Synthetic fibres and plastics

CLASS-VIII

SYNTHETIC FIBRES AND PLASTICS

ASSIGNMENT-7

Q1. Fill in the blanks:

a)     Synthetic fibres are also called _______________ or ______________ fibres.

b)    Terycot is a mixture of _________________ and _______________.

c)     ___________ is used for making bottles of soft drinks, oils etc.

d)    ______________ was the first man-made plastic.

e)     Thermosetting plastics have a __________________ arrangement of molecules.

f)      Teflon is a ________________ plastic.

g)     _______________ is a flame resistant plastic.

Q2. Match the following:

1.     Terylene                          a) Prepared by using wood

                                                  pulp

2.     Teflon                              b) Used for making

                                                  parachutes and stockings

3.     PET                                 c)  Fabrics do not wrinkle

                                                   easily

4.     Rayon                              d) Used to make non-stick

                                                  cookwares

5.     Nylon                              e)  Used for making magnetic

                                                   tapes

Q3. Synthetic fibres are not considered environment friendly. Why?

Q4. What do you understand by high tensile strength?

Q5. Define polymerization.

Q6. Why synthetic fibres are used for making dress materials?

Q7. a) What is the other name of artificial silk?

       b) How is it prepared?

       c) Why is it also called regenerated fibre?

Q8. Explain the difference between:

a)     thermoplastic and thermosetting plastic

b)    biodegradable and non-biodegradable materials

Q9. Write 3 uses of melamine.

Q10. Why is Teflon used to make non-stick cookwares?

Q11. Explain why the following are made of thermosetting plastics:

a)     Saucepan handles

b)    Electric plugs/ switches/ plug boards

Q12. Why is melamine coated on uniform of fireman?

Q13. Why electrical wires have plastic covering?  OR 

         Why handles of screw drivers are made of plastic?

Q14. Explain why plastic containers are favoured for storing food?

Q15. Write any 2 measures that can be taken to control the damage caused by plastic waste.

Sound

CLASS-VIII

SOUND

ASSIGNMENT-6

Q1. Fill in the blanks:

a)     Time taken by an object to complete one oscillation is called______________.

b)    Loudness is determined by the____________________ of vibration.

c)     The unit of frequency is____________.

d)    Unwanted sound is called_____________.

e)     Shrillness of a sound is determined by the_______________ of vibration.

f)      The three tiny bones present in the ear are_____________,____________ and______________.

g)     The effect of sound stays on the ear drum for about_________.

h)     __________is the minimum distance to hear an echo.

i)       Noise is produced by_______________ vibrations.

j)       For human ears the audible range is ___________ to ____________.

Q2. In which medium sound travels fastest: air, water or steel?

Q3. Name the type of waves which are used by astronauts to communicate with another on moon (or in outer space).

Q4. Which property of sound leads to the formation of echoes?

Q5. Choose the infrasonic waves and ultrasonic waves from the following frequencies:

a)     30,000 Hz

b)    18 Hz

c)     50,000 Hz

d)    10 Hz

Q6. Which of the following sound frequencies cannot be heard by a human ear?

a)     10 Hz

b)    100 Hz

c)     10,000 Hz

d)    15 Hz

e)     40,000 Hz

Q7. What type of sound waves is produced by a vibrating simple pendulum?

Q8. What is the name of the device which is used to find the depth of sea (or ocean) by using ultrasonic sound waves?

Q9. Name two animals which can produce ultrasonic waves.

Q10. Name the three characteristics of sound.

Q11. Name the characteristic which helps us distinguish between a man’s voice and a woman’s voice, even without seeing them.

Q12. A cricket ball is seen to hit the bat first and the sound of hitting is heard a little later. Why?

Q13. When we put our ear to a railway line, we can hear the sound of an approaching train even when the train is far off but its sound cannot be heard through the air. Why?

Q14. On which day, a hot day or a cold day, an echo is heard sooner? Give reason for your answer.

Q15. Why should we not put a pin or a pencil in our ears?

Q16. What is the function of 3 tiny bones in the middle ear?

Q17. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Q18. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Q19. A man standing 825 metres away from a cliff fires a gun. After how long will he hear its echo? Speed of sound in air is 330m/s.

Q20. The echo of a sound is heard after 3seconds. If the speed of sound in air be 330m/s, calculate the distance of the reflecting surface.

Q21. What is meant by the quality of sound?

Q22. The quality of sound produced by men, women and children is different. Why?

Q23. Explain why, if we strike a tabla lightly , we hear a soft sound but if we hit the tabla hard, a lound sound is heard.

Q24. What is the difference between noise and music? Can music become noise sometimes?

Q25. List sources of noise pollution in your surroundings.

 

Numericals based on concave/convex lens and power

CLASS-X

LIGHT

ASSIGNMENT-3­

Q1. A 4cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 20cm. If the distance of the object is 30cm from the lens, find the position, nature and size of the image. Also find its magnification.                                                  

Ans: v=60cm, h_i= -8cm, inverted, m= -2

Q2. A needle placed at 45cm from the lens forms an image on a screen placed 90cm on the other side of the lens. Identify the type of the lens and determine its focal length.

