Light Reflection and Refraction Flashcards
Light: Definition
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.
Nature of Light
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.
Light incident on a surface separating two media
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.
Reflection of light by other media
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.
Laws of Reflection
The incident ray, reflected ray and the normal all lie in the same plane. Angle of incidence = Angle of
reflection
[∠i=∠r
Propagation of light
Rectilinear propagation of light: Light travels in a straight line between any two points
Fermat’s Principle
● 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.
Plane mirror
Any flat and polished surface that has almost no irregularities on its surface that reflect light is called as a
plane mirror.
Image formation by a plane mirror
● The image formed by a plane mirror is always virtual and erect.
● Object and image are equidistant from the mirror
Characteristics of images
● 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.
Principle of Reversibility of light
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.
Spherical Mirrors
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.
Relationship between focus and radius of curvature
Focal length is half the distance between the pole and the radius of curvature.
F = R/2
Curved Mirror
A mirror (or any polished, reflective surface) with a curvature is known as a curved mirror
Important terms related to spherical mirror
● 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.
Rules of ray diagram for representation of images formed
● 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
Image formation by spherical mirrors
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.
Uses of spherical mirror based on the image formed
Concave and Convex mirrors are used in many daily purposes.
Example: Rear view mirrors in vehicles, lamps, solar cookers.
Mirror formula and Magnification
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’.
Sign convention for ray diagram
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.
Position and Size of image formed
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.
Refraction Through a Glass Slab and Refractive Index
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.
Laws of Refraction
● 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.
Absolute and Relative Refractive Index
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.
