Light Online Test 7th Science Lesson 12 Questions in English

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Light is the only source of energy for plants. So they entirely depend on light.

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People and animals derive energy from carbohydrates, protein and fat through their food.

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Plants produce food using the energy from Sun light, carbon-di-oxide and water by the process called as Photosynthesis. Sun light acts a vital role in the process of photosynthesis.

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Objects which are able to emit light are known as light sources. Light rays can come from different sources.

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There are two types of sources of light, natural sources of light and artificial sources of light.

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Sources which emit light naturally are known as natural sources of light. The Sun is the primary and the major source of natural light.

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Stars also produce light in the same way as the Sun do. However as they are much farther away than the Sun the light from them are too weak.

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Some living organisms have the ability to produce light named by bioluminescence. It is the effect of certain chemical reactions occurring in the organism.

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Fireflies, jellyfish, glow worm, certain deep sea plants and some microorganisms can emit light naturally.

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The different light sources that are able to produce light artificially can be put under three broad categories.

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The moon provides light as well but it cannot produce light by its own. The light emitted by the Moon is the light of the Sun reflected towards the Earth. When we see the Moon we see only the Moon’s lighted part. Thus half of the moon is always facing the Sun and receiving light from it. Hence, we receive light from the moon.

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Artificial sources are man – made light sources such as flame of candle, incandescent lamp, neon lamp, Sodium lamp etc.

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Incandescent Sources: When certain objects are heated to a high temperature, they begin to emit light. The glowing of hot iron rod is a kind of Incandescent light. Example: Candle, incandescent lamp.

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Gas Discharge Sources: Passing electricity through certain gases at a very low pressure (discharging) can produce light. Example: Neon lamp, Sodium lamp

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We often use a kind of gas discharge lamp that uses fluorescence to produce visible light. The electric current in the gas excites mercury vapour, which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow in visible light.

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Light has some fundamental properties which are rectilinear propagation of light, Reflection, Speed, Interaction of light with matter and Spectrum

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Interaction of light with matter describes the types of material according to permeability, Formation of shadows and Plane mirror and images.

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Light travels in straight line it cannot bend the path itself. This is called as the rectilinear propagation of light. This is one of the most important properties of light

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Al-hasan -Haytham was a scientific thinker who made important contribution to the understanding of vision optics and light. He is the first one to experiment with light and found important properties like the rectilinear propagation of light.

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Before the advancement of camera Pinhole camera was used to photograph movement of the sun over a long period of time. This type of photography is known as solography and also be used for observing and recording solar eclipses. And it was also used to take photograph of stationary objects.

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The light falling on the mirror is called as incident ray and the light reflected is called reflected ray

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Laws of reflection: The angle of incidence is always equal to the angle of reflection. i = r. The incident ray, reflected ray and the normal at the point of incidence lie on the same plane.

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Solution: Angle of incidence, i = 90 -72 = 18°

Angle of reflection, r = i = 18°

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The laws of reflection holds good for all surfaces irrespective of the shape. Vertical surfaces, angled surfaces and even over the curved surfaces, the laws of reflection holds good.

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As long as we can draw the normal perpendicular to the surface at the point can be drawn the angle of incidence at that point will be equal to angle of reflection.

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The law of reflection is always observed regardless of the orientation of the surface. If the surface is smooth, and flat all points on it have the normal in the same direction. Therefore a set of parallel rays striking the surface will be reflected at an angle but the rays themselves will still remain parallel to each other.

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Broadly we can say that there are two types of reflection. If the surface is smooth then we have specular reflection. The parallel light rays striking the surface gets reflected yet individual reflected rays remain parallel

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If the surface is rough then we have diffused reflection. Light rays after reflection go in many directions.

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Generally light is not a single ray but a bundle of rays which are called as a beam of light. A light beam can be a bundle of parallel rays’ convergent rays or divergent rays.

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The rays of sunlight are parallel. Often the headlight of car gives parallel rays. However look at the rays of light coming out of a candle. Light rays go in all directions, from the candle fire. These rays are divergent. Light rays from a flash light are also divergent.

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Using lenses we can converge light rays. Using a lens you can focus sunlight at a point. That is what we are making the light rays to converge.

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When lighting a bulb in a dark room light spreads the whole room quickly. This is because the light travels very fast. In theory nothing can travel faster than light.

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Light travels three lakh kilometers per second in air or vacuum.

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Depending upon permeability materials can be classified into three categories.

