Date of conduct: - 24/01/15
CLASS-X
Science- Answer Key
Date of Submission: - 28/01/15
1.
2.
3.
4.
5.
6.
4D
Iris
Bi-focal lens
Glass prism
Violet
A ray of white light refract into seven different colours while passing through a glass prism.
Dia:-
7. In glass slab, refracting surface are parallel to each other, hence all the colour of white light refract
at same angle, hence don’t produce spectrum.
8. A lens formed by the combination of two or more lenses is known as a composite lens. The
reciprocal of focal length of composite lens is equal to the sum of the reciprocal of the focal length
of individual lenses. The power of lens combination is equal to the sum of the powers of the
individual lenses.
9. Lens ‘X’ because the converging power of a lens vary inversely with the focal length of the lens.
10.The powers of lens depend upon the radius of curvature and optical density of surrounding
medium as well as the colour/wave length of incident light. The focal length of concave mirror will
remain unchanged but the focal length of convex lens will change.
11.Size of object (h)= 7 cm
Distance of object (u) =27 cm
Power of lens (p) =-0.5 D
∴ Focal length (f) = +100 /P=-1000/5 =200 cm
Using sign conventions of concave lensh=7 cm
u=-27 cm
f=-200 cm
Using lens formula1/f= 1/v - 1/u
1/-200= 1/v + 1/27
Or 1/v = -1/200 – 1/27
= (-27 -200) /5400= -227/5400
Or u = 5400/-227= -23.78 cm
The image of object will be formed 23.78 cm in front of it.
Also m= v/u= 5400/-227X -27= 0.88
‘+ ve ’ sign of ‘m’ shows image formed is virtual and erect.
Again m= h1/h
0.88 X 7= h1
Or size of image (h1)=6.16 cm
12. Convex Lens
Concave Lens
i. It is thick in the middle and thin at edge.
It is thin in middle and thick at edges.
ii. It is convergent in nature.
It is divergent in nature.
iii. it can form real as well as virtual image.
It form virtual image only.
13. The far point of defected eye (u) =-150 [∵ of sign conv.]
Far point of normal eye (u) = -∾
Using lens formula1/f= 1/v - 1/u
1/f= -1/150 + 1/∾
Or f=-150 cm
∴ p=100/-150 = -0.67D
The corrective lens required is concave lens and power of corrective lens is 0.67D.
14. During sun set, sun is just near horizon and when rays of light coming from sun passes through
atmosphere, it under go atmospheric refraction due to which sun appear at its apparent position
which is above then the horizon, hence we can see it even after it has actually set.
15. The sun light passing through earth’s atmosphere, scattered by the atmospheric particles the colour
of sun light having smaller wave length like violet, indigo and blue scatter more than the colour of
light having longer wave length and human eye is more sensitive for blue colour, hence sky appear
blue to us.
16. P1 = 4.5 D
P2 = -3 D
∴ P= P1+ P2 = 4.5 -3=1.5 D
∴f=100/1.5 =1000/15= 200/3 cm
Size of object (h) = 5 cm
Distance of image (v)=75 cm
Using sign convention of convex lens
h= 5 cm
v= 75 cm
f= 200/3 cm
Using lens formula1/f= 1/v - 1/u
3/200= 1/75 -1/u
Or 1/u = 1/25 -3/200
= (8-9) / 600= -1/600
Or u= -600 cm
∴ Object should be placed 600 cm or 6 m in front of it.
Also h1=(v\u)X h
h1=(75/-600)X 5
=-25/40=-0.625
∴ Size of image is 0.625 cm
17. Distance of object (u)= - 60 cm [ ∵ of sign conv.]
Distance of image (v) = 120 cm
Height of object is (h) = 5 cm
Using lens formula1/f = 1/v – 1/u
1/f = 1/120 + 1/60 = (1+2)/120 = 3/120
Or f= 40 cm
Now h1=v\u h= (120/-60)X 5 = -10 cm
∴ Size of image is 10 cm.
18. Magnification (m)= -4 [ ∵ of real image]
Distance of image (v)= 60 cm
Using m= v/u
–4 = 60/u
Or u= 60/-4=-15 cm.
Using lens formula1/f = 1/v-1/u
i/f= (1/60)+ (1+15)= (1+4)/60= 5/60
∴ f= 60/5=12 cm.
Also p= 100/f=100/12=8.33 D
19.
20. The focal length of convex lens will increase, because wave length of red light is more than the
blue light hence it refract least and meet the principal axis at large distance from optical centre.