Chapter 4 — Light Energy

What is light?
Light is a form of energy that enables vision and carries energy across space. We detect light with the eyes, but many instruments can also measure it. Light interacts with matter: it can be absorbed, transmitted, reflected or scattered. These interactions explain a wide range of everyday experiences, from the warmth felt from sunlight to how photosynthesis uses light energy in plants.

Kinds of sources
Luminous sources produce their own light. Natural luminous sources include the Sun, stars and flames. Man-made luminous sources include electric bulbs, LEDs and fluorescent tubes. Some materials emit light after absorbing energy: for example, fluorescent paints and glow-in-the-dark materials release light after being charged by sunlight. Bioluminescence is light produced by living organisms, such as fireflies and some deep-sea animals.

How light is produced
Different sources make light by different processes. In a heated filament bulb, thermal energy makes the metal glow (incandescence). In fluorescent tubes, electrical energy excites gas atoms which then emit ultraviolet light; a coating converts it to visible light. LEDs emit light when electrons move across a semiconductor material. Flames produce light from hot gases and solid particles that radiate across many colours. Understanding these differences helps explain brightness, colour and safety concerns for each source.

Seeing non-luminous objects
Most objects we see are non-luminous; they become visible because they reflect light from luminous sources. For example, the Moon is non-luminous but bright because it reflects sunlight. The apparent brightness of an object depends on the source intensity, distance and reflective properties of the object's surface. Matte surfaces scatter light and appear less shiny, while polished surfaces reflect more directionally and look glossy.

Why it matters
Knowing types of sources and how light is produced helps for choosing lamps at home, protecting eyes, and understanding technology such as LEDs and optical devices. Students will practice identifying sources in experiments and learn safe handling when working with strong light sources.

Basic idea
Rectilinear propagation means light travels in straight lines in a uniform medium. This idea is helpful for explaining sharp shadows, the direction of light beams and the paths used in simple ray diagrams. For ordinary everyday distances and objects much larger than the wavelength of light, the straight-line model accurately predicts where light will travel.

Simple observations
Classroom demonstrations show rectilinear propagation clearly. For example, darken the room and make a small round hole in a cardboard screen; a round bright spot appears on the opposite wall showing light moves in straight paths from the hole. Another experiment uses three cards with holes: when holes are aligned in a straight line a beam of light passes through, but moving the middle card slightly blocks the beam. These activities give direct visual evidence and help students learn to draw straight rays with arrows to show direction.

Shadows and pinhole camera
Rectilinear travel explains shadow shapes and the pinhole camera. When an opaque object blocks the light, the shape of the shadow on a screen depends on the straight paths that are blocked. A pinhole camera uses a tiny aperture so only straight rays from each point of the object reach the screen, forming an inverted image. The smaller the hole (to a limit), the sharper the image because fewer different paths overlap.

Limitations and note
Rectilinear propagation is an excellent approximation for many problems in Class 7. There are cases like diffraction and interference where light shows wave behaviour and does not travel strictly in straight lines near edges or very small openings. These effects appear only when openings or obstacles have sizes comparable to the wavelength of light and are studied in higher classes. For now, practise using straight rays for drawing diagrams, predicting shadow positions and explaining basic optical devices such as periscopes and simple lenses.

Shadow formation
A shadow forms when an opaque object blocks light from a source. The shadow is the region where direct light is absent. If the light source is small or far away, the shadow has a sharp edge; if the source is extended, the shadow has two parts: the dark central umbra and the lighter surrounding penumbra. The umbra is the place of complete darkness where all rays are blocked. The penumbra receives some rays from parts of the source and is therefore only partially dark.

How geometry affects shadow
Shadow size and sharpness change with the relative positions of source, object and screen. Move the object closer to the source and the shadow on the screen becomes larger and more blurred; move the object closer to the screen and the shadow becomes smaller and sharper. These changes can be explained by tracing straight-line rays from different parts of the extended source to the screen and noting which are blocked by the object.

