PRASHANT KIRAD
Class 10th - SCIENCE
Light
PRASHANT KIRAD
Light is a form of energy that enables the sensation of vision.
Speed of light waves depends on the medium through which they pass. The speed of light in
vacuum is 3 × 10⁸ m/s.
Rectilinear Propagation of Light – Light travels in a straight line. A ray of light is the
straight line along which light travels, and a bundle of light rays is called a beam of light.
REFLECTION OF LIGHT
The process of sending back light rays which falls on the surface of an object is
called Reflection of light.
Incident Ray: The incoming ray of light that strikes the surface
is called the incident ray.
Reflected Ray: The ray that bounces off the surface is called
the reflected ray.
Normal: The imaginary line perpendicular to the surface at the
point of incidence is called the normal.
PRASHANT KIRAD
The Angle of incidence (∠i) = The angle of reflection (∠r)
The incident ray, the reflected ray and the normal to the mirror at the point of incidence
all lie in the same plane.
Lateral Inversion – A phenomenon where an image appears reversed from left to right.
This effect is commonly seen in mirrors, where your right hand appears as the left hand in the
mirror image.
CHARACTERISTICS OF A IMAGE FORMED BY A PLANE MIRROR:
Plane mirror: A smooth and polished surface that reflects light uniformly.
The image obtained is virtual.
The image is laterally inverted.
The image is erect.
The size of the image is the same as the size of the object.
The distance between the image obtained from the mirror is the
same as the distance between the object from the mirror.
PRASHANT KIRAD
Q. State the laws of reflection and draw a labeled diagram to illustrate these laws.
Answer. 1. The law of reflection states that: 1.The angle of incidence is equal to the angle of
reflection.
2.The incident ray, the reflected ray, and the normal all lie in the same plane
PRASHANT KIRAD
Terms Definition
The center point of the reflecting surface of a spherical
Pole (P)
mirror.
The center of the sphere of which the mirror's reflecting
Centre of Curvature (C)
surface forms a part.
The radius of the sphere of which the mirror's reflecting
Radius of Curvature (R)
surface forms a part. R = 2f CONCAVE MIRROR
The straight line passing through the pole and the center of
Principal Axis
curvature of the mirror.
The point where parallel rays of light either converge or
Principal Focus (F)
appear to diverge after reflecting from the mirror.
Focal Length (f) The distance between the pole and the principal focus.
The diameter of the reflecting surface of the spherical
Aperture
mirror.
CONVEX MIRROR
PRASHANT KIRAD
Ray Diagrams
(i) A ray parallel to principal axis will (iii) A ray passing through center of curvature
pass through focus after reflection. will follow the same path back after reflection.
(ii) A ray passing through the principal focus will (iv) Ray incident at pole is reflected back
become parallel to principal axis after reflection making same angle with principal axis.
PRASHANT KIRAD
Concave Mirror Convex Mirror
PRASHANT KIRAD
Q. The image shows the path of incident rays to a concave mirror.
Where would the reflected rays meet for the image formation to take place?
(a) Behind the mirror
(b) Between F and O
(c) Between C and F
(d) Beyond C
PRASHANT KIRAD
Q. Draw ray diagrams for the following cases :
(i) passing through centre of curvature of a concave mirror is incident on it.
(ii) an object is placed between infinity and the pole of a convex mirror.
(iii) object is placed between focus and pole of concave mirror
PRASHANT KIRAD
Concave Mirror
Uses of Concave Mirrors:
Torches, Search-lights, and Vehicle
Headlights:
Shaving Mirrors
Dentist's Mirrors
Solar Furnaces
Uses of Convex Mirrors:
Convex Mirror Rear-View Mirrors in Vehicles:
Preferred in vehicles as they provide
erect but diminished images.
Have a wider field of view due to their
outward curve.
Allow drivers to see a larger area
compared to plane mirrors.
PRASHANT KIRAD
Q. (a) Name the type of mirrors used in the design of solar furnaces. Explain how high
temperatures are achieved by this device.
(b) State the types of mirrors used for:
(i) Headlights of vehicles
(ii) Rear-view mirrors of motorcycles. Justify your answer in each case.
PRASHANT KIRAD
Q. (a) Define the following terms in the context of spherical mirror:
(i) Pole
(ii) Centre of curvature
(iii) Principal axis
(iv) Principal focus
(b) Consider the following diagram in which M is a mirror and P is an object and Q is its
magnified image formed by the mirror. State the type of the mirror M and one characteristic
property of the image Q.
Answer. (i) Pole: The middle point of the reflecting surface of a spherical mirror is called pole.
The letter P represents pole,
(ii) Centre of curvature: It is the centre of the sphere of glass of which the mirror is a part. The
letter C represents the centre of curvature.
