Light, Shadow and Reflection for Class 6

Light helps us see things around us. When light hits an object, it can be blocked, pass through, or bounce off. Shadows are created when an object blocks the light, making a dark area behind it. The shape of the shadow depends on where the light is. Reflection happens when light bounces off surfaces like mirrors, letting us see images. Together, light, shadows, and reflection help us understand how we see the world.

What is Light?

As already said, light is a type of energy that helps us see. It moves in waves and is critical to sight. Without light, we would be unable to see anything. Some facts about the speed of light in a vacuum, which is approximately 299,792 km/s. Light travels in air, water, glass, and so on.

Sources of Light

Light emanates from sources that emit light waves. Most of this is deprecating and divide sources into two types:

1. Natural Sources – These are sources of light that exist naturally without human intervention.
   Examples:

• Sun (The primary source of light on Earth)

• Stars

• Fireflies

2. Artificial Sources – These are the man-made sources that provide light.
   Examples:

• Light bulbs

• Streetlights

• Torches

 

Transparent, Translucent, and Opaque Objects

Light interacts with objects differently, and this depends upon the material and architecture of the object. If an object passes light through it, we can classify it into three categories:

1. Transparent Objects

Definition: Transparent objects transmit all or most of the light through the object without scattering. These are transparent objects — that is, we can see through them.

How They Work: Instead, when light rays go in one side of some transparent object and out the other side, those rays (within limits) continue in the same direction without scattering. This is due to having a uniform structure that absorbs or reflects very little light.

Examples:

• Glass: It’s in windows, eyeglass lenses, and drinking glasses and lets us see through it easily.

• Clear Plastic: Some packaging materials and plastic water bottles pass light.

• Clean Water: Clean lakes, rivers, or a glass of water are transparent; they let light pass.

 

2. Translucent Objects

Definition: Translucent objects transmit some light but in a diffuse manner. As a result, we can't see through them as clearly as transparent objects.

How They Work: When light travels through a semitransparent medium, part of it is absorbed, and part is scattered in different directions. We cannot see clear images through them because this effect blurs our vision.

Examples:

• Frosted Glass: Often found in bathroom windows and decoration panels, it allows light to penetrate while keeping things private.

• Wax Paper: Light passes throug this cooking and food wrapping item, but objects behind it are blurry.

• Thin Fabric: It lets the light through - some use thin fabric like curtains and veils but will not allow a clear natural view.

 

3. Opaque Objects

Definition: Opaque objects absorb all light that strikes them. Such objects are opaque and will block light and cast shadows.

How They Work: If light hits an opaque thing, it is absorbed or reflected. Furthermore, light cannot pass through the body, which means you can't look through it.

Examples:

• Wood: That means there are a large variety of objects & places around us that are opaque materials like Wood: Doors, furniture, and trees are opaque.

• Metal: This is when cars, iron gates, and utensils reflect some light but do not permit it to pass through.

• Stone: Walls and rocks are not transparent.

 

Shadow Formation

When an opaque object obstructs the path of light, a shadow is created behind the object. The position, size , and intensity of the light source affect the shadow, as does the size and shape of the object.

Rectilinear Propagation of Light

In a vacuum, light travels in a straight line (rectilinear propagation). When the light falls on an opaque body, (obstacle) can not go through that body which causes the light to block from passing through it and there is a formation of a shadow.

Characteristics of a Shadow

• Shape: The shape of the shadow depends on the object blocking the light source.

• Size: The distance of the object from the light source determines the scale of the shadow. The nearer the object is to the light source, the larger the shadow.

• Clarity: Shadows can be sharp or blurry. The brightness depends upon how much distance the light source is from the object.

 

Shadows Formed Due to Sunlight

In the daytime, the Sun is the main source of illumination, and when it shines on objects, it produces shadows. These shadows, which grow larger and smaller when the earth rotates, arrive at different points.

• Morning: The Sun is low in the sky, and shadows are long.

• Afternoon: Shadows become shorter as the Sun rises higher in the sky.

• Midday: Shadows are at their shortest as the Sun is directly overhead.

 

Umbra and Penumbra

When a solid object blocks a source of light, the area behind that object cannot be illuminated, thus resulting in the creation of a shadow. But shadows aren't always pure black and tend to vary in areas according to how much light gets blocked. There are two main components of a shadow:

 

Umbra – The Darkest Part of the Shadow

Definition: The umbra is the darkest core of a shadow from which no light from the source can reach. This is because the light is prevented from passing through this area due to the opaque object.

How It Forms:

• Light travels in straight rays and cannot bend around an opaque object like a wall.

• The region immediately behind the object into which no light can penetrate is the area known as the umbra.

• The object is between the light source (sun) and the ground.

Example:

• If you have ever been standing under a streetlight at night and looked around, you have noticed that there is a dark shadow (or the absence of light) directly underneath you; that is the umbra.

• A total solar eclipse occurs when the Moon fully covers the Sun, causing total darkness for observers within the umbra.

 

Penumbra – The Lighter Part of the Shadow

Definition: The outer, lighter part of a shadow where some light from the source still reaches is called the penumbra. A sharp shadow, however, is not as straightforward. A light source is not a single point but a surface area, so some light can bend around the object's edges.

