NCERT Solutions for Class 8 Science Chapter 13 Light

Q1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.

Answer:

No, we cannot see the objects inside the room but we might see the objects outside the room. An object is visible to us when light reaches it, gets reflected and reaches our eyes. Inside the darkroom there is no light present to be reflected from the objects and reach our eyes and therefore we won't be able to see them but if there is light present outside the room and if it is reflected from objects outside the room and if this light reaches our eyes then we would be able to see the objects outside the room.

Q2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Answer:

In regular reflection, the reflected rays are parallel to each other whereas in diffused reflection, the reflected rays are not parallel to each other. In diffused reflection, the laws of reflection are not violated. In the case of diffused reflection, the surface from which reflection takes place has microscopical irregularities which means the surfaces at the points where each ray gets reflected from are not parallel to each other.

Q3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

(a) Polished wooden table

Answer:

Regular reflection since the polished wooden table has a smooth surface.

Q3 (b). Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

Chalk powder

Answer:

Diffused reflection since chalk powder has an irregular or rough surface.

Q3 (c). Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

Cardboard surface

Answer:

Diffused reflection since cardboard surface is rough.

Q3 (d). Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

Marble floor with water spread over it

Answer:

Regular reflection since marble with water spread over it is a smooth surface.

Q3 (e). Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

(e) Mirror

Answer:

Regular reflection since mirror is a smooth surface.

Q3 (f). Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

Piece of paper

Answer:

Diffused reflection since a piece of paper is an irregular surface.

Q4. State the laws of reflection.

Answer:

The laws of reflection are:

(i) The angle of reflection is always equal to the angle of incidence.

(ii) The incident ray, the reflected ray and the normal at the point of incidence to the surface on which reflection takes place all 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 incidence lie in the same plane.

Answer:

Place a plane mirror on the table. Take a piece of paper and make a small hole in it. Ensure that the light in the room is dim. Hold the sheet normal to the table. Take another piece of paper and place it on the table in contact with the mirror. Draw a normal from the mirror on the paper kept on the table Now, light a torch on the mirror through the small hole such that the ray of light falls on the normal at the bottom of the mirror. When the ray from this hole is incident on the mirror, it gets reflected in a certain direction. You can easily observe the incident ray, reflected ray and the normal to the mirror at the point of incidence on the sheet placed on the table. This shows that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.

Q6. Fill in the blanks in the following.

(a) A person 1 m in front of a plane mirror seems to be _______________ m away from his image.
(b) If you touch your ____________ ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with ____________.
(c) The size of the pupil becomes ____________ when you see in dim light.
(d) Night birds have ____________ cones than rods in their eyes.

Answer:

(a) A person 1 m in front of a plane mirror seems to be 2 m away from his image.
(b) If you touch your left ear with the right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with the left hand .
(c) The size of the pupil becomes large when you see in dim light.
(d) Night birds have fewer cones than rods in their eyes.

Choose the correct option in Question 7

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

(a) Always (b) Sometimes (c) Under special conditions (d) Never

Answer:

The angle of incidence is always equal to the angle of reflection. This is the first law of reflection, therefore, option (a) is correct.

Choose the correct option in Question 8

Q8. Image formed by a plane mirror is

(a) virtual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of the same size as the object.
(c) real at the surface of the mirror and enlarged.
(d) real, behind the mirror and of the same size as the object.

Answer:

The image formed by a plane mirror is virtual, behind the mirror and of the same size as the object . Therefore option (b) is correct.

Q9. Describe the construction of a kaleidoscope.

Answer:

To make a kaleidoscope, get three rectangular mirror strips each about 15 cm long and 4 cm wide. Join them together to form a prism as shown in Fig. (a). Fix this arrangement of mirrors in a circular cardboard tube or tube of a thick chart paper. 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 you can see [Fig (b)]. To make the disc durable, paste a piece of the transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, fix a circular plane glass plate [Fig (c)]. Place on this glass plate several small pieces of coloured glass (broken pieces of coloured bangles). Close this end of the tube by a ground glass plate. Allow enough space for the colour pieces to move around.

Q10. Draw a labelled sketch of the human eye.

Answer:

The human eye consists of the retina, lens, cornea, optic nerve, iris, ciliary muscle

Q11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advice?

Answer:

Laser light can harm the eye if looked at directly because of its high intensity and therefore Gurmit's teacher advised her to not use it.

Q12. Explain how you can take care of your eyes.

Answer:

We can take care of our eyes by following the following steps:

(i) Keeping a distance of at least 25 cm between our eyes and a book while reading it.

(ii) Not looking at a laptop or TV screen for a long time.

(iii) Regularly cleaning our eyes.

(iv) Visiting an ophthalmologist 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:

The angle of incidence = angle of reflection. since the angle between the incident and reflected ray is 90 degrees. The angle of the incidents will be 90/2 =45 degrees

$$\begin{gathered} Angleofincidence=Angleofreflection \\ Angleofincidence+Angleofreflection={90}^{\circ } \\ 2\times Angleofincidence={90}^{\circ } \\ Angleofincidence={90}^{\circ }÷\text{2} \\ Angleofincidence=\text{4}{5}^{\circ } \end{gathered}$$

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

Answer:

An infinite number of images of the candle will be formed because when an object is placed between to parallel plane mirrors, an infinite number of images are formed.

Q15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 13.19. Draw the reflected ray from the second mirror.

Answer:

We use the first law of reflection to draw the path of the ray. It states the angle of incidence is equal to the angle of reflection.

Q16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 13.20. Can he see himself in the mirror? Also, can he see the image of objects situated at P, Q and R?

Answer:

Boojho cannot see himself in the mirror because of the short length of the mirror. He would be able to see objects at position P and Q but not at R.

Q17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 13.21).

(b) Can Paheli at B see this image?
(c) Can Boojho at C see this image?
(d) When Paheli moves from B to C, where does the image of A move?

Answer:

(a) The image of object A forms behind the mirror at a distance equal from the mirror equal to the distance between the object and the mirror.

(b) Yes. Paheli at B can see this image.

(c) Yes. Boojho at C can see this image.

(d) Image of the object at A will not move. It will remain in the same position when Paheli moves from B to C.