Chapter 15 — Light

What is light? Light is a form of energy that enables us to see objects. It travels through space and transparent media and interacts with objects by reflection, refraction and absorption.

Nature of light (basic points for Class 7)

• Travels in straight lines: Light rays travel in straight paths. You can verify this by the formation of sharp shadows when an obstacle blocks light.
• Can be represented as rays: For simple experiments and diagrams we draw straight lines called light rays to show the direction of light.
• Visible part of electromagnetic spectrum: Visible light is the small part of the electromagnetic spectrum that human eyes can detect (roughly 400–700 nm).
• Reflection, refraction and dispersion: When light strikes a surface it may reflect (bounce back), refract (change direction in another medium) or split into colours (dispersion) as in a prism.
• Luminous vs non-luminous objects: Luminous objects produce their own light (e.g., Sun, bulb). Non-luminous objects do not produce light but may reflect it (e.g., moon, table).

Types of sources

• Natural sources: Objects that emit light naturally — the Sun, stars, fire, lightning, bioluminescent organisms (fireflies, some deep-sea creatures).
• Artificial sources: Man-made devices or processes that produce light — electric bulbs, LEDs, torches, candles, neon signs.
• Point source vs extended source: A point source is small compared to the distance (light seems to come from a single point) whereas an extended source has a noticeable size (e.g., a fluorescent tube).

Simple classroom experiments (ideas)

• Shadow formation: Use a torch and an opaque object to show that light travels in straight lines.
• Pin-hole camera: Show how inverted images form because of straight-line travel of light through a small aperture.
• Prism dispersion: Pass white light through a glass prism to separate it into colours.

Important notes

• Brightness observed depends on the distance from the source and the nature (power) of the source.
• The Sun is the primary natural source of light on Earth and supports life by providing energy and daylight for photosynthesis.

Definitions: Materials are classified by how they allow light to pass through them.

• Transparent bodies: Allow most of the incident light to pass through in straight lines so objects seen through them are clear. Example behavior: clear glass, clean water, air.
• Translucent bodies: Allow some light to pass but scatter it so objects seen through them appear blurred or not clearly visible. Example behavior: frosted glass, thin paper, wax paper.
• Opaque bodies: Do not allow light to pass through; they either absorb or reflect the incident light so no image is seen through them. Example behavior: wood, metal, brick.

How to test: Place the material between a light source and an object. If the object is seen clearly → transparent; seen vaguely or blurred → translucent; not seen at all (a shadow forms) → opaque.

Why these behaviours occur: Interaction of light with matter involves transmission, absorption, reflection and scattering. Transparent materials transmit light with little absorption and little scattering. Translucent materials scatter light: photons change direction inside the material, so transmitted light loses directional information. Opaque materials absorb or reflect most incident light; transmitted intensity is essentially zero.

Notes: The classification can depend on wavelength and thickness — a material may be transparent to some wavelengths (e.g., visible) but opaque to others (e.g., glass is opaque to some infrared). Also, very thin pieces of an otherwise opaque material can become partly transparent.

Applications: Windows and lenses use transparent materials; lamp shades and diffusers use translucent materials to soften light; walls and doors use opaque materials for privacy and blocking light.

Summary: Transparent = clear transmission; Translucent = scattered transmission (blurred); Opaque = no transmission (shadow).

Definition: Rectilinear propagation of light means that in a homogeneous medium (like air or vacuum) light travels along straight lines. We represent these straight-line paths by rays.

Explanation & evidence:

• When a small (or point) light source is placed behind an opaque object, a sharp shadow is formed on a screen. The straight edges of that shadow show that light travels in straight lines from the source to the screen.
• Simple classroom experiments: poke a small hole in a cardboard and let sunlight or a bulb beam through the hole — the patch of light on the opposite wall is circular and aligned with the hole because rays travel straight. Place an obstacle in the beam and you get a sharp shadow.
• If the light source is extended (not a point), each point on the source casts its own shadow; their overlap produces a dark central region (umbra) and a lighter surrounding region (penumbra). The geometry of umbra/penumbra still follows straight-line ray paths.

