Chapter 16: Light

Q1)

Imagine a person is in a dark room, will he be able to see any object which is kept inside the room and any other object which is kept out of the room? Elaborate.

Ans-

If a person is inside the room where there is no light, it is then impossible to visualize the object inside the room but the object out of the room can be seen easily.

When light falls on eyes after reflecting from the object, then an object becomes visible. If the room is dark, then the object which is in the room reflects no light. Hence, the person is not able to see in the room where there is no light. If the light is present out of the room, it is possible to see objects out of the room.

Q2)

Write down the difference between diffused and regular reflection. Have the laws of reflection failed in the case of diffused reflection?

Ans-

  

 

The laws of reflection have not failed because each ray obeys the law of reflection. As shown above the laws of reflection and diffusion are followed and the diagrams indicate that the path take is correct as per the laws – in regular reflection all the reflected rays are parallel to each other, where as in diffused reflection the rays aren’t parallel to the incident rays.

Q3)

Write along side the following statements and give reason to whether the reflection is regular or a diffused reflection and what will appear when light touches the surface.

a) Surface of the Cardboard –

b)Paper Pieces-

c) Polish wooden table –

d) Mirror –

e) Chalk powder –

f) Ceramic floor which has been mopped recently –

Ans-

a) Cardboard surface- Diffused reflection

The surface of the cardboard is a kind of irregular surface. Hence, diffused reflection will take place from a cardboard surface.

b) Bits of paper- Diffused reflection

Although a piece of paper may look smooth, but it has many irregularities on its surface. Because of this reason, it will give a diffused reflection.

c) Wooden table that has been polished-Regular reflection

Surface that has been recently polished can be a good example of smooth surface. Wooden table that has been polished has surface that is smooth.

d) Mirror- Regular reflection

A mirror has a very smooth surface. That is why it gives a regular reflection.

e) White Chalk powder that is used in school- Diffused reflection

Chalk powder spread on a surface is an example of an irregular surface. Hence, it is rough. Therefore, diffused reflection will appear from chalk powder.

f) Ceramic floor which has been mopped recently- Regular reflection

The ceramic floor which has been recently mopped can be a good example of surface that is regular. Because water makes the ceramic glossy. Thus, thereflections that are regular takes occurs on this surface.

Q4)

What is “law of reflection”?

Ans-

The law of reflection states that

a) The angle of reflection and the angle of incidence both arealways equal to one another.

b) The reflected ray,the incident ray, and the normal to the reflective surface at the point of incidence all comes on the same plane.

Q5)

Elaborate experiment to justify that the reflected ray, the incident ray, and the normal ray lies in the same plane at the point of incidence.

Ans-  

On a table place a plane mirror perpendicular to the plane of the table.Make a small hole in a paper and hold it perpendicular to the plane of the table. Try to do this experiment in a dark room. Take one more piece of paper and place it on the table so that it makes contact with the mirror. Draw a line perpendicular to the mirror on the piece of paper which is on the table. Now beam light rays with the help of a torch through the small hole such that the beam of light hits the normal at the bottom of the mirror. The ray of light will be reflected if the light rays from the hole are incident on the mirror. Looking at the piece of paper on the table, we can easily show that the incident ray, the normal line and the reflected ray at the point of incidence lie in the same plane.

 

Q6)

Fill the blanks with appropriate words:

a) When you see in the dull light, then the pupil’s size becomes____.

b) In plane mirror person seems to be___ m away, if he is standing 1m away.

c) Night birds have_____ cones than rods in their eyes.

d) If you hold your ______ ear with your right hand standing ahead of a plane mirror, it will be seen in the mirror that your right ear is holding with your _______.

Ans.)

a) When you see in the dull light, then the pupil’s size becomes large.

(A Very little amount of light entering into the eyes in dull light, pupil expands in order to increase the amount of light.)

b) In plane mirror person seems to be 2 m away, if he is standing 1 m away.

(From a plane mirror the image distance and the object distance are the same. The image of a person 1m in front of a mirror is 1m back to the mirror. Hence, the image is 1+1=2m away from the person)

c) Night birds have fewer cones than rods in their eyes.

(Night birds are not able to see during the day time but can able to see inthenight. They have a large number of rod cells and only a few cones on their retina.)

d) If you hold your left ear with your right hand standing ahead ofaplane mirror, it will be seen in the mirror that your right ear is held with your left hand.

