Light is fundamental to how we perceive the world around us. From the natural illumination of the sun and stars to artificial sources, light allows us to see objects and appreciate phenomena like the dance of fireflies or the glow of the moon. This chapter explores some key properties of light, how shadows are formed, and how light interacts with surfaces, leading to reflections and the formation of images.

Reflecting on observations, like moonlight or beams from headlights, can lead to questions about where light originates and how it travels. Does the moon produce its own light, or is its glow borrowed? Do all objects shine by themselves?

11.1 Sources Of Light

Objects that produce or give out their own light are called luminous objects.

Examples of natural luminous objects include:

• The Sun (the primary source of light for Earth).
• Other Stars.
• Lightning.
• Natural fire.
• Certain animals (like fireflies) that produce light through a biological process (bioluminescence).

Humans have also developed artificial luminous objects. Early artificial lighting was fire, created using various fuels. With the invention of electricity, a wide range of electric light sources became available.

Objects that do not produce their own light are called non-luminous objects. We see non-luminous objects because light from a luminous source bounces off them and enters our eyes.

The Moon is a classic example of a non-luminous object. It appears bright because it reflects sunlight that falls on its surface.

Science and Society: Modern light sources like Light Emitting Diodes (LEDs) are increasingly popular due to their energy efficiency, brightness, and longer lifespan compared to older lamps. This transition is promoted for energy saving and environmental benefits, although proper recycling of LEDs is important.

11.2 Does Light Travel In A Straight Line?

Everyday observations, like sharp shadows, suggest that light might travel in a straight path. Let's test this idea.

Activity 11.1: Let Us Investigate

Take three matchboxes and make holes at the same position in each. Arrange them in a straight line so the holes align. Shine a torch through the aligned holes onto a screen.

A spot of light will appear on the screen only when all three holes are perfectly aligned in a straight line. If even one matchbox is moved slightly, breaking the straight line of holes, the light spot on the screen disappears. This strongly suggests that light travels in a straight path.

Activity 11.2: Let Us Explore

Try looking at a lighted candle flame through a straight hollow pipe and then through the same pipe bent in the middle.

You can see the flame clearly through the straight pipe, but you cannot see it through the bent pipe. This observation further supports the idea that light travels in a straight line.

This property is sometimes referred to as the rectilinear propagation of light.

Interesting Note: While generally true in common circumstances, light can sometimes bend slightly around obstacles or corners, a phenomenon called diffraction, which you learn about in higher grades. Also, a laser beam in a cloudy medium (like water with a drop of milk) shows a visible straight path.

11.3 Light Through Transparent, Translucent, And Opaque Materials

Materials differ in how they interact with light when placed in its path.

Activity 11.3: Let Us Experiment

Shine a torch onto a screen (like a wall or cardboard). Then, place objects made of different materials between the torch and the screen. Observe how much light passes through and reaches the screen.

Based on how much light passes through them, materials are classified as:

• Transparent materials: Light passes through almost completely. You can see clearly through them (e.g., clear glass, air, pure water).
• Translucent materials: Light passes through partially. You can see light through them, but objects on the other side appear blurry or indistinct (e.g., tracing paper, frosted glass, some plastics).
• Opaque materials: Light does not pass through them at all. You cannot see through them (e.g., wood, metal, cardboard, thick paper).

Material	Classification (Transparent/Translucent/Opaque)	Prediction (Light passes fully/partially/not at all)	Observation (Light passes fully/partially/not at all)
Glass	Transparent	Fully	Fully
Tracing paper	Translucent	Partially	Partially
Cardboard	Opaque	Not at all	Not at all
Paper	Opaque/Translucent (depends on thickness)	Not at all / Partially	Not at all / Partially
Thick cloth	Opaque	Not at all	Not at all
Clear plastic sheet	Transparent	Fully	Fully
Frosted glass	Translucent	Partially	Partially

What happens on the screen when an opaque object is placed in the path of light?

