Download Q-1 what is cornea? Ans- cornea is a thin membrane through which

Q-1 what is cornea?
Ans- cornea is a thin membrane through which the light enters the eye. It forms the
transparent bulge on the front surface of the eyeball.
Q-2 what forms the transparent bulge on the front surface of the eyeball?
Ans- cornea
Q.3 what is an average diameter of the eyeball?
Ans – 2.3 cm (23mm)
Q.4 where does most of the refraction of light in eye takes place?
Ans – most of the refraction for the light rays entering the eye occurs at the outer
surface of the cornea.
Q.5 what is the function of crystalline lens?
Ans – the crystalline lens merely provides the finer adjustment of focal length
required to focus objects at different distances on the retina.
Q.6 what is iris?
Ans – iris is a dark muscular diaphragm, found behind the cornea, which controls the
size of the pupil.
Q. 7 what is the function of pupil?
Ans – the pupil regulates and controls the amount of light entering the eye.
Q.8 what is retina?
Ans – the eye lens forms an inverted real image of the object on the retina. Retina is a
delicate membrane having enormous number of light sensitive cell which get
activated upon illumination and generate electrical signals.
Q.9 what is the function of optic nerves?
Ans –the signals generated by the retina are sent to the brain via the optic nerves.
Q.10 the image formed on the retina is inverted. Then why do we see an erect image
of our surroundings?
Ans – when the brain interprets the image, it again inverts the image formed on the
retina, to make the objects visible in the same way as they are in our surroundings.
Q -11 how is the focal length of eye lens changed?
Ans - the focal length of eye lens is changed by changing of the curvature of the eye
lens by ciliary muscles.
- When the ciliary muscles relax, the lens becomes thin and its focal length
increases.
- When the ciliary muscles contract, the curvature of lens increases and its
focal length decreases.
Q.12 why can we see both distant objects as well as nearby objects clearly?
Ans – when we are looking at distant objects, the ciliary muscles relax to decrease the
curvature of eye lens which increases the focal length of the lens. This enables us to
see distant objects clearly.
-
When we are looking at closer objects, the ciliary muscles contract to
increase the curvature of the eye lens which decreases the focal length.
This enables us to see nearby objects clearly.
Q.13 what is accommodation?
Ans – the ability of the eye lens to adjust its focal length is called accommodation.
Q.14 what do you mean by least distance of distinct vision?
Ans – the minimum distance, at which objects can be seen most distinctly without
strain, is called the least distance of distinct vision, or near point of the eye.
Q -15 what is the near point for a normal eye?
Ans – for a young adult with normal vision, the near point is about 25 cm (.25 m)
Q.16 what is far point of the eye?
Ans – the farthest point upto which the eye can see objects clearly is called the far
point of the eye. It is infinity for a normal eye.
Q.17 what is cataract?
Ans. Cataract is a condition of partial or complete loss of vision. This generally
happens to people at old age when the crystalline lens becomes milky and cloudy.
Q.18 explain the three main refractive defects of vision.
Ans. 1) Myopia (near-sightedness): a person with myopia can see nearby objects
clearly but cannot see distant objects distinctly.
2) Hypermetropia (far-sightedness): a person with hypermetropia can see
distant objects clearly but cannot see nearby objects distinctly.
3) Presbyopia: a person suffering from presbyopia can neither see nearby
objects clearly, nor he/she can see distant objects distinctly.
Q.19 where is the far point of a person suffering from myopia?
Ans. A person with myopia has the far point nearer than infinity.
Q.20 where is the image formed when a person has myopia?
Ans. In a myopic eye, the image of a distant object is formed in front of the retina.
Q.21 what can be the cause of myopia?
Ans. 1) Excessive curvature of the eye lens, or
2) Elongation of the eyeball.
Q.22 how can myopia be corrected?
Ans – myopia can be corrected by using a concave lens of suitable power.
Q.23 how does a concave lens correct myopia?
Ans. A concave lens of suitable power will bring the image back on to the retina (by
enlarging the image) and thus the defect is corrected.
Q.24 draw a diagrams for correction of (1) myopia (2) hypermetropia.