Why it matters: Rectilinear propagation is the basis for ray diagrams used to explain shadows, pinhole cameras, periscopes, and basic reflection/refraction reasoning.

Key ideas shown by ray diagrams:

• Straight rays from a point source to the top and bottom of an object determine the position and size of its shadow.
• For a very distant source (like the Sun), rays are nearly parallel, producing long, well-defined shadows whose direction depends on the Sun’s position.

Simple geometry (using similar triangles): If you draw rays from a point source through the top of an object to the ground/screen, you can use similar triangles to relate object height, distances, and shadow length (see formulas below).

Limitations: Rectilinear propagation is valid inside a uniform medium; light can change direction when it reflects from surfaces or refracts at boundaries between media (those are separate principles).

What is a shadow?
A shadow is a dark region formed on a surface when an opaque object blocks light from a source. Light travels in straight lines (rays); where rays are obstructed, no light reaches the region and a shadow appears.

How shadows are formed (basic idea)
If a light source, an object and a screen (or surface) are placed so that the object lies between the source and the screen, rays from the source that hit the object cannot go beyond it. The rays that pass by the edges of the object determine the boundary of the shadow on the screen. Using geometry (similar triangles) we can relate the sizes and distances of the object and its shadow.

Types of shadow

• Umbra: The fully dark central region where the light source is completely blocked (no direct light). This is the darkest part of the shadow.
• Penumbra: The partially shaded outer region where only part of the light source is blocked (some rays reach this region). It appears lighter and fuzzy.
• Antumbra: Occurs in some eclipse-like geometries when the occluding object is smaller than the light source as seen from the screen; the observer inside antumbra sees the light source as a ring around the occluding object (relevant in solar eclipses).

Factors affecting the size and sharpness of a shadow

• Size of the light source: A point source gives a sharp shadow (only umbra). A large/extended source gives a fuzzy shadow with a penumbra.
• Distance between the light source and the object: For a fixed screen position, moving the source closer to the object increases the shadow size on the screen.
• Distance between the object and the screen: Moving the screen farther from the object (keeping source fixed) makes the shadow larger.
• Size of the object: A taller/wider object casts a larger shadow (proportional).

Derivation using similar triangles (point source)
Consider a point light source S, an object of height h placed at distance a from S, and a screen placed at distance b from S (b>a). Rays from S to the top of the object and to the top of the shadow make similar triangles. From similarity:

H / h = b / a   so    H = h * (b / a)

Here H is the height (size) of the shadow on the screen, h is the object height, a = distance from source to object, and b = distance from source to screen.

Qualitative consequences from the formula

• Shadow size H is directly proportional to object height h.
• For fixed object and source position (fixed a), H is directly proportional to b (screen farther from source -> larger shadow).
• For fixed h and b, H decreases if a increases (moving object closer to the source reduces shadow size on a fixed screen).

Simple classroom experiments

1. Point source experiment: Use a small bulb or LED (approximate point source), a vertical stick of known height and a screen. Measure H while varying b and verify H = h * (b / a).
2. Extended source experiment: Use a wide lamp and observe umbra and penumbra. Move the lamp closer/further to see how penumbra size changes.
3. Shadow length at different times of day: Observe a vertical pole outdoors — shadow is shortest at local noon and longer in morning/evening (sun’s elevation changes).

Uses and real-life examples
Sundials (tell time by shadow position), solar and lunar eclipses (umbra and penumbra explain partial/total eclipses), shadow puppetry, measuring heights using similar triangles (measure shadow of a stick and an object), photographic lighting (control of shadows for sharpness or softness).

Safety note
When using the Sun as a light source, never look directly at the Sun. Use indirect observation methods for eclipse/shadow studies.

What is reflection of light?
Reflection of light is the phenomenon in which light rays bounce back when they strike the surface of a body. The bounced rays are called reflected rays. Reflection helps us see objects that do not produce their own light.

Important terms
• Incident ray: the incoming light ray that strikes a surface.
• Reflected ray: the ray that leaves the surface after reflection.
• Normal: an imaginary line perpendicular to the 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.

Laws of reflection
1. The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection (i = r). These angles are measured from the normal.

