Light, Shadows and Reflections

IMPORTANT TERMS

 

Reflection:- Bouncing back of a beam of light after hitting any surface is known as reflection of light

 

Umbra:- The dark part of the shadow is known as umbra as it does not receive any light from the source.

 

Penumbra:- The partially dark region of a shadow is called penumbra.

 

Lateral Inversion:- The interchange of left and right sides of image is called lateral inversion. The right side of object appears as its left side and vice versa.

 

Eclipse:- A shadow cast by heavenly bodies is known as an eclipse.

 

 

LIGHT

Light is a form of energy which enables us to see things around us. Light travels along a straight line. This property of light is called rectilinear propagation of light.

 

Luminous Objects

The objects which emit light of their own are called luminous objects. E.g. Sun, stars, burning candle, electric bulb etc.

 

Non-luminous Objects

The objects which do not emit light of their own are called non-luminous objects. E.g. Moon, planets, tree etc.

 

SOURCES OF LIGHT

An object that gives out light is called a source of light.

 

Natural sources of light

Sun, stars and firefly are the natural sources of light.

 

Artificial Sources of light

An electric bulb, candle, torch, fluorescent lamp are some of the artificial sources of light.

 

TRANSPARENT, TRANSLUCENT AND OPAQUE OBJECTS

 

Transparent

Materials that allow light to pass through them completely are called transparent materials. We can see through these substances clearly. E.g. Glass, air, pure water, cellophane paper etc.

 

Translucent

Materials that allow light to pass through them only partially are called translucent materials. We cannot see through them clearly. E.g. Mica, tracing paper, butter paper, frosted glass and paper smeared with oil etc.

 

Opaque

Materials that do not allow light to pass through them are called opaque materials. We cannot see through them at all. E.g. Sand, wood, cement etc.

 

Shadows

A shadow is a dark outline that is formed when an opaque object blocks the light falling on it from a source of light. It creates an area of darkness on the side of the object away from the source of light. The following things are required for a shadow to form:

• A source of light is required for a shadow to form. The source can be natural such as Sun or artificial such an electric bulb or torch.
• An opaque object is required for a shadow to form.
• A screen, such as a wall is required for a shadow to form.

 

Characteristics of a shadow

 

A shadow has the following characteristics:

• A shadow is formed when the path of light is blocked by an opaque object.
• A shadow is always black regardless of the colour of the object.
• A shadow only shows the shape of the object and no other details about the object.
• Shadows are formed both during day and night.
• The size of a shadow depends on the distance between the object and the source of light. The closer the object is to the source of light, the larger the shadow it casts. When the distance between the object and the source is increased, the shadow becomes smaller.

 

ECLIPSES

 

A shadow cast by heavenly bodies is known as an eclipse. There are two kinds of eclipses depending on the position of the Sun, Moon and the Earth with relation to each other.

 

Solar Eclipse

A solar eclipse occurs at New Moon, when the Moon comes between the Earth and the Sun. The moon blocks the light of the Sun. The shadow of the Moon falls upon the surface of the Earth and the total solar eclipse occurs.

 

Lunar Eclipse

A lunar eclipse occurs on a Full Moon day when the shadow of the Earth falls on the Moon. This happens when the Earth comes between the Sun and the Moon and all three are in a straight line. The light reaching the moon from the Sun is obstructed by the Earth. So the moon cannot be seen from the Earth and thus lunar eclipse occurs.

 

FORMATION OF IMAGES

 

Image formed by a pinhole camera

A pinhole camera is an instrument with a very tiny hole on one of its sides. It forms an image of an object on its screen. The working of a pinhole camera is based on the rectilinear propagation of light which means that light travels in a straight line. Image obtained on a pinhole camera is always inverted i.e upside down, because there is no reflecting optical element to change the path of the rays of light.

 

Image formed by a plane mirror

A plane mirror has a very smooth and polished surface which reflects almost all of the light that falls on it. When rays of light strike the smooth surface of a mirror, they bounce back at the same angle. The reflected rays of light travel from the mirror to our eyes and thus we see our reflection in the mirror.

 

• An image formed by a plane mirror is of the same size as the object.
• An image formed by a plane mirror is of the same colour as the object.
• An image formed by a plane mirror is virtual and erect.
• The image is of the same shape and gives details of the object.
• The image formed by a plane mirror is laterally inverted which means that the right hand side of the object appears as the left side, and the left side appears to be right hand side.

INTERESTING FACTS

• Shadow puppetry is the art of using the shadows of puppets to entertain the audience.
• Shadows are longer during winter because of the angle of the Sun.
• A long time ago, people observed the way shadows were formed by the sun and utilized this principle in making the clocks called sundials.
• The smaller the angle between an elongated object and the direction of the light is, the shorter the shadow is. On the other hand, the smaller the angle between the direction of the light and the surface on which the shadow occurs is, the longer the shadow is.
• Some animal can see light that human beings can't. Bees can see ultraviolet light.
• The colour of an object is determined by the colour of the light it scatters – an object appears green because it scatters green and absorbs the rest of the colours.
• A black object appears black because it does not scatter any light.