11.4 Shadow Formation

When an opaque object is placed in the path of light, it blocks the light from reaching the area behind it. Since light travels in straight lines, the area where the light is blocked remains dark. This dark area formed behind an opaque object when light shines on it is called a shadow.

We see shadows of ourselves and objects daily, created by sunlight or artificial lights. Playing with hand shadows to create different shapes is a fun demonstration of how shadows mimic the shape of the object blocking the light.

Do translucent and transparent objects also form shadows? Opaque objects create the darkest and most defined shadows. Translucent objects allow some light to pass through, resulting in fainter or lighter shadows. Even some highly transparent objects can create very faint shadows under specific conditions.

Activity 11.4: Let Us Explore

Experiment with opaque objects and a light source shining onto a screen to observe how shadows change based on different factors:

Action Performed	Observations Regarding Shadow
The screen is removed.	No shadow is visible (shadow is formed on a surface).
The object is removed.	The shadow disappears (light reaches the screen).
The torch is switched off.	The shadow disappears (no light source).
The colour of the object is changed (opaque objects).	The colour of the shadow remains dark (shadow is the absence of light, not a coloured projection).
The object is moved closer to the screen, keeping the torch and screen fixed.	The shadow becomes larger and less sharp.
The object is moved closer to the torch, keeping the torch and screen fixed.	The shadow becomes larger and less sharp.
The object is tilted, keeping the torch and screen fixed.	The shape of the shadow changes, becoming distorted compared to the object's actual shape.

Key takeaways about shadow formation:

• A light source, an opaque object, and a screen are necessary to observe a shadow.
• Shadows are formed because light travels in straight lines and is blocked by opaque objects.
• The shape, size, and sharpness of a shadow depend on the size of the light source and the relative positions of the light source, the object, and the screen.
• The colour of an opaque object does not affect the colour of its shadow, which is simply a lack of light.

Fascinating Fact: Shadow puppetry is an ancient art form that uses light, opaque puppets, and a screen to create storytelling and entertainment through the manipulation of shadows.

11.5 Refl Ection Of Light

What happens when light hits a shiny or polished surface?

Activity 11.5: Let Us Investigate

Shine sunlight onto a shiny surface, like a polished metal plate or a plane mirror. By tilting the surface, you can direct a bright spot of light onto another surface that is not directly in the sunlight.

This demonstrates that the shiny surface or mirror has changed the direction of the light that fell upon it. This phenomenon, where light bounces off a surface and changes direction, is called reflection of light. Mirrors are excellent reflectors.

Activity 11.6: Let Us Experiment

Use a comb with a thin slit (created by blocking other openings with black paper) and a torch to create a thin beam of light on a sheet of white paper. Place a plane mirror in the path of this light beam.

Observe that the light beam changes its direction after hitting the mirror. This confirms that light is reflected by the mirror. The reflected light also travels in a straight line.

When you look in a mirror and see your face, it is also due to the reflection of light from your face off the mirror surface and into your eyes.

11.6 Images Formed In A Plane Mirror

Mirrors form representations of objects placed in front of them, which we call images. Let's explore the characteristics of images formed by a plane mirror (a flat mirror).

Activity 11.7: Let Us Experiment

Place an object, like a pen, in front of a plane mirror. Observe the image you see in the mirror.

Comparing the object and its image formed by a plane mirror:

• Size: The image formed by a plane mirror is the same size as the object.
• Orientation (Upright/Erect): The image is upright, meaning it is not upside down. The top of the object appears as the top of the image.
• Can it be caught on a screen? The image formed by a plane mirror cannot be obtained on a screen placed behind or in front of the mirror. Such images are called virtual images.

Activity 11.8: Let Us Experiment

Stand in front of a plane mirror and observe your image. Notice its position and how it appears reversed left-to-right.

