lightWHERE cd.courseId=12 AND cd.subId=16 AND chapterSlug='light' and status=1SELECT ex_no,page_number,question,question_no,id,chapter,solution FROM question_mgmt as q WHERE courseId='12' AND subId='16' AND chapterId='379' AND ex_no!=0 AND status=1 ORDER BY ex_no,CAST(question_no AS UNSIGNED) CBSE Class 8 Free NCERT Book Solution for Science

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Chapter 16 : Light


  • Light is reflected from all the surfaces.
  • Regular reflection takes place when light is incident on smooth, polished and regular surfaces.
  • Diffused/ irregular reflection takes place from rough surfaces.
  • Image formed in a plane mirror undergoes lateral inversion.
  • Beautiful patterns are formed in a kaleidoscope because of multiple reflection.
  • Spelling of light into seven colours are called dispersion.
  • Angle of incidence is the angle between the incident ray and the normal. It is denoted as (∠i).
  • Angle of reflection is the angle between the reflected ray and normal. It is denoted as (∠r).
  • The area on retina that does not have any light sensing nerves. This area have zero vision. It is known as blind spot.
  • Braille may be defined as the system for blind persons in which raised dots represent the letter. By this blind person can read and write by touching.
  • In retina, photoreceptor cells are present, are called as cones which are sensitive to light and colour.
  • The transparent part of the eye called as cornea which covers the most of the part of eye.
  • In plane mirror, generally the right part seems as left and left is seems as right, it is due to the phenomenon of lateral inversion.

Exercise 1 ( Page No. : 215 )
Q:
A:

We cannot see objects in the dark room due to the absence of the light. But we can see the objects outside the room due to the presence of light outside the room. The light falls on the objects and gets reflected. These reflected rays then reaches to out eyes which enables us to see the objects.


Exercise 1 ( Page No. : 215 )
Q:
A:

Difference between the regular reflection and the diffused reflection are as follows:

                              Regular Reflection

                                  Diffused Reflection

 When all the parallel rays reflected from a plane surface are parallel, the reflection is known as regular reflection.

 When all the parallel rays reflected from the surface are not parallel, the reflection is known as diffused reflection.

 The image formed by the regular reflection is sharp and clear.

 The image formed by the irregular reflection is either faint or  blurred.

 Regular reflection takes place on a smooth surface.

 Irregular reflection takes place on the round surface.

No, diffused reflection does not mean the failure of laws of reflection. The laws of reflection are also followed in diffused reflection.


Exercise 1 ( Page No. : 215 )
Q:
A:
  1. Polished wooden table: Regular reflection will take place because rays of light reflected from the polished wooden table (smooth surface) are parallel.
  2. Chalk powder: Diffused reflection will take place because rays of light reflected from the chalk powder are not parallel due to its round surface.
  3. Cardboard surface: Diffused reflection will take place because rays of light reflected from the cardboard surface; are not parallel due to its rough surface.
  4. Marble floor with water spread over it: Regular reflection will take place because rays of reflect light reflected from the marble surface/ floor spreads on it. Smooth surface are parallel.
  5. Mirror: Regular reflection will take place because rays reflected from the mirror are parallel.
  6. Piece of paper: Diffused reflection will take place because rays of reflection from the piece of paper will not parallel due to its rough surface.

Exercise 1 ( Page No. : 215 )
Q:
A:

There are two laws if reflection:

  1. The angle of incidence (∠i) is always equal to the angle of reflection (∠r).
  2. The incident ray, the normal at the point of incidence and the reflected ray, all lie in the same point.

Exercise 1 ( Page No. : 215 )
Q:
A:

The laws of reflection of light can be verified by the following activity:

experiment

  1. Pin a white sheet of plane (mirror) paper on a cardboard.
  2. Place a strip of plane mirror M’ on the plane paper.
  3. Pined a 2 pins P1 and P2. Obliquely in front of the plane mirror in a straight line so that the line joining the pins can make an angle of 30◦, 45◦ and 60◦ with the normal drawn at the point of incidence.
  4. See through the mirror from other side and observe the pins P1 and P2 seen on the mirror and pins P3 and P4 appears in the straight line.
  5. Remove the plane mirror and the pins. Encircle the points where pins were fixed on the paper.
  6. Join the P1 and P2 extended the line to meet at point O of the mirror. The ray AO so obtained is an incident ray.
  7. Similarly join P3 and P4 and extend the line to meet at point O of the mirror. The ray O so obtained is a reflected ray.
  8. Draw a normal OC.
  9. Measure the angle of incidence ∠I and the angle of reflection ∠r. We find that ∠i = ∠r

Conclusion: This activity shows that the mirror incident ray, the reflection ray and the normal at the point of incidence, all lie on the same plane. The angle of the incidence is also equal to the angle of reflection. Thus, this activity verifies the laws of reflection.  


