Light, Shadow and Reflections Chapter Notes | Eureka Plus Class 6: Book Solutions, Notes & Worksheets PDF Download

Introduction

Light is what helps us see things. When light hits an object and bounces off, that's how we see it. Things we can see fall into two groups: objects that make their own light (luminous objects) and objects that reflect light (illuminated objects).

Luminous Objects

Luminous objects are those that produce their own light. These objects are sources of light. For example, the sun and a lit candle are luminous objects.

Sources of Light

Light sources can be either natural or human-made:

• Natural Luminous Objects: The sun and other stars are natural sources of light. These celestial bodies emit light on their own and are natural luminous objects.
• Human-Made Sources of Light: Objects like electric bulbs, candles, lanterns, and torches are human-made sources of light.

Some living organisms, such as fireflies, certain bacteria, fungi, and various deep-sea fish, can produce light without generating heat. This phenomenon is called bioluminescence.

Illuminated Objects

Illuminated objects do not produce light on their own. Instead, they appear bright because light falls on them and reflects back. These objects are also known as non-luminous objects.

• The Earth is an example of an illuminated object. It does not generate its own light but is lit by the sun. The part of the Earth that receives sunlight experiences day, while the part that does not receives night.

Illuminated objects reflect the light that falls on them, which is why they appear visible to us. For instance, this page is visible to you because it reflects the light that falls on it, allowing the light to reach your eyes. Without light, we would not be able to see anything.

Light: A Form of Energy

• Light is a type of energy. The energy contained in light can vary depending on the source. For instance, sunlight contains a vast amount of energy, while the light produced by a candle has significantly less energy.
• The sun constantly generates immense amounts of light and heat energy. This energy from the sun is called solar energy. Some of this solar energy reaches the Earth in the form of light and heat rays. The light from the sun illuminates half of the Earth at any given time, while the heat helps to keep the Earth warm.

Solar Energy and Life on Earth

• Plants and Photosynthesis: Plants rely on sunlight to produce their food through a process known as photosynthesis. This process is fundamental because all animals, either directly or indirectly, depend on the food made by plants for their survival.
• Sustaining Life: The warmth and light provided by the sun are crucial for sustaining life on Earth. Solar energy is, therefore, a vital factor responsible for the existence and continuation of life on our planet.

What is a Light Year?

• A light year is a unit of distance that astronomers use to measure the vast distances in space. Since light travels at an incredible speed of approximately 300,000 kilometers per second, the distance it covers in one year is enormous.
• To calculate this, scientists consider the time it takes for light emitted by a star to reach Earth. By measuring this time, they can determine how far away the star is from our planet. The distance covered by light in one year is referred to as one light year.
• To give you an idea of how far light can travel in a year, let's first understand how many seconds are in a year:
• There are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, and about 365 days in a year (not accounting for leap years).
• So, the number of seconds in one year is: 60 × 60 × 24 × 365 = 31,536,000 seconds
• Therefore, in one light year, light travels approximately 9.46 trillion kilometers (about 5.88 trillion miles). This immense distance helps scientists measure how far away stars and other celestial objects are from Earth.

Objects and Their Shadows

Objects can be classified based on how they interact with light into three categories: opaque, translucent, and transparent objects.

• Opaque Objects: Opaque objects do not allow light to pass through them at all. Because no light passes through, we cannot see through these objects. Examples of opaque objects include wood, stones, and metals.
• Translucent Objects: Translucent objects allow only a portion of light to pass through them. This means that while some light gets through, the objects behind the translucent material are not clearly visible. Examples of translucent materials include certain types of glass and plastics.
• Transparent Objects: Transparent objects allow light to pass through them completely, which enables us to see things clearly on the other side. Examples of transparent objects include clear glass, certain types of plastic, and clean water.

Formation of Shadows

• A shadow is formed when an opaque object blocks the path of light. Since the light cannot pass through the object, a dark patch appears on the other side of the object, which is the shadow.
• For a clear shadow to form, several elements are necessary:
  • A luminous source of light to provide the light.
  • An opaque object to block the light.
  • A surface or a screen to capture the shadow.
• When an opaque object is placed in the path of light, it will always create a shadow. However, the shadow will only be visible if it is projected onto a surface or screen. Bright light is needed to create a clear and distinct shadow.