(ii) Principal axis of a spherical mirror is the straight line joining the centre of curvature and
pole of the mirroг. (iv)Principal focus: The mid-point of CP is called focus (F). It is the point on
the principal axis of spherical mirror where all incident rays parallel to the principal axis meet
or appear to diverge after reflection.
(b) The mirror used in the given diagram is a concave spherical mirror. Image formed (Q) is
virtual and magnified,
PRASHANT KIRAD
(i) The object is placed to the left of the mirror.
(ii) All distances parallel to the principal axis are measured
from the pole of the mirror.
(iii) All distances measured in the direction of incident ray
(along + X-axis) are taken as positive and those measured
against the direction of incident ray (along – X-axis) are
taken as negative.
(iv) Distance measured perpendicular to and above the
principal axis are taken as positive. Object distance = always positive
(v) Distances measured perpendicular to and below the Focal length of concave mirror = Negative
principal axis are taken as negative. Focal length of convex mirror = Positive
PRASHANT KIRAD
The mirror formula relates the object distance (u), image distance (v), and focal length (f) of a
spherical mirror.
It gives us information about the image in terms of how large or small is the image formed.
h’ = positive (virtual images)
h’ = negative (real images)
m = negative (real)
m = positive (virtual)
PRASHANT KIRAD
Q. A student wants to obtain an erect image of an object using a concave mirror of 10 cm focal
length. What will be the distance of the object from the mirror?
(a) Less than 10 cm
(b) 10 cm
(c) Between 10 cm and 20 cm
(d) More than 20 cm
PRASHANT KIRAD
Q. A student conducts an activity using a flask of height 15 cm and a concave mirror. He finds
that the image formed is 45 cm in height. What is the magnification of the image?
(a) -3 times
(b) -1/ 3 times
(c) 1/ 3 times
(d) 3 times
PRASHANT KIRAD
Q. An object is placed at a distance of 10 cm from a convex mirror of focal length 15 cm. Find
the position of the image formed by the mirror.
PRASHANT KIRAD
Q. The linear magnification produced by a spherical mirror is +3. Analyse this value and state
the (i) type of mirror and (ii) position of the object with respect to the pole of the mirror.
PRASHANT KIRAD
Q. An object 4 cm in height, is placed at 15 cm in front of a concave mirror of focal length 10
cm. At what distance from the mirror should a screen be placed to obtain a sharp image of the
object. Calculate the height of the image.
PRASHANT KIRAD
Q. An object 4 cm in height, is placed at 15 cm in front of a concave mirror of focal length 10
cm. At what distance from the mirror should a screen be placed to obtain a sharp image of the
object. Calculate the height of the image.
Answer. Given : object distance, u = -15 cm,
object height, h = 4 cm, focal length f = -10 cm;
Image distance, v = ?
Using mirror formula,
1/v+1u=1/f⇒1/v+1/(−15)=1/−10⇒1/v=1/15−1/10
or 1/v=10−15/150=−5/150=−1/30 or v = -30
In order to obtain a sharp image of the object on the screen, screen should be placed at a
distance of 30 cm in front of the mirror.
m=h/h′=−v/u
Substitute values:
m= −(−30)/−15=2
h′=m×h=2×4= +8cm
PRASHANT KIRAD
Q. Study the data given below showing the focal length of three concave mirrors A, B, and C and
the respective distances of objects placed in front of the mirrors:
Case Mirror Focal Length (cm) Object Distance (cm)
1 A 20 45
2 B 15 30
3 C 30 20
(i) In which one of the above cases will the mirror form a diminished image of the object? Justify
your answer.
(ii) List two properties of the image formed in Case 2.
(iii) (A) What is the nature and size of the image formed by mirror C? Draw a ray diagram to
justify your answer.
PRASHANT KIRAD
The phenomenon of bending of ray of light when it enters from one medium to another.
The bending of a light ray during refraction occurs because of a change in the speed of light as
it passes from one medium to another with a different refractive index.
Incident Ray: The incoming ray of light in the first medium
is called the incident ray.
Reflected Ray: The ray that bends as it enters the second
medium is called the refracted ray.
Normal: The imaginary line perpendicular to the surface at
the point of incidence is called the normal.
CAUSES OF REFRACTION:
When the light goes from air into water, it bends towards
normal because there is a reduction in its speed.
When the light goes from water to air, it bends away from
normal because there is an increase in the speed of light.
PRASHANT KIRAD
Rarer to denser medium (bends towards normal)
Denser to rarer medium (bends away from normal)
LAWS OF REFRACTION:
The incident ray, refracted ray, and the normal to the interface
of two media at the point of incidence all lie on the same plane.
