STD. 7 (2014)
Light and Associated Phenomena
Q1.What is light?
Ans. Light is a form of energy that enables us to see.
Q2. Name three examples each of natural and artificial sources of light.
Ans. The three natural sources of light are sun, stars and fireflies, while the artificial light
sources are moon, fire and lamps.
Q.3 What is rectilinear propagation of light?
The property of light which states that light always travels in a straight line, is known as
rectilinear propagation of light.
Q.4 What is reflection of light?
The bouncing of light off a surface is known as reflection of light.
Q.5 Explain the term: a) Regular reflection
b) Irregular reflection
a) Regular reflection: Reflection that takes place through a regular and smooth surface is
known as regular reflection.
b) Irregular reflection: Reflection that takes place through a rough and dull surface is known
as irregular or diffused reflection.
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Q.6 Write the characteristics of image formed in a plane mirror.
i) An image in a mirror is of the same size as the size of an object.
ii) The image formed in the mirror is at the same distance behind the mirror as the object is
in front of it.
iii) The image formed in a plane mirror is virtual.
iv) The image formed is laterally inverted. (Left side seems to be on the right and vice
versa.)
Q7. How is a mirror formed? List out all the types of mirror.
Ans. Mirror is a reflecting surface made out of glass, which is coated from one side with
metals like gold, silver or mercury. Generally Mercury is used because it has a shiny surface
and is good reflector.
There are three types of mirror
Plane Mirror
Concave Mirror and
Convex Mirror
Q8. What are the characteristics of an image formed by a plane mirror?
Ans.10. An image formed in a plane mirror shows the following characteristics:
Image is always upright or straight.
It is always virtual.
It is of the same size as the size of the object.
An image formed in the mirror is at the same distance behind the mirror as the object
is in front of it.
Q.9 What do you mean by lateral inversion?
Lateral inversion is the property of an image formed by a mirror where left side seems to be
on the right and vice versa.
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Q.11 Explain the terms used in reflection by using as appropriate diagram.
The ray of light which falls on the surface of the reflecting material or mirror is the incident
ray.
(labelled I in the diagram).
The ray of light which is sent back from the surface of the reflecting surface is the reflected
ray.
(labelled R in the diagram).
The point at which the light ray strikes the reflecting surface is the point of incidence.
At the point of incidence where the ray strikes the mirror, a line can be drawn perpendicular
to the surface of the mirror. This line is known as a normal line (labeled N in the diagram).
The normal line divides the angle between the incident ray and the reflected ray into two
equal angles.
The angle between the incident ray and the normal is known as the angle of incidence.
The angle between the reflected ray and the normal is known as the angle of reflection.
(These two angles are labeled with the Greek letter "theta" accompanied by a subscript; read
as "theta-i" for angle of incidence and "theta-r" for angle of reflection.)
Q.12 Explain the law of reflection.
The law of reflection states that,
* When a ray of light reflects off a surface, the angle of incidence is equal to the angle of
reflection. i.e. < i = <r
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*Incident ray ,reflected ray and the normal at the point of incidence lie in the same plane.
Q.13 Differentiate between: Real Image and Virtual Image
Real Image
Virtual Image
It can be taken on the screen.
It cannot be taken on the screen.
Always Inverted
Always erect but laterally inverted.
The rays of light after reflection actually
meet at a point.
The rays of light after reflection appear to
meet at a point.
Q.14 Explain ‘Spherical Mirrors’.
A spherical mirror is of two types: Convex mirror and Concave mirror.
A spherical mirror, in which the reflective surface bulges toward the light source is called
Convex Mirror.
A spherical mirror, in which the reflective surface that bulges inward is called Concave
Mirror.
Principal Focus : All the rays of light reflect from the curved surface of the concave mirror
meet at a point on the principal axis. This point is called the principal focus of a concave
mirror.
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In case of a convex mirror, the rays of light appear to diverge from a point on the
principal axis , called the principal focus.
Q 15. What is the principal focus of a Concave mirror?
Ans. All the rays of light reflect from the curved surface of the concave mirror meet at a
point. This point is called the principal focus of a concave mirror.
Q.16 Explain the nature of image formed in plane mirror, concave mirror and convex
mirror.
Type of image
formed
Plane mirror
Concave mirror
Convex mirror
Real or virtual
Virtual
Real except when the
object is near the
mirror
Virtual
Upright or inverted
Always upright
Inverted when real
and upright when
virtual
Always upright
Size of image
Image formed is of
the same size as the
object.
Diminished for
certain positions and
enlarged for others
Diminished
Q.17 What are the uses of Concave Mirrors?
 It is used as a shaving mirror.
 It is used as a reflector in torches, lighthouses, headlights o vehicles
 Since it can focus the rays of the sun at a point, this property of concave mirrors is
used in solar-powered gadgets.
 A dentist uses a concave mirror to obtain a magnified images of the teeth of the
patient.
 In astronomical telescopes, it is used to view magnified images of heavenly bodies.
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Q.18 What are the uses of Convex Mirrors?
 As a rear view mirror in vehicles such as car, bus, truck etc., since it forms a smaller
and virtual image. It can be used to see a much larger area than the area visible by a
plane mirror.
 In shopping complexes and large shops to keep an eye on customers and shoplifters, if
any.
Q.19 Define ‘Dispersion of Light’.
The splitting up of white light into its constituent colours when it passes through a prism is
known as dispersion of light.
Q.20 How do you obtain a spectrum of light?
The band of seven colours is called a spectrum and the colours of the spectrum are denoted
by VIBGYOR. When white light is passed through a transparent reflecting medium (eg
Prism )the white light splits into these seven colours which is a spectrum. When these
colours combine, they appear white.
Q.21 What is Newton’s disc? How and why is it made?
Sir Isaac Newton first observed that a thin beam of light after passing through a prism, forms
a band of seven colours that include red, orange, yellow, green, blue, indigo and violet. To
prove this observation, Newton invented Newton’s disc. This is a disc painted with the seven
colours according to the percentage of the colours in the spectrum. If the disc is rotated very
fast, we can see that the colours of the disc seem to disappear and the disc appears white.
Q22. Why are we able to see white colour in Newton’s disc when we rotate it very fast?
Ans. We are able to see white colour in Newton’s disc because the effect of the image of the
colour does not disappear immediately from the eye, but persists for a fraction of a second.
When the disc rotates , the colours form images one after the other and we see a combined
effect of all the colours which is a white colour .
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Q23. Fill in the blanks on the basis of nature of images formed in plane mirror,concave
mirror and convex mirror.
Types of images
formed
Plane mirror
Real or Virtual
Virtual
Concave Mirror
Convex Mirror
Inverted when real
and
Upright or inverted
upright when virtual
Diminished
Size of Image
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