Study Notes: Light, Shadows & Reflections - Science Class 6

Light is the energy that enables us to see. Light is emitted from a source such as the Sun.

Luminous objects: Objects that emit light on their own are called luminous objects. The light emitted by luminous objects enables us to see things around us. Examples of luminous objects are a tube light, the sun, a lit candle, a glowing bulb, a bonfire and a lit torch.

Non-luminous objects: Objects that do not emit light on their own are called non -luminous objects. The light emitted by luminous objects falls on non-luminous objects, and then bounces back to our eye, which enables us to see non-luminous objects. Examples of non-luminous objects are the moon, a book, a pen, a wooden box, a cupboard and a chair.

Transparent, Opaque and Translucent Objects

The material through which the light passes through called a medium.

There are three types of media. They are transparent, translucent and opaque.

Opaque objects: Objects through which we cannot see are called opaque objects. A medium that does not allow light to pass through it is called an opaque medium. Examples of opaque medium are a pencil box, a wooden screen, a book, a towel, a ceramic plate and chart paper.

Transparent objects: If we are able to see anything clearly through an object, then such an object is said to be a transparent object. A medium that allows all the light incident on it to pass through it is called a transparent medium. Examples of transparent objects are plain glass, a reading glass, a plastic scale, windowpanes, a soap bubble and pure water.

Translucent media: Objects that allow only a part of the light incident on them to pass through it are called translucent media. Examples of translucent media are a shower stall, smoked glass, sunglasses and butter paper. Light travels in a straight line, and its rays represent the path of light.

What Exactly are Shadows?

When an opaque object blocks the path of light, a dark region is formed behind it which is called a shadow. A shadow is a dark region, and is formed only when a light source, an opaque object and a screen are present. 

For example, during a lunar eclipse, we see a part of the earth's shadow on the surface of the moon. This happens when the earth, the sun and the moon are in a straight line, with the earth between the sun and the moon. 

Here, the sun acts as the light source, the earth as the opaque object, and the moon as the screen. 

Opaque objects form shadows because light is not able to bend around them. Shadows are seen more clearly on light screens. 

Whatever the colour of the object, its shadow is always black because it is not illuminated by light. 

The length and shape of a shadow depends on the object by which it is formed. 

In the olden days, shadows caused by objects placed in the sun were used to measure time. Such a device is called a sundial. The Jantar Mantar in Jaipur consists of a sundial or Samrat Yantra, which can be used to tell the time, as its shadow moves visibly at one millimetre per second, or roughly six centimetres every minute.

A Pinhole Camera

The image formed by a pinhole camera is inverted and smaller in size when compared to the original object. These cameras work on the principle that light travels in a straight line. Pinhole cameras are cheap and simple to make. An eclipse can be viewed using a pinhole camera.

Making a Pinhole Camera

• Take two rectangular boxes that fit into one another without leaving any gap.
• Cut open one side of each box.
• Make a small hole in the larger box at the centre of the closed end opposite the side that has been cut open.
• Cut a square of side five centimetres in the smaller box in the closed end opposite the side that has been cut open. Cover this square with tracing paper.
• Finally, slide the smaller box into the larger box, ensuring that the pinhole and the tracing paper are in line with one another, but at the opposite ends.
• Slide the smaller box to adjust the focus so that you can capture the image of any object you want.

This is the basic structure of the pinhole camera. However, the developed form of a pinhole camera uses photosensitive film instead of translucent paper to capture images. The film can be developed later to obtain photo prints. Nowadays, of course, we use digital cameras that store images in a computer chip. However, the basic principle of capturing the image remains the same.
An area that is not or is only partially irradiated or illuminated because of the interception of radiation by an opaque object between the area and the source of radiation is called a shadow. Shadows take the form of the outline of the object that intercepts the light in its path. Thus, the dark outline of the opaque object that you see is a shadow.
To form a shadow, we need a source of light, an opaque object in the path of light, and a screen on which a shadow is formed.

The shape of the shadow of an object is dependent neither on the colour of the object nor on the colour of the screen.

If the distance of the object from the source is decreased, then the size of the shadow increases, provided the distance between the source and the screen is constant.

Maintaining the distance of the screen from the source, if the object is moved away from the source, then the size of its shadow decreases.

If the distance between the object and the screen is constant, and if the light source is moved towards the object, then the size of its shadow increases. On the other hand, if the distance between the object and the screen is constant, and if the light source is moved away from the object, then the size of its shadow decreases.

Another factor affecting the size of the shadow is the distance between the object and the screen. If the distance between the object and the source of light is maintained constant, and if the screen is moved away from the object, then the size of the object's shadow decreases. On the other hand, if the screen moves towards the object, then the size of its shadow increases.

The size and shape of the shadow of an object depends on the orientation of the object with respect to the beam of light. The shadow changes in size and shape when the object is placed in a different way in the path of light.

Mirrors and Reflections

Image

Image

The likeness of an object carried and formed by light in a mirror is called an image. For example, your image is a reflection of your face in the mirror. An image can be seen in the mirror because the light reflected from an object falls on the mirror and it is reflected. So, light incident on any smooth shiny surface like a mirror bounces back into the same medium. This bouncing of light by any smooth surface is called reflection of light. Mirrors change the direction of light incident on them. The image in a plane mirror is the same size and colour as that of the object. Moreover, the distance between the image and the mirror, is the same as the distance between the mirror and the object.

Shadow

When an opaque object blocks the path of light, a shadow is formed. A shadow is a black dark outline. Its length changes with a change in the position of the source of light. Shadows can be seen only on a surface or a screen.

Two-Way Mirror

A glass that acts as a mirror on one side and plain glass on the other is called a two-way mirror. A two-way mirror is used when you want to observe people without letting them know that they are being watched, for example, by police or in psychological institutions.

A Pinhole Camera

Images formed by a pinhole camera are entirely different from the images that are formed by a mirror. The image formed by a pinhole camera is inverted and smaller in size when compared to the object. The image is also not clear. Moreover, the distance between the object and the pinhole is not the same as the distance between the pinhole and the image. A periscope is an instrument used for observing objects from a hidden position. This instrument is mostly used in submarines, and works on the concept of reflection in plane mirrors. Using mirrors, it enables people in a submarine to view objects on the surface of the water.

Reflection is the phenomenon in which light incident on an object bounces back into the same medium through which it was propagating while incident on the object, following some laws. The path along which light propagates is called a light ray. When a light ray is incident on an object, it is called the incident ray. The ray that bounces back by the object into the same medium called the reflected ray.

A smooth plane surface that reflects the entire light incident on it is called a plane mirror. Light incident on an object is reflected by that object. That reflected light incidents on the mirror, and is reflected again. It is because of this reflection that we see images. An image is the likeness of an object carried and formed by light in a mirror. In other words, your image is a reflection of your face in the mirror. In ancient times, people mostly used pools of still water, or water collected in a vessel as mirrors. The first man-made mirrors were in the form of a polished stone made by using obsidian, a naturally occurring volcanic glass.
The similarity between a shadow and an image is that neither can exist without light. However, there are some differences between shadows and images. Shadows are black, whereas images are colourful. Shadows of objects placed in the sun change in length with the position of the sun through the day. Your shadow will be at its longest at dawn or early morning, and at dusk or early evening, whereas it will be at its shortest at noon. An image in a plane mirror doesn't change in size at all; it is always the same size as that of the object. An image in a mirror can be seen without a screen, whereas it is mandatory to have a screen to form a shadow.