NCERT solution for class 8 ||Science || Chapter 16 - Light

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NCERT solution for class 8 ||Science || Chapter 16 - Light

Chapter 16- Light 

Exercise 

Q1. Suppose you are in a dark room can you see objects in the room can you see objects outside the room explain? 

Answer - If we are in a dark room then it is not possible to see the object in the room while the object outside the room are visible to us because the object becomes visible only in presence of light . The light reaches to our eyes after being reflected from the object and the object becomes visible to us

Q2. Differentiate between regular and diffuse reflection does diffuse reflection mean the failure of the laws of reflection ? 

Answer - Regular reflection 

• It is takes place from a smooth or regular surface 
• In this reflection all reflected rays are parallel to each other  

Diffuse reflection 

• Diffuse reflection takes place from an irregular surface .
• The reflected rays  are not parallel to each other 
• This happened because of the presence of irregular microscopic surfaces The parallel incident rays are reflected in different direction and the laws of reflection is not violated in diffuse reflection 
• 

Q3. Mention against each of the following whether regular or diffuse reflection will take place when a beam of light strike justify your answer in each case .

a) Polished wooden table 

b) Chalk powder 

c) Cardboard surface . 

d) Marble floor with water spread over it 

e) Mirror 

f) Piece of paper 

Answer - a) Polished wooden table regular reflection 

Reason - The polished table has a smooth surface and reflection from the smooth surface will be regular 

b) Chalk powder diffuse reflection 

Chalk powder sprayed on the surface is the example of irregular surface and it shows the diffuse reflection 

c) Cardboard surface diffuse reflection 

Reason - Cardboard surface is also the example of irregular surface so it shows the diffuse reflection 

d) Marble floor with water spread over it regular reflection 

Reason - Marble flower has a smooth surface while the water is also has a smooth surface so it shows the regular reflection 

e) Mirror regular reflection 

Reason - Because mirror has a smooth surface 

f) Piece of paper diffused reflection 

Reason - Piece of paper look smooth but it has many irregularities on its surface it shows the diffuse reflection

Q4 . State the laws of reflection . 

Answer - laws of reflection 

a) The angle of reflection is always equal to the angle of incident 

b) The incident ray the reflected ray and the normal to the relative surface all are lie in the same plane 

Q5. Describe an activity to show that the incident ray the reflected ray and the normal at the point of incident light in the same plane ? 

Answer - 

i) Place of a plane mirror on the table 

ii) Take a paper sheet and make a small hole in the centre 

iii) Hold the sheet normal to the table 

iv) Take the another sheet and place it in this way that it is in contact with vertical mirror 

v) Draw a normal line on the second sheet from the mirror 

vi) Now light a torch on the mirror through a small hole in such a way that the Ray of light falls on the normal

vii) When the ray from this whole is incident on the mirror then it gets reflected in a certain direction 

viii) and you can easily observe the incident ray reflected ray and normal to the mirror at the point of incident ray on the sheet placed on the table 

ix) It prove that all the rays are placed in a same plane

Q6. Fill in the blanks in the following. 

a) A person 1 m in front of a plane mirror seems to be 2 metre away from his image .

b) If you touch your left  ear with front hand in front of a plane mirror it will be same in the mirror that you are right ear is touched with left hand 

c) The size of The pupil becomes large when you see in dim light .

d) Night birds have less cones than rods in their eyes .

Choose the correct option in question 7 and 8 

Q7. Angle of incidence is equal to the angle of reflection .   

a) always  b) sometimes   c) under special conditions.     d)  never

Answer - always

Q8. Image formed by a plane mirror is 

a) virtual behind the mirror and enlarged 

b) virtual behind the mirror and the same size of the object .

c) real at the surface of the mirror and enlarged 

d) real behind the mirror and of the same size of the  object 

Answer- b) Virtual behind the mirror and the same size of the object .

Q9. Describe the construction of a kaleidoscope 

Answer- Kaleidoscope is made up of by following way  

• Get three rectangular mirror strip each about 50 cm long and 4 cm wide 
• Join them together to form a prism shape 
• Fix this arrangement of mirror in a circular cardboard tube 
• Make sure that the tube is slightly longer than the mirror strips 
• Close one end of the tube by a cardboard disc having a hole in the centre Through which we can see 
• Paste a piece of transparent plastic sheet under the cardboard disc 
• At the other end fix a circular plane glass plate 
• Please on this glass plate several small pieces of coloured broken  bangles  
• Close this end of the tube by a ground glass plate and from it allow space for the coloured pieces to move around

Q10. Draw a labelled sketch of the human eye .

Answer-  

Q11. Gurmeet wanted to perform activities 16.8 using a laser torch her teacher advised her not to do so can you explain the basis of the teachers advice? 

Answer- No the laser light is very harmful to human eyes because its intensity is very high and it can damage the retina and also leads to blindness so it is advisable to not to look at the laser beam directly

Q12. Explain how you can take care of your eyes 

Answer- We should protect our eyes by following way 

a) Avoid reading in very dim light and very bright light 

b) Do not rub your eyes 

c) Clean your eyes with cold water if any dust particle enter in it 

d) Maintain a distance at least 25 CM between the book and your eyes while reading  

c) Brightness of the mobile and computer should be very dim whenever you use it 

d) Check up your eyes regularly

Q13. What is the angle of incidence of a Ray if the reflected ray is at an angle of 90 ° to the incident ray ? 

Answer- We know that angle of incidence is equal to angle of reflection 

And angle is twice of the incidence angle is equal to 90° 

Does the angle of incidence i= 90°/2 

                                i = 45 ° 

Q14. How many images of a candle will be formed if it is placed between two parallel plane mirror separated by 40 cm ? 

Answer- Infinite number of images are formed between two parallel plane mirrors separated by 40 centimeter

Q15. Two mirrors meet at right angles A Ray of light is incident on one at an angle of 30° as shown in figure 16.99 draw a reflected ray from the second mirror ?  

Answer- The first law of equation used to observed the path of reflected light .  It can be observed that the given Ray of light is reflected from the second mirror of 60 °

Q16. Boojho stands at A just on the side of a plane mirror as shown in figure 16.20 can he see himself in the mirror also can he see the image of the object situated at P,Q and R 

Answer- A cannot see his image because the length of the mirror is very short on his side we can see the image of object at P and  Q but cannot see the object at R

Q17. a)  Find out the position of the image of an object situated at A in the plane mirror  .

b) Can Paheli at B see  this image 

c) When Paheli moves from B to C where does the image of A move .

Answer-  a ) The image of A is formed behind the mirror at the same distance as that of A from the mirror 

b) Yes Paheli can see the image at B 

c) The image of A is unaffected by the moment of B and C so paheli will be able to see the image of a while moving from B to C