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## Friday, February 19, 2016

### class8 light and vision (Refraction and eyes)

Class 8 light and vision (reflection and eye) leanwell science/Basic science

Q. Why are multiple images are formed when two mirror are placed at an angle to each other ?

Ans: This happen because image formed by first mirror act as an object for 2nd mirror.

No of image = (360/q) -1[0 degree or Parallel to each other --> Infinite number]

Q. What do you mean by persistence of vision?

Ans : Image of an object retain on retina for 1/16 th of a sec even after removed is called persistence of vision. This form the basis of the principle of cinematography.

Q. How persistence of vision form basis of the principle of cinematography?

Ans: in moving cinema, we take still pictures. The sequence of still pictures are projected on screen at rate of 24 images per second (persistence of vision is 1/16th of sec.)The successive images merge smoothly and give us feeling of movie.

Q. A person whose retina is damaged cannot see why?

Ans: This happens because retina acts as seen on which image is formed by eye lens.

Q. Does diffused reflection means failure of the law of reflection?

Ans: NO. Law of reflection apply to all reflecting surfaces whether plane or curved.

Q. Why are you not able to see complete rainbow at all places?

Ans: this is because rainbow is formed due to dispersion of light through water droplets in atmosphere.

Q. Why are red signals used in traffic signals to stop vehicles?

Ans: This is because red light scattered least through atmosphere due to longer wave length and seen from distance.

Q. When two prisms are placed in succession we can get white light back. Why?

Ans: This happen because the dispersed colour of first prism when pass through the inverted prism converge back into white.

Q. An object is placed in front of a plane mirror. If the mirror is moved away from the object through a distance x, by how much distance will the image move?

Ans: 2x meter

Q. A concave mirror is made up by cutting a portion of a hollow glass sphere of radius 30 cm. Calculate the focal length of the mirror.

Ans: f = r/2 = 15 cm

Q. An object 0.5 m tall is in front of a plane mirror at a distance of 0.2 m. what is the size of the image formed

And: Hi = Ho = 0.5m

Q. Write uses of concave mirror?

Ans: Concave mirrors are used in torches, search lights and head lights of vehicles to get parallel beams of light. They are used as shaving mirrors to see larger image of the face. They are used by dentists to see larger images of the teeth. Large concave mirrors are used to concentrate sunlight to produce heat in solar furnaces

Q.Why does convex mirror used as rear-view mirrors in vehicles?

Ans: Convex mirrors give erect diminished images of objects. They also have a wider field of view than plane mirrors.

Q. Why concave mirrors are used In torches, search lights and headlights of vehicles?

Ans: In torches, search lights and headlights of vehicles the bulb is placed very near to the focus of the reflector to get parallel beams of light.

Q. What is the angle of incidence, when a ray of light falls on the spherical mirror from its centre of curvature?

Ans: The angle of incidence is zero, when a ray of light falls on the spherical mirror from its centre of curvature.

Q. What is Astigmatism?

Ans: Astigmatism is a condition in which the surface of the cornea is not spherical, it causes a blurred or slightly twisted image to be received at the retina. It is quite common to have only one eye affected, whilst the other is normal.

Q. Choose correct one

1) A stick in water appears broken due to

(a) Reflection of light
(b) Refraction of light
(c) total internal reflection
(d) none of these

Ans: (b) Refraction of light

2) Refractive index of diamond with respect to vacuum is 2.5, then the velocity of light in diamond is.

(a) 25×10^8 m/s
(b)2.5×10^8m/s
(c)1.2×10^8m/s
(d)2.1×10^8m/s

Ans: (c)1.2×10^8m/s

3) Lateral displacement is directly proportional to:

(a) Angle of incidence
(b) angle of refraction
(c) Thickness of the slab
(d) velocity of light

Ans: (a) Angle of incidence  and (c) Thickness of the slab

4) A drop of water appears like pearl due to

(a) refraction
(b) total internal reflection
(c) reflection
(d) none

Ans: (b) total internal reflection

5) Myopia can be corrected using

(a) concave lens
(b) convex lens
(c) cylindrical lens
(d) Plano convex lens

Ans: (a) concave lens

CBSE NCERT Formative Assessment : MCQ ( Reflection and refraction)

8th Physics: Light and Our Eyes links

Class 8 light and vision Solved CBSE Test paper - 01           Download File

Class 8 light and vision Solved CBSE Test paper - 02           Download File

Class 8 light and vision Solved CBSE Test paper -03            Download File

Class 8 light and vision Solved CBSE Test paper - 04           Download File