Ans. (refer to ncert pg 189 – 190)
Q.25 where is the near point of a person suffering from hypermetropia?
Ans. A person with hypermetropia has the near point beyond 25 cm.
Q.26 where is the image formed when a person has myopia?
Ans. In a myopic eye, the image of a close by object is formed behind the retina.
Q.27 what can be the cause of hypermetropia?
Ans. 1) The focal length of the eye lens is too long, or
2) The eyeball has become too small.
Q.28 how can hypermetropia be corrected?
Ans – myopia can be corrected by using a convex lens of suitable power.
Q.29 how does a convex lens correct hypermetropia?
Ans. A convex lens of suitable power will bring the image back on to the retina (by
providing additional focussing power) and thus the defect is corrected.
Q.30 how does presbyopia arise?
Ans. It arises due to the gradual weakening of the ciliary muscles and diminishing
flexibility of the eye lens.
Q. 31 how is presbyopia corrected?
Ans. Presbyopia is corrected by using a bi-focal lens, in which the upper portion
consists of a concave lens (to facilitate distant vision), and the lower part is a convex
lens (to facilitate near vision).
Q.32 for parallel refracting surfaces, as in a glass slab, the emergent ray is equal to the
___ ray.
Ans. Incident
Q.33 what is a prism?
Ans – a prism is a three dimensional shape with two triangular bases and three
rectangular lateral surfaces.
Q.34 what is a glass prism?
Ans – a glass prism is a 3d transparent structure made up of glass, with two triangular
bases and three rectangular lateral surfaces.
Q-35 what do you mean by angle of the prism?
Ans. Angle of the prism is the angle between its two lateral faces.
Q-36 what do you mean by angle of deviation?
Ans. The angle made by the emergent ray with the direction of the incident ray is
called angle of deviation.
Q-37 what is spectrum?
Ans – the band of the coloured components of a light beam is called its spectrum?
Q. 38 what is vibgyor?
Ans. Vibgyor is an acronym for the seven colours of a rainbow, in order1 – violet,
indigo, blue, green, yellow, orange and red.
Q.39 what is dispersion of light?
Ans. The splitting of lights into its component colours is called dispersion.
Q.40 which ray bends the least and which ray bends the most in a prism?
Ans the red light bends the least while the violet the most.
Q.41 who first used glass prism to obtain the spectrum of sunlight?
Ans. Isaac Newton
Q.42 how did Newton come to know that sunlight is made up of seven colours?
Ans . -Newton first used a glass prism to obtain the spectrum of sunlight.
-he tried to split the colours further by placing another prism.
- However he could not get further colours. So he placed a second identical
prism in an inverted position with respect to the first prism.
-he found a beam of white light emerging from the other side of the second
prism.
- This observation gave Newton the idea that the sunlight is made up of seven
colours.
Q.43 give an example of a natural spectrum.
Ans. Rainbow
Q.44 how is rainbow caused?
Ans. Rainbow is caused by the dispersion of sunlight by tiny water droplets present in
the atmosphere.
Q. 45 what are the phenomena involved in the formation of rainbow?
Ans. 1) Dispersion of light
2) Internal reflection.
Q. 46 explain the formation of rainbow.
Ans. - the water droplets in the atmosphere act like small prisms.
- They refract and disperse the incident sunlight, then reflect it internally,
and finally refract it again when it comes out of the raindrop.
- Due to the dispersion of light and internal reflection, different colours
reach the observer’s eye, and the observer can see a rainbow.
Q.47
Ans.
why do the objects seen through air above fire appear flickering?
- The air above the fire is hotter than surrounding air.
- The hotter air is lighter(less dense) than the cooler air above it, and has a
refractive index slightly less than that of the cooler air above it.
- Since the physical conditions of the refracting medium (air) are not
stationary, the apparent position of the objects, as seen through the hot air,
fluctuates.
Q.48 name some phenomena associated with atmospheric refraction.
Ans.
1) Twinkling of stars
2) Wavering of objects seen through a turbulent stream of hot air.
3) Advance sunrise and delayed sunset.