Types of reflection
• Regular (specular) reflection: occurs from smooth, polished surfaces (like a plane mirror or calm water). Parallel incident rays remain parallel after reflection and produce clear images.
• Diffuse reflection: occurs from rough surfaces (like paper or unpolished wood). Parallel incident rays scatter in many directions, so no clear image is formed.

Plane mirror image characteristics (simple points)
• Image appears behind the mirror (virtual).
• Image is of the same size as the object and erect.
• Left and right are interchanged (lateral inversion).
• Distance of the image from the mirror equals the distance of the object from the mirror.

Simple classroom demonstration
Place a ray box or torch so that a narrow beam hits a plane mirror at some angle. Draw the incident and reflected rays and the normal on a paper. Measure the angles of incidence and reflection; you will find i = r.

Introduction

Reflection of light is the return of light rays when they strike a surface. The most common example is a plane (flat) mirror. To describe reflection we use these terms:

• Incident ray: the incoming light ray that strikes a surface.
• Point of incidence: the point where the incident ray meets the surface.
• Normal: an imaginary line perpendicular to the surface at the point of incidence.
• Reflected ray: the ray that bounces off the surface.
• Angle of incidence (θi): the angle between the incident ray and the normal.
• Angle of reflection (θr): the angle between the reflected ray and the normal.

The two laws of reflection

1. First law: The angle of incidence is equal to the angle of reflection. In symbols, θi = θr. This is measured from the normal, not from the surface.
2. Second law: The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane (called the plane of incidence).

Why these laws hold (intuition)

When a light wavefront meets a smooth surface, each point on the wavefront produces secondary wavelets. The direction in which the reflected wavefront advances is such that the path lengths satisfy equal angles with the normal — this leads to θi = θr. The geometry of the surface and the incoming wavefront ensures all involved rays and the normal lie in one plane.

How to verify experimentally (simple activity)

1. Place a small plane mirror on a table and draw a straight baseline on paper where the mirror sits.
2. Draw a normal (straight line) at a chosen point on the mirror line.
3. Use a ray (pencil and ruler or a laser pointer) to draw an incident ray making a measurable angle with the normal.
4. Observe and mark the reflected ray. Use a protractor to measure θi and θr — they should be equal.

Connection to images in plane mirrors

Because of these laws, a plane mirror forms a virtual image that appears as far behind the mirror as the object is in front. The image is upright and laterally inverted (left-right swapped).

Reflection of light is the bouncing back of light rays when they fall on a surface. There are two main kinds of reflection — regular (specular) and diffused (diffuse).

Regular (specular) reflection:

• Occurs on smooth, polished surfaces (plane mirror, calm water, polished metal).
• Parallel incident rays remain parallel after reflection. Each ray follows the law of reflection: the angle of incidence equals the angle of reflection measured from the normal at the point of incidence.
• Because reflected rays stay ordered, a clear image is formed (for example, a sharp image in a mirror).
• Used in mirrors, periscopes, reflecting telescopes, and polished instruments.

Diffused (diffuse) reflection:

• Occurs on rough or irregular surfaces at the microscopic level (paper, unpolished wood, painted walls, cloth).
• Although the law of reflection (locally: i = r) still holds at each tiny patch, the surface normals vary from point to point, so parallel incident rays are reflected in many different directions.
• No clear image is formed because reflected rays are scattered; however, diffused reflection is what allows us to see most objects from many angles (we see a book or a wall because light is diffused into our eyes).
• Useful for matte finishes, reducing glare, and lighting a room uniformly.

Why both obey the law of reflection: The law i = r is true for each small area of the surface. Regular reflection appears ordered when the microscopic normals are all nearly the same; diffused reflection appears scattered when those normals differ widely.

Key observable differences:

• Image: regular reflection produces a clear image; diffused reflection does not.
• Direction: regular gives predictable single direction for a given incident ray; diffuse gives many directions.
• Surface requirement: regular needs a smooth surface; diffuse happens on rough surfaces.

Simple classroom demonstration: Shine a narrow beam (torch) on a mirror and on a white wall. On the mirror you will see a bright reflected spot at the predicted reflected direction; on the wall the light spreads and you cannot get a sharp reflected spot visible from a single direction.

