Light Reflection and Refraction

Question 1

Light: Definition

Answer 1

Light is a form of energy that enables us to see things. Light starts from a source and bounces off objects which are perceived by our eyes and our brain processes this signal, which eventually enables us to see.
Maxwell predicted that magnetic and electric fields travel in the form of waves and these waves move at the speed of light. This led Maxwell to predict that light itself was carried by electromagnetic waves which
means that light is a form of electromagnetic radiation.

Question 2

Nature of Light

Answer 2

Light behaves as a:
● ray, e.g. reflection
● wave, e.g. interference and diffraction
● particle, e.g. photoelectric effect

According to the concept of wave-particle duality in quantum mechanics light exhibits both particle and wave nature, depending upon the circumstances. Phenomena like diffraction, polarisation and interference
could be explained by considering light as a wave. The phenomenon of the photoelectric effect is explained
by assuming that light consists of particles called photons.

Question 3

Light incident on a surface separating two media

Answer 3

When light travels from one medium to another medium it either:
● gets absorbed (absorption)
● bounces back (reflection)
● passes through or bends (refraction)
When light is incident on a plane mirror, most of it gets reflected, and some of it gets absorbed in the
medium.
● Speed of light is given as c=λμ, where λ is its wavelength and μ is its frequency.
● Speed of light is a constant which is 2.998×108m/s or approximately 3.0×108m/s.

Question 4

Reflection of light by other media

Answer 4

A medium that is polished well without any irregularities on its surface will cause regular reflection of light.
For example, a plane mirror. But even then some light gets absorbed by the surface.

Question 5

Laws of Reflection

Answer 5

The incident ray, reflected ray and the normal all lie in the same plane. Angle of incidence = Angle of
reflection
[∠i=∠r

Question 6

Propagation of light

Answer 6

Rectilinear propagation of light: Light travels in a straight line between any two points

Question 7

Fermat’s Principle

Answer 7

● The principle of least time: Light always takes the quickest path between any two points (which may
not be the shortest path).
● Rectilinear propagation of light and the law of reflection [∠i=∠r] can be validated by Fermat’s
principle of least time.
Applications of Fermat’s Principle
We can make several observations as a result of Fermat’s Principle which will prove useful as we explore the
realm of geometric optics:
● In a homogeneous medium, light rays are rectilinear. That is, in any medium where the index of
refraction is constant, light travels in a straight line.
● The angle of reflection of a surface is equal to the angle of incidence. This is the Law of Reflection.
Example of Fermat’s Principle
Mirage is an example of this phenomenon. Sometime, we feel like we are seeing water on the road, but when
we get there, the road is dry. What we really witness is the light of the sky which is reflected on the road.
Since the air is very hot just above the road but it is cooler up higher. Hot air expands more than cool air and
is thinner, this leads to less decrease in the speed of light.

Question 8

Plane mirror

Answer 8

Any flat and polished surface that has almost no irregularities on its surface that reflect light is called as a
plane mirror.

Question 9

Image formation by a plane mirror

Answer 9

● The image formed by a plane mirror is always virtual and erect.
● Object and image are equidistant from the mirror

Question 10

Characteristics of images

Answer 10

● Images can be real or virtual, erect or inverted, magnified or diminished. A real image is formed by
the actual convergence of light rays. A virtual image is the apparent convergence of diverging light
rays.
● If an image formed is upside down then it is called inverted or else it is an erect image. If the image
formed is bigger than the object, then it is called magnified. If the image formed is smaller than the
object, then it is diminished.

Question 11

Principle of Reversibility of light

Answer 11

Principle of Reversibility of light
If the direction of a ray of light is reversed due to reflection off a surface, then it will retrace its path.

Question 12

Spherical Mirrors

Answer 12

Consider a hollow sphere with a very smooth and polished inside surface and an outer surface with a
coating of mercury so that no light can come out. Then if we cut a thin slice out of the shell, we get a curved
mirror, which is called a spherical mirror.

Question 13

Relationship between focus and radius of curvature

Answer 13

Focal length is half the distance between the pole and the radius of curvature.
F = R/2

Question 14

Curved Mirror

Answer 14

A mirror (or any polished, reflective surface) with a curvature is known as a curved mirror

Question 15

Important terms related to spherical mirror

Answer 15

● Pole (P): The midpoint of a spherical mirror.
● Centre of curvature (C): The centre of the sphere that the spherical mirror was a part of
● The radius of curvature (r): The distance between the centre of curvature and the spherical mirror.
This radius will intersect the mirror at the pole (P).
● Principal Axis: The line passing through the pole and the centre of curvature is the main or principal
axis.
● Concave Mirror: A spherical mirror with the reflecting surface that bulges inwards.
● Convex Mirror: A spherical mirror with the reflecting surface that bulges outwards.
● Focus (F): Take a concave mirror. All rays parallel to the principal axis converge at a point between
the pole and the centre of curvature. This point is called as the focal point or focus.
● Focal length: Distance between pole and focus.

Question 16

Rules of ray diagram for representation of images formed

Answer 16

● A ray passing through the centre of curvature hits the concave spherical mirror and retraces its path.
● Rays parallel to the principal axis passes through the focal point or focus

Question 17

Image formation by spherical mirrors

Answer 17

For objects at various positions, the image formed can be found using the ray diagrams for the special two
rays. The following table is for a concave mirror.