The ratio of the sine of the angle of incidence to the sine of the
angle of refraction is a constant. This is also known as Snell’s
law of refraction
Snell’s law of refraction.
PRASHANT KIRAD
The refractive index is a measure of how much light is bent or
refracted when it enters a new medium. It is denoted by the
symbol "n."
velocity of light in medium 1
n=
velocity of light in medium 2
Speed of light in medium 1
n 21 = Refractive index of medium 2
with respect to medium 1
Speed of light in medium 2
Speed of light in air =c If medium 1 is vacuum or air, then the
nm = refractive index of medium m is considered
Speed of light in medium v with respect to vacuum. This is called the
absolute refractive index of the medium.
PRASHANT KIRAD
When an incident ray enters a glass slab from air, it makes an
angle of incidence (i) with the normal and bends towards the
normal as it moves from a rarer to a denser medium.
After passing through the glass slab, the refracted ray makes an
angle of refraction (r) at the other surface.
The emergent ray then bends away from the normal as it exits
from glass (denser) to air (rarer), forming an angle of
emergence (e) with the normal.
The emergent ray is parallel to the incident ray, with the
perpendicular distance between them known as lateral
displacement.
Since the angle of incidence equals the angle of emergence, the
emergent ray remains parallel to the incident ray.
In a glass slab, light is refracted twice: first from a rarer to a
denser medium, and then from denser to rarer.
This refraction causes the lateral displacement of the emergent
ray.
PRASHANT KIRAD
Q. A light ray passes from air into a glass slab. The angle of incidence is 30°, and the
refractive index of glass with respect to air is 1.5. What is the angle of refraction?
(A) 19.47°
(B) 30°
(C) 45°
(D) 60°
PRASHANT KIRAD
Q. (a) Water has refractive index 1.33 and alcohol has refractive index 1.36. Which of the two
medium is optically denser? Give reason for your answer.
(b) Draw a ray diagram to show the path of a ray of light passing obliquely from water to
alcohol.
(c) State the relationship between angle of incidence and angle of refraction in the above case
PRASHANT KIRAD
Q. (a) Water has refractive index 1.33 and alcohol has refractive index 1.36. Which of the two
medium is optically denser? Give reason for your answer.
(b) Draw a ray diagram to show the path of a ray of light passing obliquely from water to
alcohol.
(c) State the relationship between angle of incidence and angle of refraction in the above case
Answer:(a) Here, alcohol is optically denser medium as its refractive index is higher than that
of water. When we compare the two media, the one with larger refractive index is called the
optically denser medium than the other as the speed of light is lower in this medium.
(b) Since light is travelling from water (rarer medium) to alcohol (denser medium), it slows
down and bends towards the normal.
PRASHANT KIRAD
Q. Q17: A ray of light is incident as shown. If A, B and C are three different transparent
media, then which among the following options is true for the given diagram?
(a) ∠1 > ∠4
(b) ∠1 < ∠2
(c) ∠3 = ∠2
(d) ∠3 > ∠4
PRASHANT KIRAD
A transparent material bound by two surfaces, of which one
or both surfaces are spherical.
Types of lenses:
Convex (thicker in the middle, converging light rays)
Concave (thinner in the middle, diverging light rays).
Term Meaning
Convex Lens A lens with two spherical surfaces bulging outwards, thicker in the middle than at the edges. (Converging Lens)
Concave Lens A lens with two spherical surfaces curved inwards, thicker at the edges than at the middle. (Diverging Lens)
Centre of Curvature (C, C1, C2) The center of the sphere from which the lens surface is a part.
Principal Axis An imaginary straight line passing through the two centers of curvature of a lens.
Optical Centre (O) The central point of a lens where a ray of light passes without deviation.
Aperture The effective diameter of the circular outline of a spherical lens.
Principal Focus (F, F1, F2) The point where rays of light parallel to the principal axis converge (convex) or appear to diverge (concave).
Focal Length (f) The distance between the principal focus and the optical centre of a lens.
PRASHANT KIRAD
Rules to obtain image
(i) A ray of light from the object, (iii) A ray of light passing through the optical
parallel to the principal axis centre of a lens
(ii) A ray of light passing through a
principal focus
PRASHANT KIRAD
Convex Lens Concave Lens
PRASHANT KIRAD
Convex Lens
Uses of Convex Lens:
overhead projector
camera
focus sunlight
simple telescope
projector microscope
magnifying glasses
Uses of Concave Lens:
Concave Lens spy holes in the doors
glasses
some telescopes
PRASHANT KIRAD
Q. Draw a ray diagram to show the path of the refracted ray in each of the following cases:
A ray of light incident on a concave lens
(i) parallel to its principal axis, and
(ii) is directed towards its principal focus.
(i) parallel to its principal axis (ii) is directed towards its principal focus.
