NCERT Solutions For Class 8 Science Chapter – 16
Q1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer:-
We can see any object, when light reflected by that objects reaches our eyes. But in the dark room, no light is reflected by the object so we are unable to see objects in dark room. If there is light present outside the the room, then we can 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:-
| Regular Reflection | Diffuse reflection |
| When all the parallel rays from a plane surfaces are parallel, the reflection is known as regular reflection | When all the parallel rays reflected from a plane surface are not parallel,the reflection is known as diffused or irregular reflection |
| It takes place from a smooth or regular surface. | It takes place from a rough surface. |
| Images are formed by regular reflection. | Images are not formed by irregular reflection. |
The diffused reflection is not due to the failure of laws of reflection but caused by irregularities in the reflecting surface.
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.
- Polished wooden table
- Chalk powder
- Cardboard surface
- Marble floor with water spread over it.
- Mirror
- Piece of paper
Answer:-
- Regular reflection – A polished surface is an example of smooth surface. A polished wooden table has a smooth surfaces Hence, reflections from the polished table will be regular.
- Diffused reflection – Chalk powder spread on a surface is an irregular surface. It is not smooth. Therefore, diffused reflection will take place from chalk powder.
- Diffused reflection.
- Regular reflection.
- Regular Reflection
- Diffused Reflection
Q4. State the laws of reflection.
Answer:-
Laws of reflection are:
The angle of reflection is always equal to the angle of incidence.
The incident ray,the reflected ray and the normal to the reflective surface at the point of incidence 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 paper sheet and make a small hole in its center. Make sure that the light in the room is not bright. Hold the sheet normal to the table. Take another sheet and place it on the table in contact with the vertical mirror. Draw a normal line on the second sheet from the mirror. 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 place on the table. This shows that the incident ray, the reflected rays, 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 person 1m in front of a plane mirror seems to be _________ m away from his image.
- If you touch your _______ ear with your right hand in front of a plane mirror , it will be seen in the mirror that your right ear is touched with your ___________.
- The size of the pupil becomes ______ when you see in dim light.
- Night birds have ______ cones than rods in their eyes.
Answer:-
- 2
- Left , Left hand
- Large
- Less
Q7. Angle of incidence is equal to the angle of reflection.
- Always
- Sometimes
- Under special conditions
- Never
Answer:-
- Always
Q8. Image formed by a plane mirror is
- Virtual,behind the mirror and enlarged
- Virtual,behind the mirror and f the same size as the object.
- Real at the surface of the mirror and enlarged.
- Real,behind the mirror and of the same size as the object.
Answer:-
- Virtual,behind the mirror and of the same size as the object.
Q9. Describe the construction of a kaleidoscope.
Answer:-
To make a kaleidoscope , get three rectangular mirrors strips each about 15 cm long and 4 cm wide. Join them together to form a prism as shown in Figure (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 . 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 . 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 labelled sketch of the human eye.
Answer:-
The human eye consists of retinae,lens,cornea,optic nerve,iris,ciliary muscle.
Q11. Gurmit wanted to perform Acitvity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher advice ?
Answer:-
Laser light harm the eye if looked at directly because of its high intensity and therefore Gurmit 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 steps:
- Keeping a distance of at least 25 cm between our eyes and a book while reading it.
- Not looking at a laptop or TV screen for a long time
- Regularly cleaning our eyes.
- Visiting an ophthalmologist regularly.
Q13. What is the angle of incidence of a ray if the reflected ray is at an angle of 900 to the incident ray ?
Answer:-
The angle of incidence = angle of reflection. Since the angle between the incident and reflected rays is 90 0 the angles of incidents will be 90/2 = 45 degrees
Angle of incidence = Angle of reflection
Angle of incidence + Angle of reflection = 900
2 x Angle of incidence = 900
Angle of incidence = 90/2
Angle of incidence = 450
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 300 as shown in 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 . Can he see himself in the mirror ? Also , can he see the images 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 image of an object situated at A in the plane mirror
(b)Can Paheli at B see this image ?
(c ) Can Boojho at C see this mages ?
( d) When Paheli moves from B to C where does the image of A move ?
Answer:-
- The image of objects A forms behind the mirrors 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.