What is a plane mirror? A plane mirror is a flat, polished surface (usually glass with a reflective coating) that reflects light according to the law of reflection.

Law of reflection: The angle of incidence is equal to the angle of reflection (∠i = ∠r). Both angles are measured with respect to the normal to the mirror at the point of incidence.

How an image is formed by a plane mirror: When light rays from an object strike a plane mirror they reflect. The reflected rays appear to come from a point behind the mirror. Your eye intercepts the reflected rays and the brain traces them back in straight lines; the apparent meeting point behind the mirror is the image. Because the actual reflected rays do not meet behind the mirror, the image is called a virtual image.

Steps to draw a simple ray diagram (object: small upright arrow):

• Draw the mirror as a vertical line and the object (arrow) in front of it.
• Draw two rays from the top of the object: (a) one ray striking the mirror normally (if drawn) reflects back on itself; (b) one ray striking the mirror at some angle reflects such that ∠i = ∠r.
• Extend the reflected rays backward (dashed lines) behind the mirror; their intersection gives the top of the image.
• Complete the image as an arrow upright behind the mirror at the same distance from the mirror as the object.

Properties of the image formed by a plane mirror:

• Virtual (cannot be formed on a screen)
• Erect (upright)
• Same size as the object
• Laterally inverted (left and right are swapped)
• Distance from mirror to image = distance from mirror to object

Simple activity to observe image formation: Place a small object (coin or toy) on a table and hold a plane mirror vertically a few centimetres away. Move your eye so the reflected rays reach it; you will see the coin’s image appear behind the mirror at a distance equal to the coin’s distance in front of the mirror.

Why the image is virtual: Because the reflected rays diverge after reflection. They only appear to converge behind the mirror when traced backward; real light does not actually pass through the image location.

Uses and examples: Plane mirrors are used in dressing mirrors, bathroom mirrors, dentist mirrors, some periscopes (two plane mirrors at 45°), and inside cupboards or elevators for seeing your reflection.

What is a plane mirror? A plane mirror is a flat reflecting surface. When light rays from an object fall on it, they reflect according to the law of reflection (angle of incidence = angle of reflection) and form an image.

How is the image formed? To locate the image, draw at least two reflected rays from a point on the object. Extend the reflected rays backward behind the mirror. The point where these extensions meet is the image of that object point. Because the extensions meet behind the mirror, the image is virtual (not formed by real rays) and cannot be caught on a screen.

Main characteristics of the image in a plane mirror

• Virtual: The image is formed behind the mirror by extensions of reflected rays; it cannot be projected on a screen.
• Erect (upright): The image has the same orientation as the object (not upside down).
• Same size as the object: The linear dimensions of image and object are equal.
• Laterally inverted (left-right reversal): Left and right appear swapped in the image.
• Distance from mirror equal: The image appears to be as far behind the mirror as the object is in front of it.
• Magnification = 1: The image is neither magnified nor diminished.
• Only one image: A single plane mirror produces one image of an object (unless multiple mirrors are used).

Simple classroom ray-diagram description: Place an object (e.g., an arrow) in front of a vertical mirror. From the tip of the arrow, draw two incident rays to the mirror. Reflect them using the law of reflection. Extend the reflected rays behind the mirror; they meet at the tip of the virtual image arrow located the same distance behind the mirror as the object is in front.

Notes on sign conventions and physics form: In standard mirror sign convention for a plane mirror the image distance v = -u (image behind mirror, object in front), which leads to |v| = |u| and magnification m = +1 (image height = object height and upright).

Overview: Ray diagrams are drawings that show the path of light rays as they travel, meet surfaces and form images. They help us visualise how objects are seen (by reflection or refraction) and let us measure angles and distances involved.