Exercise 1 ( Page No. : 215 )
Q:
A:

(a) 2m

Plane mirror formed images eject as of the image. The distance between the object and image are same here. So, the images 1 + 1 = 2 m. away from the person.

(b) Left, left hand

This is due to the lateral inversion of images which is formed by the plane mirror.

(c) Large

The amount of light entering in the eyes is very little in dim light. Pupil expands to increase the light.

(d) Less

Night birds cannot see in day time because they have large number of rod cells and a few cone cells on their retina.


Exercise 1 ( Page No. : 215 )
Q:
A:

(a) Always

According to the first law of reflection, the angle of the incidence is always equal to the angle of reflection (∠i = ∠r).


Exercise 1 ( Page No. : 215 )
Q:
A:

(b) Virtual, behind the mirror and of the same size as the object

Plane mirror formed the image eject as the size of the object behind the mirror. Images formed by the plane mirror are virtual that is why they cannot be obtained on the screen.


Exercise 1 ( Page No. : 215 )
Q:
A:

A simple device is used to obtain the colorful designs by multiple reflections is called kaleidoscope.

Principle: It works on the principle of multiple reflection of light.

       experiment

Construction:

  1. Get three rectangular plane mirror strips, each about 15 cm long and 4 cm wide.
  2. Join them together to form a prism.
  3. Pin them in a cylindrical cardboard tube, and make sure that the tube is slightly longer than the mirror strips.
  4. Close the one end of the tube by a cardboard disc having a hole in its center through which you can see.
  5. At the other end, touching the mirrors fix circular glass plate.
  6. Place several small pieces of the coloured glass on this glass plate.
  7. Close (disc) this end of the tube by ground glass plate. Allow enough space for the coloured pieces to move around. This is kaleidoscope.

When seen through the hole, we will be able to see a variety of patterns in the tube.


Exercise 1 ( Page No. : 215 )
Q:
A:

eye


Exercise 1 ( Page No. : 215 )
Q:
A:

Gurmit’s teacher advised her not to do so because laser torch can injure the retina of her friends and she may loss her eye sight permanently or temporarily. The intensity of laser light is very high.


Exercise 1 ( Page No. : 215 )
Q:
A:

We should take care of our eyes properly. If there is any problem in the eyes then we should consult eye specialist.

  1. We should not read in the too much light or too little light.
  2. Never see sun or other powerful source of light directly.
  3. We should not rub the eyes if the dust or some other contaminants get falls into the eyes.
  4. Always read and write by keeping books at a normal distance for a vision.

Exercise 1 ( Page No. : 215 )
Q:
A:

angle

∠i + ∠r = 90◦

But, by the first law of reflection;

∠i = ∠r

∠i + ∠r = 90◦

∠i + ∠i = 90◦

2∠i = 90◦

∠i = 90◦ = 45◦                                                                                                                                                                                                                                              2


Exercise 1 ( Page No. : 215 )
Q:
A:

Infinite number of images will be formed due to multiple reflections of light. The number of images can be determined as;

solution


Exercise 1 ( Page No. : 215 )
Q:
A:

The first law of reflection is used to obtain the ray of reflected light.

          rays

Here, we can see that the given ray of light will reflect from the second mirror at an angle of 60 degree.

 


Exercise 1 ( Page No. : 215 )
Q:
A:

   rays

Boojho cannot see his image as he is not standing in front of the mirror. He can see the objects situated at P and Q as the light from these two points get reflected by the mirror and reaches his eyes. He cannot see the object at R.


Exercise 1 ( Page No. : 215 )
Q:
A:

rays

  1. This image of the object place at the A, will be formed behind the mirror and it will be at the same distance from the mirror as the object A is placed or present.
  2. Yes, Paheli at B can see the image as the light from B gets reflected by the mirror and reaches A.
  3. Yes, Boojho at C point can see the image as the light from C gets reflected by the mirror and reaches A.
  4. The image A will be stationary even if Paheli moves from B to C.

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