Eclipses in space
Eclipses are large-scale shadow events involving the Sun, Earth and Moon. A solar eclipse happens when the Moon comes between the Sun and Earth so that the Moon's shadow falls on parts of Earth. A lunar eclipse happens when the Earth comes between Sun and Moon and Earth's shadow falls on the Moon. Total eclipses occur when alignment is precise and one body completely covers the other; partial eclipses occur when only part is covered. Eclipses are not monthly events because the Moon's orbit is tilted with respect to Earth's orbit, so perfect alignment is not always obtained.

Applications and observations
Understanding shadows helps in everyday life – from reading sunset times to designing theatres and planning stage lighting. Safe observation of solar eclipses requires projection methods or approved solar filters; never look directly at the Sun. Classroom activities include measuring how shadow size varies with distance, making pinhole projections of the Sun, and drawing ray diagrams to explain umbra and penumbra formation.

Meaning of reflection
Reflection is the change in direction of light when it bounces off a surface. When a light ray strikes a reflective surface, part of its energy returns into the original medium. To describe reflection we use three rays: the incident ray (approaching the surface), the reflected ray (leaving the surface) and the normal (an imaginary line at right angles to the surface at the point of incidence).

Laws of reflection
Two simple laws govern reflection. First, the incident ray, the reflected ray and the normal all lie in the same plane. Second, the angle of incidence is equal to the angle of reflection, with both angles measured from the normal. These laws hold for reflection from plane mirrors and for small surface patches on curved mirrors. They allow accurate prediction of the direction of reflected rays using simple geometry.

Specular and diffuse reflection
Surfaces differ in how they reflect light. Smooth surfaces such as polished metal or glass produce specular reflection where parallel incident rays remain parallel after reflection, producing a clear image. Rough surfaces like paper or painted walls cause diffuse reflection; incident rays scatter in many directions and although no clear image forms, the surface is visible from many viewing angles. Everyday visibility of objects relies mainly on diffuse reflection from their surfaces.

Practical uses and experiments
Students should practise drawing incident and reflected rays, measuring angles with a protractor and verifying the laws of reflection. Simple demonstrations include reflecting a torch beam from a plane mirror and using it to locate an image. Reflection explains how flat mirrors form virtual images, why shiny surfaces show highlights, and the working of instruments like periscopes where plane mirrors at 45 degrees change the direction of rays. Safety note: avoid reflecting intense beams into anyone's eyes.

Image properties
A plane mirror always produces a virtual image that appears behind the mirror. The image is the same size as the object and is upright. One important characteristic is lateral inversion: the image appears flipped left-to-right. Also, the distance of the image behind the mirror equals the distance of the object in front of it. These features follow from the straight-line paths of light and the laws of reflection.

Constructing ray diagrams
To find the image location, draw two or more rays from a point on the object. Typical rays: one heading to the mirror and reflecting with an equal angle; one perpendicular to the mirror which reflects back on itself; or a ray aimed at any other point on the mirror reflecting accordingly. Extend the reflected rays behind the mirror as dotted lines; their intersection marks the virtual image point. Accurate construction with a ruler and protractor helps visualise why the image distance equals the object distance.

Uses of plane mirrors
Plane mirrors are used where a simple, undistorted image is needed without magnification: grooming mirrors, decorative mirrors and part of optical instruments where light needs to be redirected. Two plane mirrors placed at angles can produce multiple images and are used in periscopes to allow viewing from concealed positions. Students should practice drawing ray diagrams for objects at different heights and observe lateral inversion by comparing left-right placement.

Limitations
Plane mirrors do not magnify or reduce size and cannot form real images that can be projected onto a screen. For projection or magnification, curved mirrors or lenses are required. For Class 7, focus on neat diagrams and clear labelling of object, image, and rays to build skills for more advanced optics topics later.

Types and key points
Spherical mirrors are sections of a sphere. A concave mirror has its reflecting surface curved inward and can converge parallel rays to a point; a convex mirror has the reflecting surface curved outward and diverges parallel rays. Important points are the pole (P) at the mirror centre, the centre of curvature (C) which is the centre of the parent sphere, and the focal point (F) which lies midway between P and C for small-angle rays.