Key terms (with short definitions):

• Ray of light: A straight line showing the direction of light travel.
• Incident ray: The ray that strikes a surface.
• Point of incidence: The point where the incident ray meets the surface.
• Normal: An imaginary line perpendicular to the surface at the point of incidence.
• Angle of incidence (i): The angle between the incident ray and the normal.
• Reflected ray: The ray that leaves the surface after reflection.
• Angle of reflection (r): The angle between the reflected ray and the normal.
• Object: The real thing that emits or reflects light and whose image is formed.
• Image: The appearance or reproduction of the object formed by reflected or refracted rays. It can be real (rays actually meet) or virtual (rays appear to come from a point).
• Mirror surface / Principal axis: For plane mirrors the surface is flat; for curved mirrors a principal axis is the centre line used in drawing rays.
• Focal point and centre of curvature: Terms used for curved mirrors/lenses (introduced later), but useful to know when drawing curved-surface ray diagrams.

Measurements in ray diagrams:

• Use a ruler to measure distances along the perpendicular to the mirror to get object distance (u) and image distance (v).
• Use a protractor to measure angles: place the protractor centre at the point of incidence, align 0° along the normal, and read angles between rays and the normal.
• When drawing diagrams to scale, choose a convenient scale (for example 1 cm = 1 cm actual) so distances and heights can be compared and measured directly from the diagram.
• To locate a virtual image (e.g., in a plane mirror) extend the reflected rays backwards with dashed lines; their intersection shows the image position behind the mirror.

Steps to draw a basic ray diagram for a plane mirror:

1. Draw the mirror as a straight vertical line. Mark a point on the left as the object (an arrow pointing up is usual).
2. From the top of the object draw at least two incident rays to two points on the mirror surface.
3. At each point of incidence draw the normal (perpendicular) and then draw the reflected ray making the same angle with the normal as the incident ray (i = r).
4. Extend the reflected rays backwards behind the mirror (dashed lines); their intersection gives the position of the virtual image. The image will be erect and the same size as the object and at the same distance behind the mirror as the object is in front.

Practical tips for measurement experiments:

• Use a ray box or a torch with a slit to get narrow rays for accurate drawing.
• Fix the mirror on a sheet of paper, mark points of incidence clearly, and draw normals with a set square for accuracy.
• Repeat measurements for different angles to verify the law of reflection.

Definition: Lateral inversion is the apparent left–right reversal of an object’s image in a plane mirror. When you look into a mirror, what was on your left appears on the right in the mirror image and vice versa.

How it happens:

• Light from every point of an object reflects from the mirror according to the law of reflection: angle of incidence = angle of reflection. The reflected rays appear to come from points behind the mirror, forming a virtual image.
• For a plane mirror the image is virtual, erect, the same size as the object, and at the same distance behind the mirror as the object is in front. Because the mirror reverses the direction perpendicular to its surface, the coordinates of a point in front of the mirror are mirrored to the opposite side (for example, if the mirror lies in the yz-plane, a point at x becomes -x behind the mirror).
• This reversal of the front–back coordinate produces what we perceive as left–right interchange. Note: the mirror actually reverses front and back; the apparent left/right swap arises because we normally turn to face a mirror (or compare the image to ourselves while facing it).

Important properties (plane mirror):

• Image is virtual (cannot be projected on a screen).
• Image is erect (not inverted top-to-bottom).
• Image size = object size (no magnification).
• Distance of image from mirror = distance of object from mirror.
• Lateral inversion: left-right positions appear swapped in the mirror image.

Quick conceptual example: If you raise your right hand, the mirror image appears to raise its left hand. That is lateral inversion in everyday experience.

Multiple reflections — definition and cause
Multiple reflections occur when light reflects more than once before reaching the eye. This happens when two or more mirrors face each other or are inclined so that a reflected ray from one mirror falls on another. Each reflection follows the laws of reflection, so repeated reflections produce several virtual images of the same object.

Key cases

• Two parallel plane mirrors: Rays bounce back and forth producing a seemingly infinite series of images fading into the distance (a long corridor of images). The images are virtual, upright, and successively displaced along the line between the mirrors.
• Two mirrors inclined at an angle θ: A finite number of images is seen. The number depends on θ (see formulas below). Images are arranged symmetrically around the line of symmetry formed by the two mirrors.

Why images form: Each time light reflects from a mirror, the reflected rays that appear to come from behind the mirror form a virtual image. When more mirrors are present, each virtual image can act as a source for further reflections, producing additional images.