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Transparent Material: Eye glasses, clear drinking glass, clear water, face glasses used in buses. Translucent Material, Opaque Material: Wall, thick card board, stone, etc

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Light travels in a straight line. Hence it cannot go around such objects. That is why we see shadow. Shadow is always against, opposite side of light source. It is caused by opaque objects that stop light from propagating.

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When an opaque object is placed in the path of light from a point source a uniform dark shadow will appear on the screen. This is shadow is called as umbra.

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When an opaque object is placed in the path of light coming from a broad source of light a small umbra will appear on the screen and an illuminated shadow area appears around umbra. This illuminated shadow area is called as penumbra

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The penumbra always surrounds the umbra. The umbra is the darkest part of a shadow. In this part light rays are completely prevented by the opaque object. The lighter shade of shadow is the penumbra.

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Properties of shadow: All objects do not form shadows. Only opaque objects form shadows. Shadows will be formed in the opposite side of light source. It cannot be determined the characteristics of an object by its shadow. The shadow will be always darker whatever may be the color of light rays.

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Light source, opaque object are shadow all are in a straight line. The size of shadow depends upon the distance between light source and object and the distance between object and the screen.

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An eclipse is an incident when any astronomical object is partially or fully obscured due to the placement of another astronomical object in the presence of light. Thus solar and lunar eclipses  are occurring that are due to the property of light known as the rectilinear propagation of light.

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Solar eclipse occurs, when the moon arrives between the sun (S) and the earth (E). The shadow of the moon appears on the earth at A as shown in picture. Hence those who are at the region A are unable to see the Sun instantly. This is solar eclipse.

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Lunar eclipse: Lunar eclipse occurs, when the earth (E) comes between the sun (S) and the moon (M). The earth prevents light coming from the sun and makes shadow on the moon. This is lunar eclipse

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The distance of the object from the mirror is equal to the distance of the image from the mirror (a=b).

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Optical fiber is a device that works on the principle of total internal reflection by which light signals (huge data) can be transmitted from one place to another place with a negligible loss of energy in a very short time.

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Optical fiber consists of a cable having one or more thin flexible fibers with a glass core through which light signals can be sent. Optical fiber can be twisted and bent easily

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Optical fibers have become very important in high-speed communications, such as cable TV and high-speed broadband services. Fiber optic cables are able to carry more signals than traditional copper cable telephone lines.

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The images that are obtained on a screen are called ‘real image’ and that which cannot be obtained on a screen ‘virtual image’. The image on pinhole camera was upside down. The mirror image was upright.

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Properties of Image formed in a plane mirror: Image formed in a plane mirror is upright, Image formed in a plane mirror is virtual, the image is of the same size as the object. The distance of the image from the plane mirror is equal to the distance of the object from the mirror, Image is laterally inverted.

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Color of sunlight : Light is a form of energy in the form of a wave that simulates that retina of our eyes.

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Visible light is a spectrum of a number of waves with different wavelength range from 400nm to 700nm (1nm = 10-9 meter) each wave has a definite wavelength represents a particular color.

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The band of visible light is VIBGYOR.

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Violet color has shorter wavelength and red color has longer wavelength.

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White is not a color at all. But it is the combination of all the colors of the visible light spectrum. If the entire wavelength (colors) of visible light spectrum give appearance of white similarly the observe of all there wavelength of visible light will lead appearance of black.

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A prism is an object made up of a transparent material like glass or plastic that has at least two flat surfaces that from an acute angle (less than 90 degrees).

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This is due to lateral inversion .The phenomenon due to which the left side of an object appears to be right side of the object in its image in a reflecting medium (mirror) so that drivers see the word the right way around in their rear-view mirror.

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Difference between the images formed in Pinhole camera and Plane mirror

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Red color is scattered the least by air molecules. Red color has the highest wavelength of all the other colors. So, red color is able to travel the longest distance through air, fog.

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When white light is passed through a prism as shown in the figure the colors of the rainbow emerge from the prism.

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Newton Disc is a card board disc with seven equal sectors colored red, yellow, orange, green, blue, indigo and violet. When the disc turned quickly the retina receives the sensation of the spectrum simultaneously and disc appears white. Using this disc one can explain that white is a combination of VIBGYOR.

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Synthesis of color is the method of creating color by mixing various proportion of two (or) three distinct colors of light.

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The distinct colors are Red, Green and Blue called as primary colors.

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Equal proportions of two primary color create a secondary color. Magenta, Cyan and yellow are called secondary color. Equal proportions of all three primary color create white.