Principal rays for concave mirror
To locate images we commonly use three principal rays: (1) a ray parallel to the principal axis reflects through F; (2) a ray passing through C reflects back along itself; (3) a ray towards F reflects parallel to the axis. Using these rays you can construct the image location and determine whether it is real or virtual, inverted or upright, magnified or reduced depending on object position relative to C and F.

Characteristics and examples
Concave mirrors can form real inverted images when the object is beyond the focal point; if the object is very close (within F), the mirror gives a virtual, upright and magnified image — useful in shaving mirrors and make-up mirrors. Convex mirrors always form virtual, upright and reduced images regardless of object position and thus are useful as rear-view mirrors or in shop security because they provide a wide field of view.

Practical uses and exercises
Students should practise constructing ray diagrams for various object positions: beyond C, between C and F, at C and within F for concave mirrors; for convex mirrors show the virtual image behind the mirror. Discuss safety and correct usage: concave mirrors can focus sunlight and may cause burns if misused. These exercises prepare students for mirror formula and magnification studied later.

Definition and cause
Refraction is the bending of light when it passes from one transparent medium into another because its speed changes. Light travels at different speeds in different media: usually faster in air and slower in water or glass. When a ray meets the boundary at an angle, the change in speed causes the ray to change direction.

Direction rules
When light goes from a rarer medium (less optically dense, e.g. air) into a denser medium (more optically dense, e.g. glass), it bends towards the normal. When it goes from denser to rarer, it bends away from the normal. The normal is a line drawn at right angles to the surface at the point where the ray meets it. These rules help predict the path of light in lenses and at surfaces.

Observations and everyday examples
Try placing a straight stick partly in water; it looks bent at the surface because rays from the submerged part change direction as they leave the water and reach your eye. Another example is a coin at the bottom of a cup becoming visible when water is poured in: refraction redirects rays so they enter the eye. Refraction also causes atmospheric effects like the apparent position shift of the Sun at sunrise and sunset and contributes to mirages in deserts.

Qualitative practice
Students should draw incident and refracted rays with normals at the boundary, and note the direction of bending. Experiments with a rectangular glass slab show a lateral shift of the ray but the emergent ray is parallel to the incident ray. Triangular prisms refract rays twice and lead to dispersion (separation of colours). For Class 7 focus on careful diagrams and clear language linking speed change to bending rather than on numerical laws, which come later.

Types and shape
Lenses are transparent bodies that refract light at their surfaces. A convex (converging) lens is thicker at the centre and brings parallel rays together at its focal point. A concave (diverging) lens is thinner at the centre and causes parallel rays to spread out as if they came from a focal point behind the lens. Thin-lens diagrams use the principal axis, optical centre and focal points to trace ray paths.

Principal rays and diagrams
For tracing images with a convex lens the usual three rays are: (1) a ray parallel to the axis refracts through the far focal point; (2) a ray through the optical centre goes straight without deviation; (3) a ray through the near focal point emerges parallel to the axis. Using these rays locate the image as the intersection of refracted rays. Convex lenses give a variety of images depending on object distance: real and inverted when object is beyond F, virtual and magnified when object is within F. Concave lenses always form virtual, upright and reduced images located on the same side as the object.

Applications and practice
Convex lenses are used in magnifying glasses, cameras and corrective glasses for long-sightedness. Concave lenses are used in spectacles for short-sightedness. Students should practice neat ray diagrams showing object distance, image distance and height. At Class 7 avoid algebraic lens formulas; concentrate on reasoning about why image nature changes with object position and how magnification appears in practical devices.

White light and colours
White light is a combination of many colours. Each colour has its own wavelength and bends by a slightly different amount when passing from one medium to another. Dispersion is the process in which white light splits into its component colours because different colours refract differently. The familiar sequence of colours is red, orange, yellow, green, blue, indigo and violet.

How a prism disperses light
A triangular glass prism disperses light by refraction at two surfaces. When a narrow beam of white light enters the prism it bends towards the normal at the first surface; because violet light refracts more than red, the beam begins to spread. At the second surface each colour bends again as it leaves the glass into air, increasing the separation. The result on a screen is a continuous spread of colours called a spectrum. Red deviates least and violet most so the order across the screen is red on one side and violet on the other.

