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17. Shraddha's parents took her grandfather to the eye specialist as he was 3
complaining of difficulty in seeing things. The doctor diagnosed it as cataract
and said that it is quite common in old age. What is cataract? How is it
caused? How can it be corrected?
18. A) The eyeball of a person has become slightly larger. Which kind of lens 5
should the person wear to correct the defect in the vision caused by this
change in the size of the eyeball?
B) The images formed by an ordinary convex lens suffer from a defect, called
chromatic defect, which leads to false coloured edges in the images. This
happens because light rays of different colours bend differently as they
enter and leave the lens.
If a parallel white light beam passes through a convex lens, the light of which
colour (among violet to red in the spectrum) will converge at a point closest
to the lens? Justify your answer.
(C) A person needs a lens of power -5.0 D for correction of his vision.
(i) What is the possible vision defect of the person?
(ii) What is the focal length of the corrective lens?
(iii) Draw a diagram of a human eye with this possible vision defect.
19. The image given below 5
indicates defective vision in
Ram.
(a) Based on the given
information, what could be
the nature of the eye defect?
(b) State the type of lens to
be used to correct his vision.
Justify your choice.
(c) The near point of Ram's
eye is 75 cm. Calculate the focal length and power of the lens he should use
while reading a book kept at a distance of 25 cm from the eye.
20. Everyone enjoys the spectacle of a rainbow glimmering against a dark 4
stormy sky. How does sunlight falling on clear drops of rain get broken into
the rainbow of colours we see? The same process causes white light to be
broken into colours bu a clear glass prism or a diamond. Sunlight,
considered to be white. actually appears to be a bit yellow because of its
mixture of wavelengths, but it does contain all visible wavelengths.
The sequence of colours in rainbows is the same sequence as the colours
plotted versus wavelength in figure. What this implies is that white light is
spread out according to wavelength in a rainbow.
(A) Identify the phenomena that play a role in the formation of rainbow.
(B) In which medium, the velocity of waves of all colours is same?
(C) When we go on a hill station, we generally see a strip of seven colours in
the sky. Explain the reason behind this strip.