Q.49 why does the star appear slightly higher than its actual position?
Ans. - the starlight, on entering the earth’s atmosphere, undergoes refraction
continuously before it reaches the earth.
- Since the atmosphere bends starlight towards the normal, the apparent
position of the star is slightly different from its actual position.
- So the star appears slightly above than its actual position when viewed near
horizon.
Q.50
Ans.
why do stars twinkle?
- Since the stars are very distant, they approximate point-sized sources of light.
- As the path of rays of light coming from the star goes on varying slightly
due to change in physical atmospheric conditions, the apparent position of
the star fluctuates and the amount of starlight entering the eye flickers.
- So the star appears to twinkle.
Q.51 why don’t planet twinkle like stars?
Ans. - The planets, as compared to stars, are much closer to the earth, and are thus
seen as extended sources of light.
- if we consider a planet as a collection of a large number of point-sized
sources of light, the total variation in the amount of light entering our eye from all the
individual point-sized sources will average out to be zero, thereby nullifying the
twinkling effect.
Q.52 why does sun appears to rise 2 minutes early than actual sunrise, and why does
sunset appears to take place 2 minutes after the actual sunset?
Or
Explain the cause of advance sunrise and delayed sunset.
Ans. in the early morning, before the sun actually crosses the horizon, the
sunrays reach us due to atmospheric refraction. So the sun appears to rise 2 minutes
earlier than it actually rises.
- Similarly in the evening when the sun crosses the horizon, the sunrays still
reach us for 2 minutes due to atmospheric refraction. So the sun appears to
set 2 minutes late than the actual sunset.
Q.53 state some phenomena associated with the scattering of light.
Ans. 1) the blue colour of the sky
2) Colour of water in deep sea
3) Reddening of the sun at the sunrise and sunset
Q.54 what is Tyndall effect?
Ans. The phenomenon of scattering of light by the colloidal particles gives rise to
Tyndall effect.
Q.55 what causes scattering of light in atmosphere?
Ans. Smoke, tiny water droplets, suspended particles of dust and molecules of air
cause scattering of light in the atmosphere.
Q.56 why can we see Tyndall effect when sunlight passes through a canopy of a
dense forest?
Ans. When sunlight passes through a canopy of a dense forest, tiny water droplets
in the mist scatter light. So we can see Tyndall effect.
Q.57 how does the colour of the scattered light depend on the size of the scattering
particles?
Ans. - very fine particles scatter mainly blue light while particles of larger size
scatter light of longer wavelengths.
- if the size of the scattering particles is large enough, then, the scattered light
may even appear as white.
Q.58 why is the colour of the sky blue?
Ans. -the molecules of air and other fine particles in the atmosphere have size
smaller than the wavelength of visible light.
- so these are more effective in scattering light of shorter wavelengths at the
blue end than light of longer wavelengths at the red end.
-thus, when sunlight passes through the atmosphere, the fine particles of air
scatter the blue colour (shorter wavelengths) more strongly than red. So the sky
appears to be blue in colour.
Q.59 the sky appears dark to passengers flying at very high altitudes, as scattering is
not prominent at such heights.
Ans. -at very high altitudes, the scattering of light is not prominent.
- So the actual dark colour of sky is seen by the passengers.
- But on the land, the scattering of blue light causes the sky to appear blue.
Q.60why are ‘danger’ signal lights red in colour?
Ans. -the red is least scattered by fog or smoke.
- Therefore ‘danger’ signals are red in colour so that they can be seen in the
same colour at a distance.
Q.61 why does the sun appear reddish during sunrise and sunset?
Ans. -during sunrise and sunset the sun is near the horizon.
-so the light from the sun passes through thick layers of air and larger distance
in the earth’s atmosphere before reaching our eyes.
-therefore most of the blue light and shorter wavelengths are scattered away by
the particles, and the light that reaches our eyes is of longer wavelengths.
- This gives rise to the reddish appearance of the sun.
Q. 62 why does not the sun appear reddish during noon?
Ans - at noon the sun is overhead.
- So the light travels shorter distance and appears white as only a little of blue
and violet colours are scattered.