How It Forms:

• Light sources such as the Sun or a lamp radiate light from all directions.

• Some rays reach around partially until the area is shaded, creating a lighter, blurry shadow around the umbra.

• The penumbra is always less intense and more diffuse than the umbra.

Example:

• Shining a light on your hand so that it’s close to a wall will create an inner dark shadow (umbra) and outer lighter shadow (penumbra).

• In the case of a partial solar eclipse, the moon obscures only part of the Sun, so people in the penumbra have partial darkness.

 

Shadows in Nature

In nature, creating shadows is not limited to small solids; even the Sun, Moon, and Earth can create huge shadows in space. This creates special events known as eclipses.

 

Solar Eclipse

During a solar eclipse, the Moon passes between the Sun and Earth and blocks sunlight from reaching Earth. This casts a shadow on Earth, rendering the Sun out of sight, either partially or fully.

Types of Solar Eclipses:

1. Total Solar Eclipse – The Moon fully blocks out the Sun, darkening the sky for a moment in time.

2. Partial Solar Eclipse – Part of the Sun is covered, and part of the sunlight is visible.

3. Annular Solar Eclipse – The Moon blocks the central part of the Sun, leaving a shiny ring on the edges. We refer to this as the “Ring of Fire.”

 

Fun Fact: Solar eclipses are short-lived because the Moon moves fast in orbit.

 

Lunar Eclipse

A lunar eclipse occurs when the Earth passes between the Sun and the Moon, blocking sunlight from illuminating the Moon. The non-illuminated half of the Moon is within Earth’s shadow and appears dark or reddish.

Types of Lunar Eclipses:

1. Total Lunar Eclipse – All of the Moon is covered by the shadow of the Earth, rendering it reddish (often referred to as a "Blood Moon").

2. Partial Lunar Eclipse – A part of the Moon is covered and thus part remains visible.

3. Penumbral Lunar Eclipse – The Moon passes through the lighter outer shadow (penumbra) of Earth, causing a slight dimming.

Why Do Lunar Eclipses Happen More Often?
The Earth is much larger than the Moon, so when the two align, the former’s shade frequently blocks the latter.

 

Reflection

We know that, when light strikes an object, some of it bounces back. The return trip of light is reflection. Light is reflected off all surfaces, be they walls, roads, mirrors, or water.

Have you ever imagined what the world would have looked like if it were a mirror with everything twice as reflective, and why do we see the images in mirrors and still water and not in walls or roads? The reason is that in order for an image to be formed, the reflection has to be smooth and organized.

What is an Image?

An image is a replica of an object that appears identical in color, size, and shape. If we look in a mirror, we see our reflection, which is an image of us.

There are two main types of reflection:

1. Regular (Specular) Reflection - Occurs on smooth and lustrous surfaces such as mirrors and calm water. Here, rays of light bounce off in a unified manner, creating a clear image.

2. Irregular (Diffused) Reflection - It occurs on rough surfaces like walls or roads. The rays of light scatter in different paths, so an image is not perceived. So, instead, the object just appears bright.

 

Real and Virtual Images

Real Image

It is an image that can be formed on the screen. For example:

• The image created by a projector onto a screen is a real image.

• A pinhole camera creates a real image as well. This tiny hole lets light in, projecting an inverted image onto a screen inside the camera.

 

Virtual Image

A virtual image cannot be received on a screen. It seems to exist inside this mirror or shiny surface but can be seen by our own eyes. Examples include:

• The image of your face seen in a mirror.

• Reflected image on shiny metal surfaces or still water.

 

Reflection in a Plane Mirror

We have a plane mirror which is a flat mirror as we use at our home. One side of a glass sheet is coated with a material, such as silver or mercury, that reflects light.

Properties of image formed by a plane mirror:

• It’s just like that, just as it is.

• It is exactly the same size as the object.

• It is laterally inverted or flipped sideways. For example, if you write "MIRROR" on paper, then looked in the mirror, you would see it reversed.

• This image seems to be behind the mirror, at the same distance as the object is in front.

Try moving in front of a mirror – your image mimics all your movements.

 

Uses of Plane Mirrors

Plane mirrors are used in many fields of our daily life as:

• Dressing mirrors – To check your look.

• Decorators – Used to make the rooms feel larger.

• Solar cookers – Assist in using sunlight to cook meals.

• Mirror mazes – Found in amusement parks for fun and illusions.

Plane mirrors are used in a periscope as one of the significant applications.

 

What is a Periscope?

A periscope is an instrument that allows a person to see something that would not otherwise be visible. It has two mirrors, which are positioned at an angle so that it reflects light.

Where are periscopes used?

• Submarines – Sailors peeking through periscopes to peer above the water while remaining concealed.

• Armored tanks – Soldiers use periscopes to peek outside without revealing their presence.

• Trenches in wars – Soldiers have stayed safe from enemies by using periscopes in wars in the past.

 

Characteristics of an Image Formed by a Plane Mirror

• The image is virtual.

• The image is upright.

• The image is of the same size as the object.

• The image is laterally inverted (left and right are swapped).

• The image is formed at the same distance behind the mirror as the object is in front of it.

 

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