Question 18

Uses of spherical mirror based on the image formed

Answer 18

Concave and Convex mirrors are used in many daily purposes.
Example: Rear view mirrors in vehicles, lamps, solar cookers.

Question 19

Mirror formula and Magnification

Answer 19

1/v + 1/u = 1/f
where ‘u’ is object distance, ‘v’ is the image distance and ‘f’ is the focal length of spherical mirror, which is
found by similarity of triangles.
The magnification produced by a spherical mirror is the ratio of the height of the image to the height of the
object. It is usually represented as ‘m’.

Question 20

Sign convention for ray diagram

Answer 20

Distances measured towards positive x and y axes (coordinate system) are positive and towards negative x
and y-axes are negative. Keep in mind the origin is the pole (P). Usually, the height of the object is taken as
positive as it is above the principal axis and height of the image is taken as negative as it is below the
principal axis.

Question 21

Position and Size of image formed

Answer 21

Size of image can be found using the magnification formula m = h’/h = – (v/u) If m is -ve it is a real image
and if it is +ve it is a virtual image.

Question 22

Refraction Through a Glass Slab and Refractive Index

Answer 22

Refraction
The shortest path need not be the quickest path. Since light is always in a hurry, it bends when it enters a
different medium as it is still following the quickest path. This phenomenon of light bending in a different
medium is called refraction.

Question 23

Laws of Refraction

Answer 23

● The incident ray, the refracted ray and the normal to the interface of two transparent media at the
point of incidence, all lie in the same plane.
● The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for
the light of a given colour and for the given pair of media. This law is also known as Snell’s law of
refraction.

Question 24

Absolute and Relative Refractive Index

Answer 24

Refractive index of one medium with respect to another medium is called the relative refractive index. When
taken with respect to vacuum, it’s known as an absolute refractive index.

Question 25

Refraction through a rectangular glass slab

Answer 25

When the light is incident on a rectangular glass slab, it emerges out parallel to the incident ray and is
laterally displaced. It moves from rarer to denser medium and then again to the rarer medium.

Question 26

Refraction at a planar surface

Answer 26

Following Snell’s Law:
● Light bends towards the normal when moving from rarer to denser medium at the surface of the two
media.
● Light bends away from the normal when moving from denser to rarer medium at the surface of
contact of the two media.

Question 27

Refractive Index

Answer 27

The extent to which light bends when moving from one medium to another is called the refractive index. This
depends on the ratio of the speeds in the two media. The greater the ratio, the more the bending. It is also
the ratio of the sine of the angle of incidence and the sine of the angle of refraction, which is a constant for
any given pair of media. It is denoted by:
n = sin∠i/sin∠r = speed of light in medium 1/speed of light in medium2

The ratio of the speed of light in a vacuum to the speed of monochromatic light in the substance of interest
is known as the relative refractive index. Mathematically, it is represented as:
n = c/v
where n is the refractive index of a medium, c is the velocity of light in a vacuum and v is the velocity of light
in that particular medium.

Question 28

Total internal reflection

Answer 28

● When the light goes from a denser to a rarer medium it bends away from the normal. The
angle at which the incident ray causes the refracted ray to go along the surface of the two
media parallelly is called the critical angle.
● When the incident angle is greater than the critical angle, it reflects inside the denser medium
instead of refracting. This phenomenon is known as Total Internal Reflection.
E.g mirages, optical fibres

Question 29

Refraction at curved surfaces

Answer 29

When light is incident on a curved surface and passes through, the laws of refraction still hold true. For
example lenses.

Question 30

Spherical lenses

Answer 30

Spherical lenses are lenses formed by binding two spherical transparent surfaces together. Spherical lenses
formed by binding two spherical surfaces bulging outward are known as convex lenses while the spherical
lenses formed by binding two spherical surfaces such that they are curved inward are known as concave
lenses.

Question 31

Important terms related to spherical lenses

Answer 31

● Pole (P): The midpoint or the symmetric centre of a spherical lens is known as its Optical Centre. It is
also called the pole.
● Principal Axis: The line passing through the optical centre and the centre of curvature.
● Paraxial Ray: A ray close to the principal axis and also parallel to it.
● Centre of curvature (C): The centres of the spheres that the spherical lens was a part of. A spherical
lens has two centres of curvatures.
● Focus (F): It is the point on the axis of a lens to which parallel rays of light converge or from which
they appear to diverge after refraction.
● Focal length: Distance between optical centre and focus
● Concave lens: Diverging lens
● Convex lens: Converging lens

Question 32

Lens Formula, Magnification and Power of Lens

Answer 32

Lens formula and magnification
Lens formula: 1/v = 1/u = 1/f, gives the relationship between the object distance (u), image distance (v), and
the focal length (f) of a spherical lens.

Uses of spherical lens
Applications such as visual aids: spectacles, binoculars, magnifying lenses, telescopes.

Power of a Lens
Power of a lens is the reciprocal of its focal length i.e 1/f (in metre). The SI unit of power of a lens is dioptre
(D).

Question 33

Rules of ray diagram for representation of images formed

Answer 33

● A ray of light parallel to the principal axis passes/appears to pass through the focus.
● A ray passing through the optical centre undergoes zero deviation