Light
● It is a form of energy that enables us to see objects
● Characteristics of light
○ Light is an electromagnetic wave
○ Light travels in a straight line (rectilinear propagation of light)
○ It can travel through vacuum
○ The speed of light is 3 x 10^8 m/s
○ The speed and wavelength of light changes when it travels from one medium to another
● Reflection of light
○ The phenomenon of bouncing back of light in the same medium of striking the surface of any
object
● Incidence ray
○ The ray of light which strikes the surface
● Reflected ray
○ The ray of light that bounces back from the surface
● Normal ray
○ An imaginary line (perpendicular) drawn from the point of incidence of the surface
● Angle of incidence
○ The angle between the incident ray and the normal ray
● Angle of reflection
○ The angle between the reflected ray and the normal ray
● Law of reflection
○ The incident ray reflected ray and normal ray at the point of incidence lie on the same plane
○ The angle of incidence is always equal to the angle of reflection
● Mirrors
○ Plane mirrors
○ Spherical mirrors
■ Convex )(
■ Concave ()
● Image
○ A picture that is obtained on the screen
○ Real image
■ An image that can be obtained on the screen.
■ It is inverted
○ Virtual image
■ An image that cannot be obtained on the screen
■ It is erect or upright/the same
● Characteristics of image in plane mirror
○ The image is virtual
○ The image is laterally inverted (the left right reversal of an object is lateral inversion
○ The size of the image is the same as the size of the object
○ The object distance and mirror distance is equal
○ The object is formed behind the mirror
● If angle of reflection is more than the angle of incidence, velocity decreases and vice versa
● Refractive index
○ Speed of light in air/vacuum/speed of light in medium
○ 3.0 x 10^8 m/s
○ Sin i/Sin r (rarer/denser(
● Refractive index
○ The quantity that describes how much light is slowed down
● Total internal reflection
● Two condition for total internal reflection must be met
○ The light must be going from a more dense medium towards a less dense ore
○ The angle of incidence must be greater than critical angle
● If the angle of incidence is greater than the critical angle, the light is reflected back into the same
medium. THis phenomenon of light is called total internal reflection
● Critical angle
○ The critical angle is the angle of incidence where the angle of refraction is 90 degree
○ The light must travel from an optically more dense medium to an optically less dense medium
● Refractive index and critical angle
○ The critical angle(c) of a material is related to its refractive index (n)
○ sin(c) = 1/n
○ N = 1/sin(C)
● Law of refraction
○ The incident ray, refracted ray and normal ray at the point of incidence lie on the same plane
○ The angle of incidence is always equal to the angle of reflection
● How does total internal reflection occur
○ When the light is moving from a denser medium towards a less dense one
○ All the light is reflected
○ Angle of incidence is greater than critical angle
● Optic fibres
○ Optic fibres can be made very thin and flexible
○ This can be used in medicine industry and communication
● Advantages of optic fibres
○ Optic fibres consists of hair size threads of glass or plastic through which light can travel
○ The inner part of the fibre is called core that carries the light and the outer concentric shell is
called cladding
● Lenses
○ Lenses are made up of transparent material like glass or plastic
● Types of lenses
○ Convex/ converging lens
○ Concave lens/ diverging lens
● Convex lens
○ If the lens is thicker in the centre than the edges, it is called convex lens
● Uses of convex lenses
○ Magnifying glass
○ Cameras and eyeglasses
○ Telescope and Microscopes
● Concave lens
○ If the lens is thinner in the centre than in the edges, it is called concave lens
● Centre of curvature
○ Centre of sphere
● Principal axis
○ A straight line passing through the centre of curvature of the lens
● Principle focus (convex)
○ When a parallel beam of light incident on a convex lens after refraction, all the refracted rays
meet at a common point on the principal axis
● Principle focus (concave)
○ Focus is virtual, after refraction, the reflected
rays are diverging away from the lens
● Optical centre
○ Geometric centre of the lens
● Focal length
○ It is the distance between the principal focus and optical centre
● Ray diagram rules
○ A ray diagram is a representation of the possible paths light can take to get from one place to
another
○ Rule 1 - incident light rays that are parallel to the principal axis will pass through the focal point
after refracting through the lens
○ Rule 2 - Incident light rays are directed at the optical centre will pass directly through in other
words, they are not refracted
○ Rule 3 - Incident rays that pass through the focal point before hitting the lens will become
parallel to the principal axis after refracting through the lens
Image formation at convex lens
● Object outside 2F → Real, inverted, smaller, Image formed between F’ and 2F’
● Object at 2F → same size, inverted, real, image formed at 2F’
● Object between 2F and F → inverted, larger, real, image formed outside 2F’
● Object is placed between optical centre and F → virtual, magnified, upright/erect