Periscope — construction and working
A periscope is an instrument that allows an observer to see over or around obstacles using multiple reflections. The simplest periscope uses two plane mirrors placed parallel to each other at 45° to the direction of the incoming light, inside a long tube.

• Construction: A rectangular tube with two mirrors fixed at 45° at the top and bottom, facing each other so that the reflective sides are parallel to the viewer’s line of sight.
• Working: Light from the object enters the top opening, strikes the upper mirror, reflects downwards to the lower mirror, and then reflects to the observer’s eye. Each reflection obeys the law of reflection (angle of incidence = angle of reflection).
• Result: The observer sees a virtual image of the object located as if the ray had traveled in a straight line through the mirrors. The image is upright and shifted depending on the periscope geometry.

Practical notes
Periscopes are used in submarines, tanks, trenches and as simple educational toys. Modern periscopes may use prisms instead of flat mirrors for better image quality and compactness.

Related concepts

• Virtual image in plane mirror: Image distance behind the mirror equals the object distance in front (image appears upright and laterally reversed).
• Intensity loss: Each reflection reduces intensity by the mirror’s reflectivity factor; for many reflections the brightness drops noticeably.

Tip for diagrams: Always draw the normal at the point of reflection and mark the incident and reflected angles. Trace successive reflected rays to locate virtual image positions by extending reflected rays backward where they appear to meet.

Overview

Mirrors are reflecting surfaces that form images by reflecting light. Different shaped mirrors—plane, concave (converging), and convex (diverging)—have different image-forming properties and therefore different uses. Understanding how each type forms images helps explain their practical applications in daily life, science and technology.

Image properties and common uses

• Plane mirrors
  • Image properties: virtual, erect, same size as object, image appears at the same distance behind the mirror as the object is in front.
  • Uses: dressing and bathroom mirrors, makeup mirrors, decorative mirrors, periscopes (in submarines or observation devices use pairs of plane mirrors), kaleidoscopes, laser beam alignment.
• Concave mirrors (spherical inward)
  • Image properties: can form real inverted images or virtual erect magnified images depending on object distance (relative to focal length f and centre of curvature R). They converge parallel rays to the focal point.
  • Uses: shaving and makeup mirrors (magnified virtual image when object is within f), dentist and ENT instruments, reflecting telescopes (collect and focus light to form images), solar cookers and solar concentrators (focus sunlight to a hot point), flashlight/torch and car headlight reflectors (to convert point-source light to a parallel beam), dental loupes, some medical instruments.
• Convex mirrors (spherical outward)
  • Image properties: always virtual, erect, and diminished (smaller than the object); they diverge rays making them appear to come from a focal point behind the mirror.
  • Uses: rear-view and side-view mirrors on vehicles (wider field of view, but objects appear smaller and farther away), security/safety mirrors in stores and on roads (to see around corners), road traffic mirrors at blind intersections, ATM and shop surveillance mirrors.

Why different mirrors are chosen

Choice depends on the required image (magnified, diminished, real, or virtual) and field of view. Concave mirrors can magnify and focus light (useful where intensity or magnification is needed). Convex mirrors provide a wide field of view and are safer where a broad area must be monitored. Plane mirrors give undistorted true-size virtual images and are convenient for everyday viewing.

Practical application examples

• Periscopes: two plane mirrors set parallel allow viewing over obstacles (submarines, tanks).
• Makeup and shaving mirrors: concave mirrors enlarge the face when held within the focal length for detailed work.
• Car headlights and torches: concave reflectors convert light from a bulb into a concentrated beam.
• Rear-view mirrors: convex mirrors give a wide view of traffic behind the vehicle.
• Solar furnaces and cookers: large concave mirrors concentrate sunlight to a small point to produce high temperatures.
• Security mirrors: convex mirrors in shops and at blind drives help staff and drivers see larger areas.
• Reflecting telescopes: large concave mirrors gather and focus faint light from distant stars to form images.

