Light, Shadows and Reflection
Light
• Light is defined as radiation which is visible to the human eye.
• It helps us see different colourful objects aroundus.
• The natural source of light is theSun.
• Sunlight allows us to see objects during the day. It also plays an important role in the growth of
plants and in processes such asphotosynthesis.
• Luminous objects are objects which emit light of theirown. Example:Sun
• Non-luminous objects are objects which reflect light. Examples: Moon,mirrors
Importance of Light
• Light is the main source of energy for animals and plants.
• The sunlight provides natural warmth that creates suitable conditions for the growth and development of life.
• The green plants required sunlight to make their food by the process of photosynthesis.
• The natural colour of light is white, but this white light has seven different colours: violet, indigo,
blue, green, yellow, orange and red.
• The objects absorb specific colours of light and reflect the other colours. This reflected colour declare the
colour of the object. For example, grass cannot absorb the green colour. So, the green colour is reflected that
comes to our eyes.
Sources of Light
The objects that emit light are called sources of light. There are two sources of light: natural and man-made.
For example, the sun, stars and fireflies are natural sources(Figure 1 a), while an electric bulb, a burning lamp
and tube light are man-made sources of light(Figure 1 b).
Figure 1: Sources of light: a) Firefly(natural source; b) Candle(man-made source)
The objects that become hot when they emit light are called hot sources of light. For example, the sun, a
burning candle or lamp and an electric bulb, etc(Figure 2).
Figure 2: Hot source of light: the sun.
The objects that emit light without becoming hot are called cold sources of light.
For example, fireflies(Figure 3).
Figure 3: Cold source of light: firefly.
Types of Non-Luminous Objects
Transparent Objects
• These objects allow light to pass throughthem.
• Examples: A clear empty glass, a window pane and eyelenses.
Translucent Objects
• These objects allow only some of the light to pass through them and the other light rays arescattered.
• Examples: Thin tissue paper, frosted glass, tinted car windows and waxedpaper.
Opaque Objects
• These objects do not allow light to pass throughthem.
• Example: Cardboard, bricks, wood, buildings andeyelids.
Shadows
• A shadow is created when an opaque object blockslight.
• The objects must be opaque or translucent to form ashadow.
• Light travels in a straight line. So, when it is blocked, a shadow isformed.
• Transparent objects will not produce any shadow as light will pass throughthem.
• The shape of the shadow may not be identical to the shape of the object because the shadow’s shape
depends on the position of the source of light and the place where the shadowfalls.
• If an object is moved closer to the light source, then the shadow getsbigger.
• If an object is moved further away from the light source, then the shadow getssmaller.
Nature of a Shadow
The shadow is categorized into two parts based on the brightness of the dark patches:
• Umbra: It is the darkest part of the shadow present in the middle of it, which forms due to the absence of light.
• Penumbra: It is the faintest part of the shadow present at the corners of it. It is formed when the
light is not completely blocked (Figure 7).
Eclipses
The eclipses are formed on the basis of rectilinear propagation. They are shadows formed naturally when
the sun, moon and earth aligned in a straight line.
Solar Eclipse:
• When the sun, moon and earth align in a straight line and the moon falls between the sun and the earth
casts its shadow on earth then this phenomenon is known as solar eclipse.
• During a solar eclipse, the dark side of the moon faces the earth. This phenomenon takes place on
a new moon day.
• The moon’s shadow forms a total solar eclipse on the region of the earth where the umbra of the moon falls on it,
while the partial solar eclipse takes place on the regions where the penumbra of the moon’s shadow falls(Figure 8).
Do not look directly to the sun during a solar eclipse because this will damage your eyes permanently.
However, if you really want to look at the sun then you should use eye filters such as X-ray reports.
Lunar Eclipse
When the sun, moon and earth align in a straight line and the earth is present in the middle of the sun and the
moon, casting its shadow on the moon then this phenomenon is called alunar eclipse.
The total lunar eclipse is formed in the regions of the moon where the earth’ umbra falls, whereas the partial
lunar eclipse is formed where the earth’s penumbra falls(Figure 9).
A Pinhole Camera
• A pinhole camera is a simple camera without a lens and with a single small hole or an opening through
which the lightPasses.
• Light from a scene passes through this single hole and projects an inverted image on the opposite
side of thebox.
• The human eye in bright light is similar to a pinholecamera.
How to use it
• Hold the pin hole camera look through the open face of the smaller box. You should use a piece of
black cloth to cover your head and the pinhole camera.
• Now focus on the object you want to capture in the camera. Make sure that the objects you wish to look
at through your pinhole camera are in bright sun shine.
• Move the smaller box forward or backward till you get a picture on the tracing paper pasted at the other end
Mirrors and Reflection
Mirror
• A mirror is an object which reflectslight.
• When we look into the mirror, we see the reflection of our face in themirror.
• A mirror changes the direction of light that falls onit.
Reflection
• Reflection is light which travels along a straight line and gets reflected from themirror.
Image formation by Plane Mirror
• A plane mirror reflects the light that falls on it.
• The beam of light that falls on the mirror is called incident ray, while the light that reflects called the reflected ray.
• The angle of incident ray equals to the angle of the reflected ray.
• The incident ray, reflected ray and normal ray lies at the common point of the mirror(Figure 11).
• A plane mirror has a property of lateral inversion in which the image formed in the mirror has an opposite side.
For example, when you see your image in the mirror your right hand becomes left and your left hand becomes right.
• The image formed is erect, same size and laterally inverted. It retains the same colour as the object.