Eclipses 

Eclipses occur when the shadows of celestial bodies, like the Earth and the moon, block light from the sun. The sun's light shines on half of the Earth at a time, creating day for that half, while the other half remains in darkness, experiencing night. Similarly, sunlight also illuminates the moon, which, like the Earth, is an opaque object. This causes both the Earth and the moon to cast shadows, leading to solar and lunar eclipses.

Solar Eclipse

• A solar eclipse happens when the moon positions itself directly between the sun and the Earth. When this alignment occurs, the moon's shadow falls on certain areas of the Earth. 
• Regions under the moon's full shadow experience a total solar eclipse, where the sun appears completely covered by the moon, causing daylight to momentarily turn dark. 
• In other areas, where only a partial shadow of the moon is cast, a partial solar eclipse occurs. Solar eclipses only take place on specific new-moon days and are visible only in certain regions of the Earth at any given time during the day.

Lunar Eclipse

• A lunar eclipse occurs when the Earth comes between the sun and the moon, casting a shadow on the moon. The moon is illuminated by the sun in the same way as the Earth, but because the moon orbits the Earth, we see its fully lit side only once every 28-30 days, during the full moon phase. 
• On some full-moon nights, the Earth's shadow can completely cover the bright side of the moon, leading to a total lunar eclipse, during which the moon appears red. If only part of the moon is covered by the Earth's shadow, a partial lunar eclipse occurs.

Light Travels in a Straight Line

• Light travels in a straight path, which is known as the rectilinear propagation of light. This means that light moves in straight lines called rays. 
• A ray of light is depicted as a line with an arrow indicating the direction in which the light is moving. A collection of multiple rays forms a beam of light, and these rays within a beam are typically parallel to one another.

Why Does a Mirror Form a Clear Image?

• A plane mirror, which is an opaque object with a smooth, polished surface, reflects light rather than allowing it to pass through. When a beam of light strikes the surface of a plane mirror, the light rays are reflected back. After reflection, these rays continue to travel parallel to each other, which results in the formation of a clear image on the mirror's surface.
• On the other hand, a rough surface does not produce a clear image. When light hits a rough surface, such as a rough floor, the light is still reflected, but the rays do not remain parallel after reflection. Instead, they scatter in different directions, leading to irregular reflection. Because the reflection is not uniform, no clear image is formed on a rough surface.

Differences between an image and a shadow 

Understanding the dissimilarities between an image and a shadow can help us grasp how light interacts with objects in our surroundings. Both are formed due to light but showcase distinct characteristics.

Image:

• An image is like a twin of an object, showing the same colors and details.
• It appears when light bounces off a smooth surface, creating a replica of the object.
• For instance, when you look in a mirror, the reflection you see is an image of yourself.

Shadow:

• Shadows, on the other hand, are usually dark and lack color, irrespective of the object casting them.
• They form when an opaque object blocks light, resulting in a silhouette of the obstructing object.
• Imagine standing in the sun; the shadow you cast on the ground is a representation of your shape without any color.

 A Pinhole Camera

A pinhole camera is a simple device used to create images with the help of a small hole.

Image Formation in a Pinhole Camera

• Even though the hole in a pinhole camera is tiny, light rays from every part of an object pass through it.
• These light rays travel in a straight line and create an upside-down (inverted) image.
• The image produced is an exact copy of the object, displaying its color, shape, and other details.

Explanation with Examples

• Small Hole, Big Picture: Imagine the pinhole as the eye of the camera. It captures light from all points on an object, similar to how your eye takes in all the details of a scene.
• Straight Line Travel: Picture light rays as straight arrows moving through the pinhole without bending. This movement helps in creating a clear and focused image on the opposite side.
• Inverted Image: Think of the upside-down image like a funhouse mirror reflection. The image may be flipped, but it still retains all the essential features of the original object.
• Exact Replica: Consider the image in the pinhole camera as a precise duplicate of the object, much like a photocopy that captures every detail accurately.