Further characteristics of images formed by a plane mirror:

• Distance: The image formed by a plane mirror appears to be located as far behind the mirror as the object is in front of it. As you move closer to the mirror, your image also appears to move closer.
• Lateral Inversion: In the image formed by a plane mirror, the left side of the object appears as the right side of the image, and vice versa. This left-right reversal is called lateral inversion. This is why words written normally appear reversed in a mirror. The word "AMBULANCE" is often written laterally inverted on the front of emergency vehicles so that drivers ahead see it correctly in their rearview mirrors.

Fascinating Fact: While most modern mirrors are made of glass with a reflective coating, ancient mirrors were crafted from polished stone or metal. The tradition of making metal mirrors still exists in some places, like the unique Aranmula Kannadi in Kerala.

11.7 Pinhole Camera

Can we form an image without a mirror? A pinhole camera demonstrates how light travelling in straight lines can create an image by passing through a small opening.

Activity 11.9: Let Us Explore

In a dimly lit room, make a small hole in a piece of cardboard. Place a lighted candle in front of the cardboard and hold a screen behind it.

Light rays from the candle flame travel in straight lines and pass through the small pinhole. Rays from the top of the flame travel downwards to the bottom of the screen, and rays from the bottom of the flame travel upwards to the top of the screen. This crossing of light rays at the pinhole results in an inverted image of the candle flame appearing on the screen.

Activity 11.10: Let Us Construct

A more functional pinhole camera can be made using two sliding cardboard boxes. A pinhole is made in one end of the larger box, and a translucent screen (like tracing paper) is fixed on the opposite end of the smaller box. The smaller box slides inside the larger one, with the screen inside.

When pointed at a bright object (like a tree or building in sunlight), light from the object passes through the pinhole and forms an image on the tracing paper screen. By sliding the inner box, you can focus the image. The images formed by a pinhole camera are:

• Inverted (upside down).
• Show the colours of the object.
• Can be obtained on a screen (real image).

Comparison: A pinhole camera forms an inverted image. A plane mirror forms an erect image, but it is laterally inverted. You will explore the reasons for these differences in higher grades.

11.8 Making Some Useful Items

The properties of light traveling in straight lines and undergoing reflection can be used to create useful devices.

11.8.1 Periscope

A periscope is a device that uses mirrors (or prisms) to see objects that are not in the direct line of sight. A simple periscope can be made using a Z-shaped tube and two plane mirrors placed at an angle.

Light from the object enters the top mirror, reflects downwards to the bottom mirror, and then reflects horizontally into the observer's eye. This allows seeing over obstacles (like in submarines, tanks, or bunkers).

11.8.2 Kaleidoscope

A kaleidoscope uses multiple mirrors placed at angles (typically three rectangular mirrors forming a triangle) to create beautiful, symmetrical patterns from small coloured objects.

Small coloured pieces (like beads or bangle pieces) are placed at one end. Light reflects off the mirrors multiple times, creating numerous symmetrical images of the pieces. As the kaleidoscope is rotated, the coloured pieces shift, and the reflections rearrange, creating continuously changing, unique patterns. This device leverages the principle of multiple reflections and symmetry. Designers and artists often use kaleidoscopes for inspiration.

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In a Nutshell:

• Luminous objects emit their own light (Sun, stars, fireflies), while non-luminous objects are seen by reflected light (Moon, planets, most everyday objects).
• Light generally travels in straight lines (rectilinear propagation).
• Materials are classified by how light passes through: Transparent (fully), Translucent (partially), and Opaque (not at all).
• A shadow is a dark area formed behind an opaque object blocking light on a screen. Formation requires a light source, opaque object, and screen. Shadow characteristics depend on the positions and size of the light source and object.
• Reflection is the phenomenon where light bounces off a surface, changing direction (e.g., from a mirror).
• Images formed by a plane mirror are the same size as the object, erect (upright), virtual (cannot be obtained on a screen), and laterally inverted (left-right reversed).
• A pinhole camera forms an inverted, real image on a screen by light passing through a small hole.
• Devices like periscopes and kaleidoscopes utilize the principles of reflection to create useful or visually interesting effects.

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