Concept of Light:

Light (or optics) is a form of energy that causes the sensation of vision. This energy relationship with light is termed as luminous energy and it is the energy that causes the sensation of vision if it falls on our eyes.

Nature of light:

There are mainly two views regarding the nature of light namely:

1) Light as particle (Corpuscular theory)

2) Light as matters (Wave theory)

According to the corpuscular theory light comprises of a stream of minute weightless particles given off by the source. On the basis of this concept, interference and diffraction of light can't be described while the photoelectric effect could be described satisfactorily by supposing that the light exists in discrete particles termed as photons.

On other hand, the wave theory considered light as a form of disturbance spreading out from its source. On the basis of this concept or theory, the phenomena of interference and diffraction of light could be satisfactorily described while that of the photoelectric effect can't be described by it.

Sources of Light:

Visible light is a form of energy that our eyes can detect. Visible light is categorized into two sources namely the luminous sources and the non-luminous sources.

1) Luminous sources: These are the light sources which produce and emit light through them (that is, given out light of their own). Illustrations are the stars, sun, electric lamp, burning flames, TV screen, fire-flies, some deep - sea fishes and other artificial light sources. The lights they produce enter our eyes.

2) Non-luminous sources: Such are the light sources which don't give out light on their own. They based on the natural or artificial light sources to illuminate them. They reflect light from luminous objects for them to be seen. Illustrations comprise moon, pages of book, wood, road sign. Luminous sources illuminate them and make them come out luminous.

Properties of Light:

Whenever light strikes or hits an object, it might be, based on the nature of the material:

a) Reflected, absorbed and transmitted

b) Partly absorbed, partly reflected and partly transmitted

Transmission of Light:

1) Opaque materials: They partially absorb and partially reflect light energy however don't transmit light at the surface. Illustrations are brick wall, wood and a sheet of cardboard.

2) Transparent Objects: They transmit most of the light energy falling on it in such a way that the object can be seen. It reflects and absorbs much little. Illustrations comprise glass and clear water.

3) Translucent Object: They let light to energy to pass via them in such a manner that the object can't be seen. Illustrations comprise glass sheet employed for certain windows and waxed sheet.

Rays and Beams of Light:

The direction or path all along which the light energy travels is termed as a light ray and the collection of rays is termed as a beam. There are three kinds of beams namely:

1) Parallel beams: A parallel beam is one in which the rays of light are parallel to one other. Illustrations of parallel rays are rays from the search light.

2) Convergent beam: A convergent beam is one in which the entire rays of light converge or meet up at a point. Convergent beam can be generated by employing hand lens.

3) Divergent Beam: A divergent beam is one in which the entire rays of light come from a point and spread out or diverge from that source. The lights from lamps are illustrations of divergent beam.

Rectilinear Propagation of Light:

Rectilinear Propagation of Light is the trend of light to travel in straight Lines. In a homogenous transparent medium light travels in a straight line and this is termed as a rectilinear propagation of light.

Illustration:

1) Take three (3) cardboard papers or any transparent medium of the similar kind label them A, B, and C. Now put at one side of 'A' a source of light such as candle, open a pinhole at the middle of all the cardboard papers in a straight line ensuring that the light rays can be view via all, you will observe that the candle flame will be visible via the pinholes in a straight line only. This exhibit that light is propagated rectilinearly.

2) Ray of light coming from the sun go after rectilinear propagation of light.

Shadows:

A shadow is a region in which light rays from a source can't reach. It takes place as light travels in a straight line. Shadows are generated through the obstruction of light through an opaque object. For illustration, if light rays arrives at an opaque obstacle (one which absorbs Light), the rays just graze the edges of the obstacle and generate the outline of a shadow.

The type of shadow obtained based on the size of the luminous object (that is, source of light) sending out the rays.

How shadows are formed:

A shadow is made if an opaque object blocks the path of light as light travels in straight lines. Light can't bend around the object therefore it forms a shadow.

A shadow fundamentally is a kind of image generated if light is blocked. A shadow usually takes up most of the space behind an opaque item by means of light right in front of it. The fundamental cross section of a shadow is usually a two-dimensional silhouette, or the overturned projection of the item hindering the light.

For a non-point source of light, the shadow is distributed to the umbra and penumbra. The more widespread the light source, the more hazy the shadow will be. When there are many light sources there will be multiple light sources.

Eclipses:

An eclipse occurs if one heavenly body like a moon or planet moves into the shadow of the other heavenly body. There are two kinds of eclipses on Earth: an eclipse of the moon and an eclipse of the sun.

1) Lunar Eclipse:

The moon revolves in an orbit around Earth, and at similar time, Earth orbits the sun. At times Earth moves between the sun and the moon. Whenever this occurs, Earth obstructs the sunlight which generally is reflected by the moon. (This sunlight is what the reason the moon to shine is.) Rather than light striking the surface of moon, shadow of earth falls on it. This is an eclipse of the moon - a lunar eclipse. A lunar eclipse can take place merely when the moon is full.

