Class VI : Physics : Light

Light

• Light is a form of energy.
• Light does not require a medium like air, water to travel. Therefore light can travel through space and we can see sunlight.
• Any object that produces light is called the Source of Light. 
• Light Sources: There are two types of light sources.

Natural Light Source	Artificial Light Source
These sources of light exist in nature on their own.	These sources of light are man-made.
For example Sun, lightning stars, and fire due to natural causes	For example Electric bulbs, tubes, and candles

• Luminous and Non Luminous objects

Luminous Objects	Non Luminous Objects
Objects that give out the light on their own (emit light)  are called luminous objects.	The objects that do not give out light are called non-luminous objects.
All objects mentioned above are luminous objects.	For example Chair, paper, trees, tables
	Some non-luminous objects, when heated to high temperatures, emit light and become luminous. For example, when metals are red-hot, they start emitting light.

• Bio-luminescence: The production or emission of light by a living organism as a result of the conversion of chemical energy into light energy is called bioluminescence. For example:
• Fireflies: These insects emit pale greenish-yellow light which glows in the dark
• Jellyfishes: Certain jellyfishes living deep under the sea emit light.

Is the moon a luminous object?

No, the moon is not a luminous object. It gets its light from the sun. The moonlight that reaches the earth is actually the reflected light of the Sun.

Moon and all planets are non-luminous objects. They are illuminated objects because they get light from other sources of light.

When light falls on any object, the object becomes illuminated or visible.

Transparent, translucent, and opaque materials

All materials are classified on the basis of the amount of light each material allows to pass through it, in the following three different types.

Transparent Medium (material)	Translucent Medium (material)	Opaque Medium (material)
A medium that allows most of the light to pass through it is called a transparent medium.	A medium that allows only a part of the light to pass through it is called a translucent medium.	A medium that does not allow any light to pass through it is called an opaque material.
Examples: Clear glass, clear water, air, clear plastic	Examples: Frosted glass, lampshades, butter paper, mist, fog, beeswax, certain colored glasses.	Examples:  Wood, stone, metals, books
We can see through any transparent medium. A totally transparent object is invisible.	We cannot clearly see through the translucent medium.	We cannot see through an opaque object. The opaque object when kept in the path of light, creates a shadow.

Rectilinear propagation of light: The property of light traveling in straight lines in a medium is called the rectilinear propagation of light.

The Principle of Least Time: Out of all possible paths to go from one point to another, light takes a path that requires the shortest time. That path is the straight line. This is called the principle of least time.

Ray of Light: it is a path along which light energy travels. It is shown by a straight line with an arrow indicating the direction in which the ray of light is traveling.

The beam of Light: A collection or group of rays of light is called a beam of light.  A beam of light can be parallel, convergent or divergent.

Parallel Beam of Light	Convergent Beam of Light	Divergent Beam of Light
A collection of parallel rays of light is called a parallel beam of light.	When rays of light from different directions converge at a point, then such a collection of rays is called a convergent beam of light.	When rays of light from a point diverge in different directions, then it is called a divergent beam of light.
Examples: Searchlight, the rays from a point on a very distant object, such as the sun are considered to be parallel	Examples: A source behind the lens	Examples: A lamp, a candle flame

The speed of Light:

Light travels in straight lines with a speed of 3 X 108  m/s.

Formation of Shadow:

The formation of the shadow is an effect of the rectilinear propagation of light. When the path of light gets blocked by an opaque object, a dark patch is formed which is called a shadow.

• Translucent objects cast a faint shadow.
• Transparent objects do not cast a shadow because they allow the light to pass through them.

Characteristics of Shadow

• Shadows are formed only in the presence of light.
• Shadows are always formed on the opposite side of the source of light.
• The nature (clarity and size) and the position of a shadow depend on the size and position of both the light source and the object.

A point source of light: A light source that is of the size of a pinhead.

Non-point source of light: An extended or wide source of light.

Umbra Region	Penumbra Region
The darkest part at the center of the shadow is called the umbra region.	The penumbra is the part of the shadow that surrounds the umbra region.
No light falls on the umbra region	Some rays of light fall into the penumbra region and partially illuminate it.
When an opaque object is placed between a point source of light and a screen, a shadow with only the umbra region is formed.	When an opaque object is placed between a non-point source of light and screen a shadow with an umbra region at the center and surrounding penumbra region is formed.

Pinhole Camera :

Pinhole camera works on the principle of rectilinear propagation of light. Rays of light from the object go in all directions. Some of those light rays pass through the hole of a pinhole camera and fall on-screen at the back of the pinhole camera. The rays intersect at the pinhole and crossover at this point.  Therefore the top of the object appears at the bottom of the image and the bottom of the image appears at the top. Thus the image is inverted.

Characteristics of the image formed by pinhole camera:

• The image is inverted
• Since the image can be obtained on the screen, it is a real image
• The size of the image can be reduced or enlarged.

Eclipse
https://prezi.com/view/iHARgggWDO8y3btCAOYR/

Eclipse	Lunar Eclipse: When the Earth comes in between the sun and moon, being an opaque object, it blocks the light from the sun and forms a shadow on the moon. People living on the side of the Earth-facing moon cannot see the moon. This is called Lunar Eclipse.	Penumbral Lunar Eclipse: When the moon crosses the Earth’s penumbra it is called the penumbral lunar eclipse.
		Partial Lunar Eclipse: When the moon crosses the Earth’s penumbra it is called the penumbral lunar eclipse. In this case, the moon receives some light from the sun and it reflects it to the earth. Hence the moon is partially visible to the people living on that part of the Earth.
		Total Lunar Eclipse When the moon is in the umbra region of the Earth, then it is called the total lunar eclipse. In this case, the moon does not receive any sunlight. Hence moon is invisible to the people living on that part of the Earth.
	Solar Eclipse: When the moon comes in between the sun and the Earth, the light from the sun gets blocked by the moon, forming a shadow on the Earth. People living on that part of the Earth can’t see the sun for some time. This is called a Solar Eclipse.	Partial Solar Eclipse This eclipse happens when the view of the sun is partially blocked by the moon. In this eclipse, the sun and the moon are not exactly in the straight line. The part of the Earth that falls under the penumbra region has a partial solar eclipse.
		Total Solar Eclipse A total solar eclipse takes place when the view of the sun is completely blocked by the moon. The part of the Earth that falls on the umbra region has a total solar eclipse.
		Annular Solar Eclipse When the sun appears as a black disc surrounded by a ring of light, it is called the annular solar eclipse. In this eclipse, the sun and the moon are exactly in a straight line. When the moon whose apparent size is smaller than that of the sun, comes in front of the sun, a ring-like shape of light is formed.
		Hybrid Solar Eclipse This type of eclipse occurs when people on some part of the Earth can see a total solar eclipse and some can see an annular solar eclipse.