Extra 30 short-answer questions from the Chapter:9 Light - Reflection and Refraction (Class 10 CBSE Science):
Reflection of Light
State the laws of reflection.
Answer:
The incident ray, reflected ray, and normal all lie in the same plane.
The angle of incidence is equal to the angle of reflection (
∠
i
=
∠
r
).
Differentiate between real and virtual images.
Answer:
|
Feature |
Real Image |
Virtual Image |
|---|---|---|
|
Nature |
Inverted |
Erect |
|
Formation |
Formed by actual intersection of rays |
Formed by apparent intersection of rays |
|
Mirror Type |
Concave mirror (except when object is between focus and pole) |
Plane and convex mirrors |
Why does a concave mirror form an enlarged image when the object is placed between the focus and pole?
Answer:
When an object is placed between
focus (F) and pole (P)
, the reflected rays appear to diverge from a point behind the mirror.
This forms a
virtual, erect, and magnified image
.
What happens when an object is placed at the focus of a concave mirror?
Answer:
The reflected rays become
parallel
.
The image is
formed at infinity
, is
highly magnified, real, and inverted
.
Why do we use a convex mirror for rear-view mirrors in vehicles?
Answer:
Convex mirrors always give a
virtual, erect, and diminished image
.
They provide a
wider field of view
, helping drivers see more of the road behind.
What is the mirror formula? Define each term.
Answer:
Formula:
1f=1v+1u\frac{1}{f} = \frac{1}{v} + \frac{1}{
u}f
1=v1+u1
Where:
fff
= focal length
vvv
= image distance
uuu
= object distance
Explain why concave mirrors are used in torches and searchlights.
Answer:
When a light source is placed at the
focus
, the concave mirror
reflects and directs the light rays parallel
.
This produces a
strong, focused beam of light
.
Describe the nature and position of the image formed by a convex mirror for an object at infinity.
Answer:
Nature:
Virtual, erect, and highly diminished.
Position:
Formed at the
focus behind the mirror
.
Explain the sign convention for mirrors.
Answer:
Distances measured toward the mirror (left side) are negative.
Distances measured away from the mirror (right side) are positive.
Focal length of concave mirror is negative
, while
convex mirror is positive
.
What is magnification? Give its formula for mirrors.
Answer:
Magnification (mmm) is the ratio of
image height to object height
.
Formula:
m=−
vum
= \frac{-
v}{
u}m=u−v
Negative mmm means
real and inverted image
, positive mmm means
virtual and erect image
.
State the laws of refraction.
Answer:
The incident ray, refracted ray, and normal lie in the
same plane
.
Snell’s law
: n1sini=n2sinrn_1 \sin
i
= n_2 \sin rn1
sini
=n2
sinr
, where n1n_1n1 and n2n_2n2 are refractive indices.
What is the refractive index? How is it related to the speed of light?
Answer:
Refractive index (n) =
Speed of light in vacuumSpeed of light in medium\
frac{
\text{Speed of light in vacuum}}{\text{Speed of light in medium}}Speed of light in
medium Speed
of light in vacuum.
Higher
nnn
means slower light speed in the medium.
What is total internal reflection? Give an example.
Answer:
When light
travels from denser to rarer medium
at an angle greater than the
critical angle
, it gets completely reflected.
Example:
Mirage in deserts
.
What happens when light travels from air to glass?
Answer:
It
bends towards the normal
because glass is denser than air.
What is the critical angle?
Answer:
The angle of incidence at which light
just grazes along the boundary
while moving from a denser to a rarer medium.
Why does a pencil appear bent in water?
Answer:
Due to
refraction
, light from the pencil bends at the water-air boundary, making it appear shifted.
What is a lens? Differentiate between convex and concave lenses.
Answer:
|
Feature |
Convex Lens |
Concave Lens |
|---|---|---|
|
Shape |
Bulging outward |
Curved inward |
|
Image |
Real/Virtual |
Always Virtual |
|
Use |
Magnifying glass |
Spectacles for myopia |
State the lens formula and define terms.
Answer:
Formula:
1f=1v−1u\frac{1}{f} = \frac{1}{v} - \frac{1}{
u}f
1=v1−u1
f = focal length, v = image distance, u = object distance
.
What is power of a lens? Give its unit.
Answer:
Power (P) =
1
f(
in meters)\frac{1}{f(\text{in meters})}f(in meters)1, measured in
dioptres
(D)
.
A convex lens has a focal length of 10 cm. Find its power.
Answer:
P=1f=10010=+10DP = \frac{1}{f} = \
frac{
100}{10} = +10DP=f1=10100=+10D.
Why do stars twinkle but planets do not?
Answer:
Due to
atmospheric refraction
, starlight bends as air layers change density.
Planets appear as discs, averaging out the effect.
What is a real-life example of refraction?
Answer:
Swimming pool appears shallower than actual depth
.
Why are optical
fibres
used in communication?
Answer:
They use
total internal reflection
to carry light signals over long distances.
Why do diamonds sparkle?
Answer:
Due to
total internal reflection
and
high refractive index
.
Why do we use convex lenses in magnifying glasses?
Answer:
They form
virtual, erect, and magnified images
when the object is placed between
focus and lens
.
What is the use of concave lenses in spectacles?
Answer:
Used to correct
myopia (short-sightedness)
.
Why do convex lenses form both real and virtual images?
Answer:
Real when object is beyond focus
.
Virtual when object is between focus and lens
.
What is the effect of increasing the refractive index of a medium?
Answer:
Light bends more towards the normal, decreasing the speed.
What is the effect of increasing the curvature of a convex lens?
Answer:
The
focal length decreases
, increasing power.
Why does a rainbow form?
Answer:
Due to
dispersion of light
by water droplets.
