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LIGHT NOTES CLASS 8
Q 1:- Differentiate between regular and diffused reflection. Does diffused reflection mean the failure
of the laws of reflection?
Answer :
Regular reflection
Diffused reflection
When all the parallel rays reflected from a When all the parallel rays reflected from a plane
(1.) plane surface are parallel, the reflection is surface are not parallel, the reflection is known as
known as regular reflection
diffused or irregular reflection
Reflection from a smooth surface like that
of a mirror is an example of regular
(2.)
reflection Images are formed by regular
reflection.
Reflection from a rough surface, like that of a
cardboard, is an example of irregular reflection. It
is caused by the irregularities in the reflecting
surface .
(3.) Images are formed by regular reflection.
Images are not formed by irregular reflection.
Q 2 :- State the laws of reflection.?
Answer : There are two laws of reflection as given below :
1.
(i) The angle of incidence is equal to the angle of reflection.
2.
(ii) Incident ray, reflected ray and the normal drawn at the point of incidence to the
reflecting surface, lie in the same plane
Q 3 :- Explain how you can take care of your eyes.?
Ans :- We can take proper care of our eyes in following ways :
1) In case of any problem we should go to an eye specialist. Beside, we should have a regular eye
checkup.
2) In case, after an eye checkup, if advised bye an eye specialist, we should use suitable spectacles.
3) We should avoid bright or dim light. Insufficient light causes eyestrain and headaches. Too much
light, like that of the sun, a powerful lamp or a laser torch can injure the retina.
4) We should not look at the sun or a powerful light directly.
5) We should not rub our eyes. If particles of dust go into our eyes, we should wash our eyes with clean
water. If there is no improvement then we should go to a doctor.
6) We should Wash our eyes frequently with clean water.
7) We should always read at the normal distance for vision. Do not read by bringing our book too close
to our eyes or keeping it too far.
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8) Our Diet should include food items rich in vitamin A.
Q 4:- . What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Ans :- As per the laws of reflection
The angle of incidence ∠i = angle of reflection ∠r
Here as given ∠i + ∠r = 90°
∴ ∠i + ∠i = 90 °
or ∠i = 90/2 = 45 °
Hence angle of incidence of a ray = 45 degree.
Q 5 :- What is Periscope ?
Ans :- A Periscope is a device through which a person can se the objects that are out of the
direct line of sight. For example, by using a periscope, we can see the objects on the other
side of a high wall which cannot be seen by us directly. Periscope has two plane mirrors
arranged parallel to one another. Each plane mirror, however, makes an angle of 450 with
the side of the tube.
Q 6 :- What is Kaleidoscope ?
Ans :- The kaleidoscope is an instrument which produce multiple reflections of coloured
glass pieces {or coloured plastic pieces} and create beautiful patterns.
Q7:- How can be a Kaleidoscope constructed ?
Ans :- Construction: The Kaleidoscope consists of three long and narrow strips of plane
mirrors inclined at 600 to one another forming a hollow prism, and fitted into a cardboard
tube. One end of the cardboard tube is closed by an opaque disc {cardboard disc} having a
small hole at its centre. The other end of cardboard tube is closed with circular discs of
glass. The inner disc being of transparent glass {clear glass } and the outer disc of ground
glass {translucent glass}. A number of small pieces of different coloured glass {or plastic }
and having different shapes are kept between the two glass discs {which can move around
freely in the space between the two glass discs}. When we hold the kaleidoscope tube
towards light and look inside it through the small hole, we see beautiful patterns of
coloured glass.
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Q 8 :- What is Dispersion of light?
Ans :- The splitting up of white light into seven colours on passing through a transparent
medium like a glass prism is called dispersion of light. The seven colours of the spectrum
of white light are: violet, indigo, blue, green, yellow, orange and Red.
Q9:- Describe the structure of Human Eye ?
Ans :- The main parts of Human Eye are :
The main parts of the human eye are : Cornea, Iris, Pupil, Ciliary muscles, Eye lens
{which is a flexible convex lens}, Retina and Optic nerve.

Cornea is made of a transparent substance

The light coming from an object enters into the eye hrough cornea. Functionof cornea
is to protect the eye.

Iris is the colored part of the eye. The iris has a hole at its centre which is called pupil.
Pupil appears like a dark spot in the centre of iris because no light is reflected from it.
Iris controls the amount of light entering the eye by adjusting the size of pupil.

The eye-lens is a convex lens made of a transparent and flexible material.

The eye-lens is held in position by ciliary muscles. The function of ciliary muscle is to
hold the lens & changes its size according to the need.

The retina is a screen on which the image is formed in the eye.

The optic nerve carries the image formed on retina to the brain in the form of electrical
signals.
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Q 10:- What are the defects of the Human Eye ?
Ans :- There are two defects of Human Eye :a) Myopia (short-sightedness) : In this defect the person is able to see the near
by objects but not able to see the far-off objects.
This defect can be corrected by using spectacles containing Concave lens.
b) Hypermetropia (Far-sightedness) : In this defect the person is able to see
the far-off objects but not able to see the near by objects.
This defect can be corrected by using spectacles containing Convex lens.c)
Cataract : Cataract develops when the eye lens of person becomes cloudy (or
even opaque) due to the formation of membrane over it. Cataract decreases
the vision of the eye gradually. It can even lead to total loss of vision of the
eye.
The opaque lens is removed from the eye of the person by surgical operation
& a new artificial lens is inserted in its place.
Q11:- What is Lens ?
Ans :- The transparent, elastic, curved structure behind the iris (colored part of the
eye) that helps focus light on the retina. Also refers to any device that bends light
waves.
Q12:- Describe the types of Lenses ?
Ans :- There are two types of Lenses :A) Convex Lens :- A convex lens is thicker at the centre than at the edges.
Convex lens are thicker at the middle .Rays of light that pass through the
lens are brought closer together ( they converge ) . Thus , a convex lens is a
converging lens .
When parallel rays of light pass through a convex lens the refracted rays
converge at one point called the principal focus.
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The distance between the principal focus and the center of the lens is
called the focal length.
B) Concave Lens :- A concave lens is thinner at the centre than at the edges.
Convex lenses are thinner at the middle. Rays of light that pass through the lens are spread out
(they diverge). A convex lens is adiverging lens.
When parallel rays of light pass through a concave lens the refracted rays diverge so that they
appear to come from one point called the principal focus.
The distance between the principal focus and the centre of the lens is called the focal length.
The image formed is virtual and diminished (smaller)
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Q 13 :-What is Diffused reflection?
Ans :- Diffused Reflection
1. In Diffuse reflection, a parallel beam of incident light is reflected in different
directions.
2. The diffuse reflection of light takes place from rough surfaces .
3. A sheet of paper produces diffuse reflection of light. No image is formed in
diffuse reflection of light.
Q14:- What is Multiple Images ?
Ans :- When two plane mirrors are kept inclined at an angle, they can form
multiple images of an object.
Convex lenses are thicker at the middle. Rays of light that pass through the lens are brought closer
together (they converge). A convex lens is a converging lens.
When parallel rays of light pass through a convex lens the refracted rays converge at one point called
the principal focus.
the principal focus and the centre of the lens is called the focal length.
Convex lenses are thicker at the middle. Rays of light that pass through the lens are brought closer
together (they converge). A convex lens is a converging lens.
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When parallel rays of light pass through a convex lens the refracted rays converge at one point called
the principal focus.
The distance between the principal focus and the centre of the lens is called the focal length.
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