Recombination and reversibility
Dispersion is reversible. If the spectrum is passed through another prism oriented oppositely, the colours recombine to give white light. This proves dispersion separates existing colours rather than creating them. In nature, rainbows form when sunlight disperses inside countless tiny water droplets; each droplet acts like a tiny prism dispersing and reflecting light to the observer.

Experiments and safety
Classroom experiments can use a slit to produce a narrow beam of sunlight or a torch beam and a glass prism to project a spectrum onto a screen. Students should note the order of colours and relate stronger dispersion to greater difference in refractive index for different wavelengths. Handle prisms with care to avoid cuts and prevent concentrated sunlight from causing burns or starting fires.

Major parts and their roles
The human eye is a complex organ that functions like a camera. The cornea is the transparent outer layer that begins bending light. Behind it lies the aqueous chamber and then the lens, a flexible convex structure that fine-tunes focus. The iris is the coloured ring that changes the size of the pupil, the opening that controls how much light enters. At the back is the retina, a light-sensitive layer covered with photoreceptor cells (rods and cones) which convert light into electrical signals. The optic nerve carries these signals to the brain where they are interpreted as images.

How the eye forms an image
Light from an object passes through the cornea and lens and is focused to form a real, inverted image on the retina. The brain learns to interpret this inverted image as upright. The lens changes shape — a process called accommodation — to focus on objects at different distances: the ciliary muscles make the lens thicker for nearby objects and thinner for distant objects. The pupil size also adjusts automatically: in bright light the pupil becomes small to reduce light entry; in dim light it widens to allow more light in.

Role of rods and cones
Rods are sensitive to low light levels and help with night vision but do not detect colour. Cones work in bright light and detect colours (red, green and blue types). Together they allow us to see details and colours across a range of light conditions. The fovea, a small central area of the retina, provides the sharpest vision because it has a high density of cones.

Care and simple tests
Eye care includes avoiding direct sunlight on the eyes, using proper lighting for reading, taking breaks during long near work, and maintaining hygiene to prevent infections. Regular eye tests detect problems such as refractive errors early. In the classroom students can draw labelled diagrams of the eye and explain accommodation qualitatively. Practical demonstrations may include observing pupil changes in bright and dim light under supervision.

Common refractive defects
Two common defects of vision are myopia (short-sightedness) and hypermetropia (long-sightedness). In myopia the focussed image of distant objects falls in front of the retina because the eyeball is too long or the lens system is too strong; distant objects appear blurred. In hypermetropia the image of near objects falls behind the retina because the eyeball is too short or the lens cannot become thick enough; near objects appear blurred.

How corrective lenses work
Correction uses lenses to change the path of incoming rays so the eye can focus them on the retina. A concave (diverging) lens for myopia spreads parallel rays slightly so that the eye's own lens brings them to focus on the retina. A convex (converging) lens for hypermetropia helps by converging rays before they enter the eye so the eye lens needs less adjustment to focus on the retina. The spectacles are prescribed with powers suitable for the degree of error; contact lenses serve the same optical purpose but sit directly on the eye.

Other conditions and age effects
Presbyopia is an age-related condition where the lens loses flexibility and cannot accommodate for near vision; it is corrected with reading glasses (convex) or bifocals. Some severe refractive errors and structural problems can be treated surgically, but such decisions are for specialists. Students should understand that self-prescribing spectacles is unsafe and professional eye tests are necessary.

Practical drawing and care
Practice ray diagrams to show how the defective eye forms images and how a corrective lens redirects rays to form the image on the retina. Emphasise care of spectacles and contact lenses, hygiene and that any eye pain or sudden changes require immediate medical attention. These ideas give simple but important understanding of everyday optical corrections.

Magnifying glass
A magnifying glass is a single convex lens used to see enlarged virtual images. When the object is placed between the lens and its focal point, the lens produces a virtual, upright and larger image on the same side as the object. This helps read small print or view fine details. The apparent size increases because the image occupies more of the viewer's field of vision.