Ans: f= +30cm, convex lens

Q3. An object 2cm high is placed at a distance of 64cm from a white screen. On placing a convex lens at a distance of 32cm from the object it is found that a distinct image of the object is formed on the screen. What is the focal length of the convex lens and size of the formed on the screen?

Ans: f= 16cm, h_i= -2cm (same size, inverted)

Q4. A 10cm tall object is placed perpendicular to the principal axis of a convex lens of focal length 30cm. The distance of the object from the lens is 20cm. Find the position, nature and size of the image formed.

Ans: v= -60cm, h_i= +30cm, erect

Q5. One-half of a convex lens is covered with a black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.

Q6. A concave lens of focal length 20cm forms an image at a distance of 10cm from the lens. What is the distance of the object from the lens?

Ans: u= -20cm

Q7. Find the position and nature of image formed when an object is placed at a distance of 5cm from a concave lens of focal length 15cm. What is its magnification?

Ans: v= -3.75cm, virtual and erect, m= 0.75

Q8. An object located 32cm in front of a lens forms an image on a screen 8cm behind the lens.

a)     Find the focal length of the lens.

b)    Determine its magnification.

c)     Is the lens converging or diverging?

Ans: f= 6.4cm, m= -1/4, convex lens

Q9. An object 60cm from a lens gives a virtual image at a distance of 20cm in front of the lens. What is the focal length of the lens? Is the lens converging or diverging?

Ans: f= -30cm, lens is diverging or concave

Q10. Find the power of a concave lens of focal length 2m.

Ans: P= -0.5D

Q11. A doctor has prescribed a corrective lens of power +1.5D. Find the focal length of the lens. Is the prescribed lens converging or diverging?

Ans: f= +0.67m, converging

Q12. A concave lens of power -2D is placed in contact with a convex lens of power +1.5D. What will be the resultant power?

Ans: P= -0.5D

Q13. A lens has a power of -0.25D. Identify the type of lens and what will be the focal length?

Ans: Concave, f= -4m

Q14. 3lenses of powers 2D, -0.25D, 1.75D are placed in contact. Calculate the net power of the arrangement. Will it produce a converging or a diverging effect? Will the net power change if the order of lens placement is changed?

Ans: P= +3.5D, converging effect, No

Q15. A convex lens forms a real and inverted image of a needle at a distance of 50cm from it. Where is the needle placed in front of the convex lens if the image is equal to the size of the object? Also find the power of the lens.

Ans: 50cm, P= 4D

Light-Refraction

CLASS-X

LIGHT

ASSIGNMENT-2

Q1. Define Snell’s Law of refraction.

Q2. Refractive Indices of media A, B, C, D are given below:

         Media                   Refractive Index

           A                            1.33

           B                            1.44

           C                            1.52

           D                            1.65

    In which of these four media is the speed of light

i)                   maximum

ii)                 minimum

Q3. Refractive index of a medium with respect to air is always greater than 1. Explain why?

Q4. The velocity of light in air is 3 x 10 m/s and velocity of light in glass slab is 2 x 10 m/s. Find refractive index of glass.

Q5. The refractive index of diamond is 2.42. What is the meaning of this statement?

Q6. No refraction of light occurs when light falls perpendicularly to the boundary separating two media of different refractive index. Explain. Why?

Q7.State 3 factors on which refractive index of a medium depends.

Q8. If refractive indices of water and alcohol are 1.33 and 1.36 respectively.

      Which is optically denser medium?

Q9. A ray of light falls obliquely on a glass slab. The perpendicular distance between the emergent ray of light and the original path of the incident ray is 2cm. What is the lateral shift of the ray of light?

Q10. State 3 factors on which lateral shift of the emergent ray depends.

Light

CLASS-X

LIGHT

ASSIGNMENT-1

Q1. A parallel beam of light is made to fall on a concave mirror. An image is formed at a distance of 7.5cm from the mirror. What is the focal length and radius of curvature of the mirror?

Q2. A real image, 1/5^th the size of the object is formed at a distance of 18cm from a mirror. What is the nature of the mirror? Calculate its focal length also.

Q3. An object is placed at a distance of 5m from a convex mirror of radius of curvature 20cm. Where is the image formed and what is its nature?

Q4. We wish to obtain an erect image of an object, using a concave mirror of focal length 15cm.

a)     What should be the range of distance of the object from the mirror?

b)    What is the nature of the image?

c)     Is the image larger or smaller than the object?

Q5. The magnification produced by a plane mirror is +1. What does this mean?

Q6. An object 2cm high is placed at a distance of 16cm from a concave mirror which produces a real image 3cm high.

a)     Find the position of the image.

b)    What is the focal length of the mirror?

Q7. An object is placed at a distance of 15cm from a convex mirror of focal length 30cm. Find the position and nature of the image.

Q8. A convex mirror has a focal length of 20cm. Find the position of the object for which the image will be half the size of the object.

Q9. A concave mirror of focal length ‘f’ produces an image ‘n’ times the size of the object. What would be the object distance for which the image is real?

Q10. Why do we prefer a convex mirror as a rear-view mirror in vehicles?