or convex (curving outward). For Class 6, we introduce simple observations without heavy maths. Curved mirrors change the direction of reflected rays in ways that can make images appear larger or smaller depending on shape and position.

Concave mirrors A concave mirror can make objects appear larger if the object is close to the mirror. That is why they are used as shaving mirrors or for makeup: you can see a magnified, upright view when close to the mirror. If you move far away, the image may appear inverted and smaller — details that will be explored later in higher classes.

Convex mirrors Convex mirrors always give smaller, upright images and show a wider field of view. This is why they are used as rear-view mirrors on vehicles and in shops for security; they allow the driver or shopkeeper to see a larger area behind or around them though the image is reduced in size.

Activity Use a polished spoon to observe curved reflection: the inner side acts like a concave mirror and shows a magnified face when close; the outer side acts like a convex mirror and shows a small, wide-area view of a room. Record observations about size, orientation and field of view.

Practical uses in the home Mirrors and light are part of daily life. In homes, mirrors are used for personal grooming — shaving, applying make-up and checking appearance. A well-placed mirror can reflect daylight into a room, making it appear brighter and larger. Lamps and bulbs give usable light for reading, studying and doing hobbies. Choosing the right place for a lamp helps reduce eye strain by ensuring enough light where tasks are done.

Uses in transport and safety Mirrors help drivers and riders see what is behind or beside them. Rear-view mirrors and side mirrors in cars use plane and convex surfaces to show what is approaching from behind and from the sides. Convex mirrors on roads at blind bends give a wider view so drivers can spot oncoming traffic. Street lights and reflective signs increase safety at night by making roads and pedestrians more visible.

Industrial and everyday devices Many tools and simple machines use reflection and light. Solar cookers use reflective panels to concentrate sunlight to heat food. Shopkeepers use mirrors to see corners of their shop and deter theft. Flashlights, lamps and reflectors are used in bicycles and vehicles to make them visible at night. Even in classrooms, blackboards, whiteboards and projectors rely on light being reflected effectively so students can read what is written or shown.

Art, decoration and design Designers and artists use mirrors and light to create effects. Mirrors can make small rooms appear larger, while lights of different colours set moods. Decorative lighting highlights pictures or plants. Careful use of reflective surfaces helps in interior design to increase natural light, reduce the need for artificial lighting and save energy.

Care and economy Using energy-efficient bulbs, placing mirrors to increase daylight and keeping lights and mirrors clean are simple steps to save electricity and improve visibility. Safe handling of mirrors and careful placement of lights reduce accidents, while proper bulbs reduce glare and save power.

Colour and light Colours are seen because different objects reflect different amounts and kinds of light. White light from the Sun or a bulb contains many colours mixed together. When white light falls on a coloured object, some colours are absorbed and others are reflected to our eyes; the reflected colour is what we see. For example, a red apple reflects red light and absorbs other colours.

Mixing and filters Simple experiments can show how coloured light behaves. If you view an object under different coloured lights, its appearance changes because the reflected component changes. Coloured filters let only certain colours pass; viewing white light through a blue filter makes the light appear blue. These ideas begin the study of how light and colour interact.

Rainbows — basic idea A rainbow appears when sunlight is spread out into colours by tiny water droplets in the air. This splitting of light into colours is called dispersion and will be studied later. For now, know that white sunlight contains many colours and that different conditions make those colours visible separately as a rainbow.

Activity Shine white light through a glass of water and observe any change on a screen or paper. Use coloured cellophane over a torch to see effects of coloured light on differently coloured papers. Note how the same object can look different under red, blue or green light.

Importance of careful observation Science depends on observing closely and recording what you see. In light experiments, small changes in position or angle can change the result. Learning to watch carefully, note exact positions, and draw clear ray diagrams helps you understand and explain what happens. Keep a lab notebook or ruled page where you write the aim, materials, steps, observations and conclusion for each activity.

Steps for a good experiment Before starting, list materials and set up neatly on a table with paper under the experiment to mark rays and positions. Use a ruler for straight lines, a protractor to measure angles and a pencil to mark points. Work slowly: first place the source, then the object and finally the screen or mirror. When recording, note distances (for example, distance from torch to object), times (if measuring changes) and any visible changes such as shadow size, sharpness or position of an image.

Common sources of error Identify factors that can change results: movement of the light source, shaky hands when marking, poor alignment of mirrors, or background light making shadows faint. Repeat the experiment two or three times and record each result. If results differ, think which step could be improved and try again. Good scientists expect some variation and report it honestly.

Safety rules Never look directly at bright lamps or the Sun. Use low-power torches and angle them so light does not enter your partner’s eyes. Handle mirrors and glass with care to avoid cuts; if a mirror breaks, ask an adult to clear it. When using candles, keep them away from curtains and perform experiments on a non-flammable surface under adult supervision. Wear simple protective gear if recommended and keep work area tidy.

How to record and present Draw clear diagrams showing ray directions with arrowheads and dashed lines for virtual rays. Label normals and mark measured angles. Use tables to show measurements and a short conclusion sentence linking observation to a rule (for example, "angle of incidence equals angle of reflection"). This clear method helps in both class work and exams.

Key experiments to remember Certain simple practicals appear often in tests. Be sure you can describe: (1) straight-line travel of light using a pinhole or a ray box, (2) shadow formation with torch, showing umbra and penumbra, (3) demonstration of the laws of reflection with a plane mirror and measurement of angles, and (4) building and using a simple periscope with two mirrors at 45°. For each practical, practise stating the aim, listing materials, describing steps, recording observations and writing a one-line conclusion that links the observation to an idea.

How to draw exam diagrams A neat diagram helps you score marks. Use a pencil and ruler to draw straight lines for mirrors and rays. Show arrowheads to indicate ray direction. Where an image is virtual, extend reflected rays behind the mirror as dashed lines and mark their intersection. Draw the normal as a faint dotted line at the point of incidence and label the angles clearly, using a protractor. Keep diagrams simple and avoid unnecessary clutter; a clear, labelled sketch is better than a crowded one.

Answer structure for theory and practical questions For theory, begin with a short definition (for example, "shadow is a dark region formed when light is blocked"), then give one or two sentences explaining the cause. For practical descriptions, follow the order: aim, apparatus, procedure (brief), observation and conclusion. If asked for a reason, link directly to a concept, such as "angles are equal because reflection follows the law of reflection." Include a safety note when relevant.

Common exam tasks and tips You may be asked to draw ray diagrams, explain why a shadow changes size, name parts of an experiment, or predict the position of an image in a plane mirror. Practise each kind: draw at least one ray diagram each day, write short answers to definitions and summarise experiments in three lines. Time yourself on one or two questions to build speed and clarity.

Checking your answers After writing, read your answer to ensure you labelled diagrams, included units (if any), and wrote a brief conclusion. If describing an experiment, mention one possible source of error and a safety step. Clear presentation and the correct sequence of ideas often win marks even if some detail is missing.