January 27, 2015

10th Physical Sciences Imp Questions - Human Eye - New Syllabus

In morning and evening times except red light all colours scatter more
and vanish before they reach our eye. Since scatering of red light is
very small, it reaches us. So sun appears red in colour during sun
rise and sun set.

Q: How do you appreciate the working of ciliary muscle in the eye? (4 Marks)

A: The ciliary muscle to which eye lens is attached helps the eye lens
to change its focal length by changing the radii of curvature of the
eye lens.

When the eye is focussed on a distant object, the ciliary muscles are
relaxed so that the focal length of the eye lens has its maximum value
which is equal to its distance from the retina. The parallel rays
coming into the eye are then focussed on to the retina and we see the
object clearly. When the eye is focussed on a closer object, the
ciliary muscles are strained and focal length of eye-lens decreases.

The ciliary muscles adjust the focal length in such a way that the
image is formed on retina and we see the object clearly. This process
of adjusting focal length is called "accommodation". This is a
wonderful arrangement the nature provided in the eye. If this is not
provided we would not have lead the present comfortable life with
regard to our vision.

Q: Explain briefly the reason for the blue of the sky. (2 Marks)

A: When we look at the sky in a direction perpendicular to the
direction of the Sun rays the sky appears blue. If our angle of view
is changed, the intensity of blue colour also changes. Molecules of N2
and O2 are the causes for the blue colour of the sky.

The sizes of these molecules are comparable to the wavelength of blue
colour light. So these molecules act as scattering centres for the
blue colour of light. So the sky appears blue.

Q: How can you correct the defect 'hypermetropia'? (2 Marks)

A: Eye lens can form a clear-image on the retina when any object is
placed beyond near point. To correct the defect of hypermetropia, we
need to use a lens which forms an image of an object beyond near
point, when the object is better near point and least distance of
distinct vision. This is possible only when a double convex lens is

Q: Explain the basic process in scattering of light. (1 Mark)

A: Scattering of light is a complex phenomenon. Atoms or molecules
which are exposed to light absorb light energy and emit some part of
the light energy in different directions. This is the basic process
happens in scattering of light.

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