Compound microscope (qualitative)
A compound microscope uses two convex lenses: the objective (near the object) and the eyepiece. The objective forms a real, enlarged, inverted image of the tiny object; the eyepiece acts as a magnifier to produce a much larger virtual image for the eye. This two-stage magnification lets microscopes reveal structures too small for the naked eye. At Class 7 the focus is on the idea that magnification is achieved by combining lenses rather than on formulae.

Telescope (qualitative)
A simple refracting telescope uses a large convex objective lens to collect light from a distant object and form a real image; the eyepiece magnifies this image for the observer. Telescopes gather more light and resolve fine details of distant objects like the Moon or planets. Reflecting telescopes use mirrors to avoid colour dispersion problems. Students should understand the purpose of each lens and that alignment is important for clear images.

Practical care
When using these instruments, avoid direct sunlight and do not point telescopes at the Sun without proper solar filters. Handle lenses and mirrors carefully to prevent scratches. Classroom activities can include using a magnifier to study texture or building a simple cardboard pinhole camera to compare with lens-based instruments.

Domestic and transport uses
Light is essential in homes, schools and streets for visibility and safety. Electric bulbs and LEDs illuminate rooms and roads; LEDs are energy-efficient and long-lasting. Mirrors and lenses aid grooming and vehicle safety: convex mirrors act as rear-view mirrors to provide a wider field of view, while headlights use reflectors to direct light beams. Periscopes let people see over obstacles by changing the direction of rays using mirrors.

Communication and industry
Light carries information in modern communication. Optical fibres guide light signals over long distances with low loss using repeated internal reflections; they form the backbone of fast internet and telephone systems. In industry, lasers are used for cutting, welding and precise measurements. Light-based sensors detect motion, colour and position in automated systems.

Science and medicine
Optical instruments such as microscopes and telescopes open up worlds otherwise invisible. Microscopes reveal cells and tiny organisms; telescopes study distant stars and planets. In medicine, endoscopes let doctors see inside the body; ophthalmoscopes help examine the eye. Lasers are used for eye surgery and other delicate operations due to their precise beam control.

Energy and safety
Sunlight is a key energy source: solar panels convert sunlight into electricity, and sunlight supports plant life through photosynthesis. However, sunlight also contains harmful ultraviolet (UV) radiation that can damage skin and eyes; sunglasses and sunscreen provide protection. Devices that concentrate light, such as convex lenses or mirrors, can cause burns or start fires if misused. Responsible use and understanding of principles (reflection, refraction, dispersion) let us apply light safely across technologies.

Risks of bright light
Bright light sources can harm eyes and skin. Never look directly at the Sun, even during partial eclipses, because the concentrated visible and ultraviolet radiation can damage the retina. Use projection methods or approved solar filters for safe observation. High-power lasers and focused beams can cause temporary or permanent blindness if pointed at eyes, and can distract drivers or pilots; never aim lasers at people, vehicles or reflective surfaces.

Handling optical equipment
Glass prisms, lenses and mirrors may break and cause cuts. Handle them with dry, clean hands and store them in protective cases when not in use. Use stands and clamps for optical elements to prevent slipping when aligning beams. Keep the work surface uncluttered so beams are not accidentally redirected towards someone's eyes. If a beam must be powerful, warn others in the room and use beam stops or non-reflective surfaces to terminate the ray safely.

Safe experiment practices
When using sunlight in experiments, use a pinhole projector or a white screen to project the Sun's image; do not look through magnifying lenses or telescopes at the Sun. Monitor temperatures carefully when concentrating sunlight: do not leave focused beams on flammable materials. Wear protective goggles if working with lasers or intense light sources under teacher supervision. Dispose of broken glass carefully in a designated sharps container and report accidents immediately.

Teaching responsibility
Safety is part of scientific practice. Teach students to plan experiments, identify hazards, use protective equipment and seek adult supervision for risky demonstrations. Encourage safe storage of devices like laser pointers and proper disposal of batteries from torches. Good lab habits protect curiosity and allow learning to continue without harm.