Safety and limitations

Concave mirrors can produce burning points when focusing sunlight—handle with care. Convex mirrors distort size (objects appear smaller and farther) so warnings such as "Objects in mirror are closer than they appear" are used on vehicle mirrors. Plane mirrors reverse front-to-back but do not invert top-bottom.

Summary

Mirrors are fundamental optical tools. Plane mirrors are used for life-size reflections, concave mirrors for magnification and focusing, and convex mirrors for wide-angle surveillance. Their distinct image properties determine the practical applications in daily life, safety, scientific instruments and industry.

This topic covers simple, classroom-safe activities that demonstrate basic properties of light: that it travels in straight lines, how shadows are formed (umbra and penumbra), how images form in a pinhole camera and in a plane mirror, and how materials are classified as transparent, translucent or opaque. Each activity is given with the required materials, step-by-step procedure, typical observations and the scientific explanation.

• 1. Rectilinear propagation of light

  Materials: three cardboard pieces with a small hole in each, a torch and a dark room.

  Procedure: Fix the three cardboards vertically in a straight line with holes at the same height. Turn on the torch and place it so that the light shines through the first hole.

  Observation: Light passes through all three holes and produces a bright spot on a screen placed behind the third card if the holes are aligned. If any one cardboard is slightly moved out of alignment, the spot disappears.

  Explanation: Light travels in straight lines; only when the holes are collinear do rays pass through all holes to reach the screen.

• 2. Shadow formation, umbra and penumbra

  Materials: candle or small bulb (point source), a small object (e.g., toy), a screen, and later two light sources (for penumbra demonstration).

  Procedure: Place the object between the light source and the screen and change distances of object and screen from the light source. For penumbra, use an extended source or two lamps.

  Observation: A sharp shadow (umbra) is seen with a point source. With an extended source or two sources, a lighter region (penumbra) appears around a darker central region. Shadow size changes as the distances change.

  Explanation: From a point source rays from the edges of the object form a conical shadow (umbra). An extended source acts as many point sources; some rays reach parts of the screen producing the penumbra (partial shadow).

• 3. Dependence of shadow size on distances (using similar triangles)

  Materials: torch (point source), small object of known height, screen, metre scale.

  Procedure: Keep the light fixed at distance x from the object and move the screen to different distances y from the object; measure shadow lengths.

  Observation: Shadow length increases as the screen moves further from the object.

  Explanation: Using geometry (similar triangles) we get: shadow_size / object_height = (distance from light to screen) / (distance from light to object). This explains the systematic change in shadow size.

• 4. Pinhole camera (camera obscura)

  Materials: cardboard box (shoebox), aluminium foil, pin, white screen (paper) inside the box.

  Procedure: Make a tiny hole in the foil and fix it over a window-sized opening on one side of the box. Place a white paper on the opposite inner wall. Point the pinhole side to a bright outdoor scene.

  Observation: An inverted (upside-down) real image of the outside scene appears on the paper inside the box.

  Explanation: Each point of the object emits rays; only those passing through the pinhole reach corresponding points on the screen, producing an inverted image due to crossing of rays. Image size depends on distances (object-to-pinhole and pinhole-to-screen).

• 5. Plane mirror image (location and size)

  Materials: plane mirror, pins or markers, object (small figure), board.

  Procedure: Fix the mirror on a board, place the object in front. To locate the image, use the pin method: align a pin with the image by sighting and fix another pin so the three appear collinear; repeat from another position to find the intersection behind the mirror.

  Observation: The image appears behind the mirror at the same distance as the object is in front; image is laterally inverted but of same size.

  Explanation: Plane mirrors produce virtual images that are laterally inverted and at the same distance behind the mirror as the object is in front. Rays appear to diverge from the image point.

• 6. Classifying materials by transmission

  Materials: paper, glass, tracing paper, plastic sheet, torch.

  Procedure: Shine the torch through each material and observe how much light passes through.

  Observation: Transparent (clear glass) allows almost all light; translucent (tracing paper) allows some diffuse light; opaque (cardboard) allows none.

  Explanation: Molecular and structural properties determine whether light is transmitted, scattered or absorbed.

Safety note: Never look directly at the Sun or a very bright light source. Use small lamps or shaded light sources for demonstrations.