A lunar eclipse can be observed from Earth at night. There are two kinds of lunar eclipses: Total lunar eclipses and partial lunar eclipses.

A total lunar eclipse takes place if the moon and the sun are on precise opposite sides of Earth. However the moon is in the shadow of Earth, a few sunlight reaches the moon. The sunlight passes via the atmosphere of Earth that causes atmosphere of Earth to filter out most of the blue light. This forms the moon appear red to people on Earth.

A partial lunar eclipse occurs if merely a part of the moon enters the shadow of Earth. In a partial eclipse, the shadow of Earth appears extremely dark on the side of the moon facing Earth. What people see from Earth throughout a partial lunar eclipse based on how the sun, Earth and moon are lined up.

A lunar eclipse generally lasts for some hours. At least two partial lunar eclipses occur each and every year, however total lunar eclipses are rare. This is safe to look at a lunar eclipse.

Solar Eclipse:

At times if the moon orbits Earth, it moves between the sun and Earth. If this occurs, the moon blocks the light of the sun from reaching Earth. This causes an eclipse of the sun, or solar eclipse. Throughout a solar eclipse, the moon casts a shadow onto the Earth.

There are three kinds of solar eclipses:

1) The primary is a total solar eclipse. A total solar eclipse is merely visible from a small region on Earth. The people who notice the total eclipse are in the center of the moon's shadow if it strikes Earth. The sky becomes extremely dark, as if it were night. For a total eclipse to take place, the sun, moon and Earth should be in a direct line.

2) The second kind of solar eclipse is a partial solar eclipse. This occurs if the sun, moon and Earth are not precisely lined up. The sun seems to encompass a dark shadow on just a small part of its surface.

3) The third kind is an annular solar eclipse. An annular eclipse occurs if the moon is farthest from the Earth. As the moon is farther away from Earth, it appears smaller. It doesn't block the whole view of the sun. The moon in front of the sun looks like a dark disk the top of a larger sun-colored disk. This makes what looks similar to a ring around the moon.

Throughout a solar eclipse, the moon casts two shadows on Earth. The first shadow is termed as the umbra (UM bruh). This shadow gets smaller as it reaches Earth. It is the dark center of the moon's shadow. The second shadow is termed as the penumbra (pe NUM bruh). The penumbra gets bigger as it reaches Earth. People standing in the penumbra will see a partial eclipse. People standing in the umbra will observe a total eclipse.

Solar eclipses happen once each and every 18 months. Dissimilar to lunar eclipses, solar eclipses merely last for a few minutes.

The Pinhole Camera:

A pinhole camera is a simple camera with no lens and having a single small aperture, a pinhole - efficiently a light-proof box having a small hole in one side. Light from the scene passes via this single point and projects an inverted image on the opposite side of the box.

This is totally dark on all the other sides of the box comprising the side where the point is made. This portion is generally painted black; however black boxes are as well employed for this aim. There is as well a thin screen that looks similar to a projector sheet, and is put in between the dark side adjacent to the pinhole.

Up to a specific point, the smaller the hole, the sharper the image, however dimmer the projected image. Optimally, the size of the aperture must be 1/100 or less of the distance between it and the projected image.

As the pinhole camera needs a lengthy exposure, its shutter might be manually operated, as by a flap made up of light-proof material to cover and uncover the pinhole. Typical exposures range from five seconds to some hours.

A common utilization of the pinhole camera is to capture the movement of the sun over a long time period. This kind of photography is termed as solargraphy.

Linear magnification produced by the pinhole camera:

Definition: Magnification is the ratio of the size and height of the image to the size (or height) of the object.

We can compare the size of an object and its image through using the ratio:

Linear magnification = Size of the image/Size of the object

Linear magnification = Height of the image/Height of the object

Linear magnification = Distance of the image from pinhole/Distance of the object from pinhole

Linear magnification = Length of camera/Distance of object from pinhole

In symbolic form,

M = h_i/h_o = v/u

Here,

M = linear magnification

h_i = height of the image

h_o = height of the object

v = perpendicular distance of the image to the pinhole

u = perpendicular distance of the object to the pinhole

Reflection of Light at Plane Surfaces:

If light falls on an object, a few of it bounces off the object. The bouncing off of light at a surface is termed as reflection. All the surfaces reflect light. How well a surface reflects light based on the nature of the surface. Shiny, smooth surfaces like a mirror reflect light better than dull, rough surfaces like a wall or a sheet of paper.

If a beam of light falls on the smooth surface, the rays of the beam are reflected in a specific direction and remain parallel to one other. This is termed as regular reflection. As the rays are reflected in an arranged fashion, the image formed is clear.

If a beam hits an uneven surface, its rays are reflected in various directions. This is termed as diffuse reflection or irregular reflection. As the reflected rays get diffused or spread, in various directions, we observe a hazy image or no image at all.

The laws of reflection at plane surfaces are as follows:

1) Incident ray and reflected ray both lie on either sides of the normal.

2) Incident ray, Reflected ray and the normal lie in one plane that is termed as the plane of incidence.

3) Angle of incidence = Angle of reflection

i = r

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