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11. LIGHT, SHADOWS AND REFLECTIONS

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Chapter-reviews:

• Opaque objects: Objects which do not allow passing the light through them are known as opaque objects. Example: wood, wall, book etc.

• Transparent objects:Objects which allow light passing through them are known as transparent objects.  Example: glass, water etc. 
• ​Translucent objects: Objects which allow the light passing partially not completely through them are known as translucent.  

  Examples: oily paper, polythene and plastic etc. 

• Luminous objects: Objects like the sun that give out or emit light of their own are called luminous objects. Examples: Sun, Bulb, lamp and candle etc.
• Screens: Objects on which shadows form are called screens. 
• Shadow: When light passes through an opaque object it forms a shadow. 
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Exercise

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Que1: Rearrange the boxes given below to make a sentence that helps us understand opaque object.

Ans: 

Ques2:  Classify the objects or materials given below as opaque, transparent or translucent and luminous or non-luminous: Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden board, a sheet of polythene, a CD, smoke, a sheet of plane glass, fog, a piece of red hot iron, an umbrella, a lighted fluorescent tube, a wall, a sheet of carbon paper,b the flame of a gas burner, a sheet of cardboard, a lighted torch, a sheet of cellophane, a wire mesh, kerosene stove, sun, firefly, moon.

Ans: 

Opaque : A piece of rock, a sheet of aluminium, a mirror, a wooden board, a CD, a piece of red hot iron, a umbrella, a lighted fluorescent tube, a wall, a sheet of carbon paper, the flame of gas burner, a sheet of cardboard, a lighted tourch, kerosene stove, sun, firefly, moon.

Transparent: Air, water.

Translucent: A sheet of polythene, smoke, a sheet of plane gas, gas, a sheet of cellophone, a wire mesh.

Luminous: A piece of red hot iron, a lighted fluoressent tube, the flame of a gas burner, a lighted torch, sun, firefly.

Non - luminous: Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden, a sheet of carbon paper, a sheet of cardboard, a sheet of cellophone, a wire mesh, kerosene stove, moon. 

Que3: Can you think of creating a shape that would give a circular shadow if held in one way and a rectangular shadow if held in another way?

Ans: A cylinder object can cast shadows in two ways. When the top circular view faces the shining object. a circular shape shadow is formed, When its curved side faces the shining object. it casts a rectangular shadow.

Que4: In a completely dark room, if you hold up a mirror in front of you, will you see a reflection of yourself in the mirror?

Ans: No, to see reflection, source of light is required. We can see only in the presence of light.

Important Question with solution

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Question: How is the path of light?

Answer: The path of light is straightforward i.e. that it travels in a straight line.

Question: Give an example of a luminous object.

Answer: Candle

Question: What happens when there is no light?

Answer: When light is not there, we will not be able to see anything.

Question: Why do not we see any object kept in the dark room?

Answer: There is no light in the dark room, so we do not see any object kept in it.

Question: What kinds of objects form shadow?

Answer: Opaque objects.

Question: What is the use of the camera in the pin hole?

Answer: The pin hole camera is used to see the image of an object.

Question: Why should not we see the reflection of the Sun or the Sun light directly?

Answer: We should not see the reflection of the sun or the sun light directly, because the radiation coming with the sunlight can make our eyes impaired.

Question: Give an example of pin hole camera created in the nature.

Answer: When light passes through the leaves of trees, then the sunlight spots appear under the tree. The hole in which the light turns out, acts like pin hole camera.

Question : What is the reflection of light?

Answer: When the ray of light strikes a smooth opaque surface of an object, then it turns back and goes back to the direction from which it comes is called reflection of light.

Question: Give an example of an object that reflects the light?

Answer: Mirror

Important Question with solution

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Q1: Does the flame of a gas stove emit light?

Ans: Yes. 

Q2: What is rectilinear propagation of light?

Ans: Light travels in a straight line.

Q3: Write the names of 4 different sources of light?

Ans: Bulb, sun, stars, firefly

Q4: Give one example of living thing which emits light?

Ans: Jugnu {firefly)

Q5: Sometimes you are able to see sun or moon behind the clouds .What can you say about the ability of such clouds to transmit light?

Ans: Yes, clouds transmit light.

Q6: Image formed in a pinhole camera is inverted .Why?

Ans: It is inverted because of the rectilinear propagation of light.

Q7: Can you suggest the shape of the shadows?

Ans: It is usually similar to the object.

Q8: Does the length of shadow change from season to season?

Ans: Yes if observed at same time .

Q9: Define reflection of light?

Ans: The process of returning/ bouncing back the light to the same medium after Striking a surface is called reflection of light.

Q10: What is a reflector?

Ans: A surface which reflects the light is called reflector.

Q11: What is shadow?

Ans: A dark patch formed behind an opaque object when it is placed in the path of light is called shadow.

Q12: What do you mean by reflection of light?

Ans:Coming back of light ray incident on a surface is called reflection.

Q13: What is the principle of working f a pin hole camera?

Ans: A pinhole camera is based on rectilinear propagation of light?

Q14: Can light pass through opaque objects?

Ans: No

Q15: What is an artificial source of light?

Ans: Man made sources of light. For example: electric bulb, candle etc.

Q16: Classify the following into transparent, translucent, and opaque objects.

Glass, air, oil, oily paper, rubber sheet, polythine, water, football, table

Ans:

Transparent: air, glass, water

Translucent: oil, oily paper, polythine

Opaque: rubber sheet, football, table

Q17: Name two artificial sources of light?

Ans: Electric bulb, candle

Q18: Name one transparent and one opaque body?

Ans: Transparent: air Opaque: stone

Q19: Give one example of most commonly used reflector?

Ans: Looking glass or plane mirror

Q20: Does the reflection of light from the surface similar to the bouncing back of a rubber ball after it strikes from a ball? Explain.

Ans: Yes, because of reflection, light falling on a surface bounces back to the same medium. When a light strikes a surface after the reflection it turns back in the same medium.

Q22: Give the properties of the image formed by the plane mirror?

Ans: The basic properties of plane mirror is as follow; 

(i) The plane mirror alsways forms a virtual and errect image.

(ii) Size of the image formed by plane mirror is equal to the size of the object.