(Due to the lateral inversion of image formed in a plane mirror)

Q7)

The angle of reflection and the angle of incidence are equal.

a) Sometimes b) always c) never d) Under special conditions

Ans-

b) The angle of incidence is always equal to the angle of reflection. This is the first law of reflection.

Q8)

An image formed by the plane mirror is

a) Virtual, behind the mirror and of the same size as the object.

b) Real, behind the mirror and of the same size as the object.

c) Virtual, behind the mirror and enlarged

d) Real at the surface of the mirror and enlarged.

Ans-

a) A virtual image is formed by the plane mirror, behind the mirror and of the same size as the object. The plane mirror formed an image is of the same size as the object. The image is formed behind the mirror. On a screen the image cannot be obtained and hence, it is a virtual image.

Q9)

Elaborate the construction of a kaleidoscope:

Ans-

The construction of a kaleidoscope:

Take three rectangular mirror strips of dimensions 15cm x 4cm (l x b) are joined together to form a prism as shown in (fig a). below. A prism is fixed into a circular cardboard tube. The circular cardboard tube should be slightly longer than the prism (fig b). This circular tube is now closed at one end with a cardboard disc. This disc has a hole in it through which we can see (fig c). At the other end of the circular tube, a plane glass plate is fixed. It is important that this glass plate touches the prism mirrors. On this glass plate, several small and broken pieces of colored glass are placed. This end is now closed by a round glass plate allowing enough space for the colored glass pieces to move (fig d).

Q10)

Draw and labeled the diagram of the human eye.

Ans-

 

 Q11)

Avatar wants to perform an activity using a laser torch but his teacher advised him not to do so. Can you explain the basis of the teacher’s advice?

Ans-

Because of the intensity of the laser light is very high, it is harmful to the human eyes. It can cause damage to the retina and leads to blindness. Hence, it is advisable not to look at a laser beam directly.

Q12)

How can Avatar take care of his eyes? Explain.

Ans-

The following points help a person to take care of his eyes:

a) Reading should not be done in bright light as well as in dim light.

b) He should visit an eye specialist on a regular interval of time.

c) If any small insects or a dust particles enters his eyes, do not rub them but clean them immediately with cold water.

d) He should avoid direct exposure of sunlight to the eye.

e) While reading, there should be a distance of at least 25 cm between the eyes and the book.

Q13)

If the reflected ray is at the angle of 90o to the incident ray, then what is the angle of incidence of a ray?

Ans-

If the reflected ray is at the angle of 90o to the incident ray, then the angle of incidence is 45o. According to the law of reflection, the angle of incidence and the angle of reflection are equal. Therefore, the angle of incidence and the angle of reflection both are 90/2=45o.

Q14)

If a candle is placed between two parallel plane mirror separated by 40 cm, how many images will be formed?

Ans-

If a candle is placed between two parallel plane mirror separated by 40 cm, then the multiple and infinite images will be formed due to the multiple reflections between the mirrors. The infinite numbers of images are formed when two mirrors are placed parallel to each other.

Q15)

Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30o as shown in dia. 16.19. Draw the reflected ray from the second mirror.

  

Ans-

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

It can be observed that the given ray of light will reflect from the second mirror at an angle 60o .

Q16)

David stands at A just on the side of the plane mirror as shown in fig. 16.20. Can he see himself in the mirror? Also, can he see the image of an object situated at P, Q, and R?

Ans-

Behind the mirror, a plane mirror forms a virtual image. The image is as far behind the mirror as the object is in front of it. A cannot see his image because the length of the mirror is too short on his side. However, he can see the object placed at points P and Q, but cannot see the object placed at point R (as shown in a diagram below)

 

Q17)

i) Find out the position of the image of an object situated at A in the plane mirror (fig. 16.21).

ii) Can Joy at B see this image?

iii) Can David at C see this image?

iv) When Joy moves from B to C, where does the image of A move?

Ans-

i) Image of an object placed at A is formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror. Image of A is shown in the given figure.

ii) Yes. Joy at B can see this image.

iii) Yes David at C can see this image.

iv) Image of the object at A will not move. It will remain at the same position when Joy moves from B to C.