LIGHT
Very short type answer questions
1. What is the image formed by a plane mirror known as?
Virtual and erect
2. What are spherical mirrors?
Concave and convex mirrors are collectively known as spherical mirrors because
of their curved reflecting surfaces. A spherical mirror is a part of a sphere or
spherical surface, silvered on one side. If the inside is silvered, it is called a convex
mirror and if the outside is silvered, it is a concave mirror. The reflecting surface
of the convex mirror curves outwards. In a concave mirror, the reflecting surface
curves inwards. Refer Fig 15.9 page 196.
3. What is the nature of the image formed by a concave mirror?
Concave mirrors produce real and virtual, erect and inverted and diminished and
magnified images.
4. What is the nature of the image formed by a convex mirror?
Convex mirrors produce only virtual, erect, and diminished images.
5. What is a lens?
Lens is an optical device formed by the combination of two curved surfaces with
a common axis. The two types of lenses are concave and convex lenses.
6. Expand ‘VIBGYOR’.
The constituent colours of white light are Violet, Indigo, Blue, Green, Yellow,
Orange and Red (VIBGYOR).
7. What are the two types of mirrors
Ans: The two types of mirrors are
 Convex mirrors
 Concave mirrors
8.What is known as reflection of light?
Ans: The bouncing back and change of direction of light by polished or
shiny surfaces such as mirror is known as reflection of light.
9. What are images?
Ans: The copy of object as reflected from a mirror is called images.
10. What is erect image?
Ans: The upright image of an object is called as erect image.
11. What is known as rectilinear propagation of light?
Ans: The property that light travels in a straight line is known as rectilinear
propagation of light.
12. Differentiate between concave and convex mirrors? And give one example.
Concave mirrors
Concave mirror is a spherical mirror
whose reflecting surface is towards
the centre of the sphere of which the
mirror is a part.
The focus lies in front of the mirror
Concave mirror is a converging mirror
Concave mirror forms different types
of images depending on the position
of the object.
Eg : used in telescopes
Convex mirrors
Convex mirror is a spherical mirror
whose reflecting surface is away from
the centre of the sphere of which the
mirror is a part.
The focus lies behind the mirror
Convex mirror is a diverging mirror
Convex mirror always forms real
inverted and diminished images
Eg : used in rear view mirrors in
vehicles
13.What is Newtons disc? What is it used for.
A Newton disc, invented by Isaac Newton, is a disc with segments in rainbow colors.
When the disc is rotated, the colors fade to white. In this way Isaac Newton
demonstrated that white light is a combination of the seven different colors found in a
rainbow. With this instrument one can demonstrate the white light that can be broken
down into separate elements.
14. What is the angle of incidence and the angle of reflection and what is common in
them? How are they related?
Ans: The angle which an incident line or ray makes with a perpendicular to the reflecting
surface at the point of incidence is the angle of incidence.
The angle made by a reflected ray with a perpendicular to the reflecting surface at the
point of incidence is called the angle of reflection.
The point where the incident ray strikes the mirror, a line can be drawn perpendicular to
the surface of the mirror. This line is known as a normal line. The normal line divides
the angle between the incident ray and the reflected ray into two equal angles. This is
common in both angle of incidence and reflection.
Relation between Angle of incidence and Angle of Reflection:
Angle of incidence is always equal to the angle of Reflection. This is a law of Reflection.
15. How is an image formed by plane mirror?
Ans: The image formed by a plane mirror is of the same size as the object, virtual
and laterally inverted. A virtual image is a copy of an object formed at the location
from which the light rays appear to come. However, the image is a laterally-inverted
"mirror image" of the object. If a person is reflected in a plane mirror, the image of
his right hand appears to be the left hand of the image.
16. Differentiate between real and Virtual Image
No.
Real image
A real image is that image
which is formed when the light
1. rays coming from an object
actually meet each other after
reflection or refraction.
Virtual image
A virtual image is that image
which is formed when the light
rays coming from an object do
not actually meet, but appear to
meet
when
produced
backwards.
A real image can be obtained These images cannot be
on the screen.
obtained on the screen.
The real image is always The virtual image is always erect
3.
inverted.
but laterally inverted.
The common example of real
image is the image formed on The common example of virtual
the cinema screen. You can image is the image formed in
4.
see rays of light coming from the mirror when we stand in
the projector and falling on the front of that mirror.
screen
2.
17.What do you understand by Lateral Inversion?
The inversion of left and right by which the left side of the object appears to be
the right side of the image and vice versa is called lateral inversion.
4.What is dispersion of Light?
The splitting of white light into seven colours is called dispersion.
18. What is a spectrum?
When white light passes through a triangular glass prism, it is dispersed or split
into seven colours. The band of colours obtained because of dispersion of light is
called a spectrum. The constituent colours of white light that forms a spectrum
are Violet, Indigo, Blue, Green, Yellow, Orange and Red (VIBGYOR).
19. What are the uses of spherical mirrors?
Uses of spherical mirrors
Concave mirrors
i. Concave mirrors are used as shaving mirrors and compact mirrors to get an
erect enlarged image of the face when the mirror is held close to the face.
ii. They are used in the headlights of vehicles, torch lights, search lights and
projectors to get a strong parallel beam of light.
Convex mirrors
i. They are used as rear view mirrors in vehicles to view the traffic behind.
ii. Convex mirrors are used in street-lights as reflectors to spread light over a large
area.
iii. They are used by security personnel as surveillance mirrors to watch over a
large area.
20. Describe the image formed by a convex lens.
A magnifying glass is a convex lens that produces a magnified image of an object.
For a magnified image to be observed the distance between the object and the
lens has to be shorter than the focal length of the lens. The image formed is
upright, magnified and virtual.
21. Describe the image formed by a concave lens.
Concave mirrors can produce both real and virtual images; they can be upright (if
virtual) or inverted (if real); they can be behind the mirror (if virtual) or in front
of the mirror (if real); they can also be enlarged, reduced, or the same size as
object.
22. Describe some examples that support the concept of rectilinear propagation
of light.
1.
2.
Ray of light coming from Sun.
Activity 1 on page 193
Convex lens
1. It is thick in the middle and thin at the
edges
Concave lens
1. It is thin in the middle and thick at the
edges
2. It converges the incident rays
towards the principal axis.
2. It diverges the incident rays away
from the principal axis.
3. It has a real focus
3. It has a virtual focus
Convex Mirror
1. Convex mirror is curved outwards.
Concave Mirror
1. Concave mirror is curved inwards.
2. The focal point of convex mirror is
behind the mirror
2. The focal point of concave mirror is
in front of the mirror.
3. In convex mirror the image is always
virtual, upright and smaller than the
object
3. In case of concave mirror different
types of images are formed on different
location of the object. The image is
upside down (inverted) and far away
but if we bring the object close to the
mirror then image will be larger and
upright.
4. Convex mirrors are used in cars (as
passenger-side mirror since they provide
upright and wide view), they are also
used in camera phones, for safety
measures there are also used in roads
and drive ways
Besides these convex mirrors are found
in many hospitals, schools etc. as
hallway safety mirror.
4. Concave mirrors are used in
telescope. These are also used as
make up and shaving mirrors since
these provide larger images.
Besides these concave mirrors are
used by dentists and also used in
headlights of cars, solar devices,
satellite dishes etc.
5. It has a virtual focus
5. It has a real focus