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Light is a form of radiant energy that you can detect with your eyes. Light energy comes from chemical energy, electrical energy and nuclear energy. It is a combination of electrical and magnetic energy that

travels approximately 300 000 km per second. It can pass through anything that is transparent, more or

less passes through translucent objects (frosted window) but doesn't make it through opaque objects

such as a brick wall.Light is made-up of photons. When these photons

move or the light travels, it travels in straight lines but in small waves. 

There are two types of light:

Luminous Objects that emit their own light.

For example; Sun, Candle etc.

Non-LuminousObjects that do not emit light and

therefore reflect light emitted from luminous objects

For example; Moon, Book etc.

An image is a picture or appearance of a real object, formed by light that passes through a lens or is reflected from a mirror. There are two kinds of images:

1. Real ImagesAn image formed by a lens or mirror that may be projected onto a screen. It is formed by the convergence of light. Examples of real images include the images formed by projectors, cameras (the negative) and photographic enlargers.

2. Virtual image An image that can be seen by looking into

the lens or mirror, but not by projection onto a screen. The image in your bathroom mirror (a plane mirror) is a virtual image.

If an object does not emit its own light, it must reflect light in order to be seen. Reflection involves two rays –

an incoming ray or incident ray and an outgoing or reflected ray. If we draw a line perpendicular to a flat

surface, this line is said to be “normal”. The angle between the incident ray and the normal will be

identical in measurement as the angle between the reflected ray and the normal.

Reflection from a smooth, mirror-like surface is called “specular”. If the surface is rough the rays of light are reflected in many different directions. The angle of incidence and reflection are still equal but the rays appear to

be scattered. This is known as diffuse reflection.

Laws of Reflection1. The incident ray, the reflected ray, and the normal to the

reflecting interface at the point of incidence all lie in the same plane.

2. The angle of incidence equals the angle of reflection.

Refraction is the turning or bending of any wave, such as a light or sound wave, when it passes from one medium into another of different optical density. Light travelling through air and then going through water is an example of a light ray changing medium. Due to the difference in optical density the speed of light changes as it enters the other medium. The light ray may either speed up or slow down depending on the new medium and this causes it to bend. If the light travels slower, then this medium is called the “Denser Medium”. If the light ray travels faster, then this medium is called the “Rarer Medium”. When light enters a denser medium the ray bends towards the normal – when light enters a rarer medium the light is bent away from the normal.

Law 1 : The incident ray, the normal and the refracted ray all lie in the same plane.

Law 2 : The ratio sine of the angle of incidence (i)               sine of the angle of refraction (r)is constant for light of a given colour (frequency) crossing from one given media to another. This ratio is called the refractive index (n).     n = sin i            sin r This is known as “Snell’s Law”.

The refraction of light is explained by the fact that light has a wave nature

Laws of Refraction

Refraction is the bending of light through a substance. Different substances bend light at

different angles. White light is made up of a full spectrum of colors. Each color has a different

wavelength, and bends at a different angle. This is the same effect that produces rainbows in the

atmosphere. The most common illustration of this is a glass prism as shown below.

When we talk about the speed of light, we're usually talking about it in a vacuum. When light travels

through something else, such as glass, diamond, or plastic, it travels at a different speed. The speed of

light in a given material is related to a quantity called the index of refraction, n, which is defined as the

speed of light in vacuum divided by the speed of light in the medium: 

index of refraction : n = c / v 

When light travels from one medium to another, the speed changes, as does the wavelength. Although the

speed changes and wavelength changes, the frequency of the light will be constant.

The greater the refractive index of a medium, the lower is the speed of light. The more light is slowed

the more it is bent.

Total internal reflection is the complete reflection of a ray of light in a medium such as water or glass, from the

surrounding surfaces back into the medium. When light rays travel at an angle greater than the "critical" angle, which is determined by the medium, the light reflects back into the

medium. At all angles less than the critical angle, both reflection and refraction occur. Total internal reflection is

responsible for rainbows, atmospheric halos, the sparkle of a diamond, and the path of light through optical fibres.

The critical angle can be calculated by using the following equation:

n = 1 / Sin C

Where n= refractive index

C= critical angle

Total internal reflection is often used in telecommunications, periscopes, local area networks, endoscopes etc.

A critical angle is the angle of incidence of a ray where it meets a substance, or boundary, which creates a refraction angle of 90 degrees.For total internal reflection to take place, light must be travelling from a denser to a rarer medium. The angle of incidence should also be greater than the critical angle of the medium.

A lens is a transparent material, such as glass, that has either one curved surface and one flat surface or two curved surfaces. Lenses can be either convex or concave. Convex lenses (converging lenses) are thicker at the middle than the edges and concave lenses (diverging lenses) are thicker at the edges than the middle.

When light travels through lenses, refraction occurs. The light bends either inwards or outwards depending on the lens.

Optical CentreIt is the centre of a lens. It is denoted by the letter O. A

ray of light passing through the optical

centre of a lens does not suffer any deviation. It is also referred to as optic

centre.

Principal AxisPrincipal axis is the straight line

joining the centers of

curvatures of the two curved surfaces of a

lens.

Principal FociRays of light can pass through the lens in any direction and hence there will be two principal foci on either side of the lens and they are referred to as the first principal focus and the second principal focus of a lens.First Principal Focus (F1)It is a point on the principal axis of the lens such that the rays of light starting from it (convex lens) or appearing to meet at the point (concave lens) after refraction from the two surfaces of the lens become parallel to the principal axis of the lens.

The distance from the optic centre to the first focus is called the first focal length (f1) of the lens.

Second Principal Focus (F2)It is a point on the principal axis of the lens such that the rays of light parallel to the principal axis of the lens after refraction from both the surfaces of the lens pass through this point (convex lens) or appear to be coming from this point (concave lens).

The distance from the optic centre to the second principal focus is called the second focal length (f2) of the lens.

If the medium on both sides of the lens is same then the first and the second focal lengths will be equal. Focus of a convex

lens is real whereas that of the concave lens is virtual.