Energy is the ability or capability to do work. Light, sound, electricity, heat, chemical energy, mechanical energy, and so on are all different kinds of energy. You will learn about light energy in this lesson.


When light hits a surface and bounces back, this is called reflection. How something reflects depends on how smooth or rough it is. A reflection from a smooth surface like a mirror is regular, but a reflection from a rough surface like a wall or stone is irregular or diffuse.

Forms of Reflection

Regular reflection and diffuse reflection are the two types of reflection.

Regular Reflection

Regular reflection happens when a parallel beam of light hits a smooth or polished surface, like glass, and is reflected in the same direction.

Diffuse reflection

In contrast to specular reflection, which only occurs at one angle, diffuse reflection occurs when light is reflected from a surface at multiple angles.

Applications Of Reflection In Some Optical Instruments 

  • We use the plane mirror as a dressing mirror because it shows us how we look.
  • The simple periscope has two flat mirrors that are turned at an angle of 450 degrees to the horizontal and face each other. It lets you see past obstacles. For example, simple periscopes are used in submarines to see ships on the ocean.
  • The sextant is used to figure out how high the sun is in the sky. It also has two mirrors.
  • The kaleidoscope: When this is turned, it makes different colour patterns. It uses flat mirrors that are tilted at an angle of 600 degrees to each other

Refraction of light

When light rays go from a thin medium to a thick medium or vice versa, this is called refraction. During this time, the light rays are bent. For example, light that comes back from an object submerged in water travels first through the water and then through the air before it reaches the eyes of the observer. At some angles, the part of the object that is in the water will look like it curves where it meets the water. This is because light from that part of the object that is in the water bends.

Light moves at different speeds through different things, so where two different things meet, its speed changes. When a beam of light hits a boundary at an angle, the light on the side that gets there first is fired to speed up or slow down, and then the light from the other side touches the new materials. It causes the beam to bend or scatter at the edge.

When rays of light hit curved, smooth, or polished surfaces called lenses, they can also bend. There are two kinds of lenses: convex or converging lenses and concave or diverging lenses.

Light rays spread out when they hit a lens with a concave or diverging surface. When light hits concave or diverging lenses, it spreads out from a point called the focal point.

Rays of light bend inward when they pass through a convex or converging lens. When light rays hit a convex

Apparent Depth

Refraction is a natural process that can explain the idea of “apparent depth.”

When a coin is dropped into a cup of water and viewed straight on from above, it looks like it is closer to the surface of the water.

The real depth is the distance between where the coins are and where the surface of the water. The apparent depth is the distance between where the coins seem to be and where the surface of the water is. We also seem to have depth in the following areas:

It always looks like a swimming pool is shallower than it really is. This could be dangerous for a swimmer who doesn’t know how to swim well and thinks the pool isn’t deep enough to drown in.

When seen straight on from above, a thick glass slab, like a rectangular glass block, doesn’t look as thick as it really is.

Laws of refraction

The first law of refraction 

This means that the incident ray, the refracted ray, and the normal to the interface are all on the same plane.

The second law of refraction

The angle of the incident ray = i°

Sine of the angle of incident ray = sin i° Angle of refracted ray = r°

Sine of the angle of refracted ray = sin r

n = Refractive index

Therefore, n =

sin r Example

Calculate the refractive index, n, if the incident angle is 30°and the refracted angle is 60’’.


Sini°= sin 30° = 0.5 Sinr° = sin 60° = 0.867 sin i° 0.50

n =——- = —

sinr® 0.87

Therefore, n = 0.577

The Eyes and Vision

Vision means being able to see. The eyes are what let us see.

The eye is made up of the following:

Eye lens

Eye lens It shows real images of things that are upside down on the retina.


Retina The nerves for seeing starts at the retina.


Cornea It starts to bend the light that comes into the eye.


It starts to bend the light that comes into the eye.

The pupil is a round hole in the eye that lets light in. When it’s dark, the opening gets bigger so lighter can get in. When it’s bright, the opening gets smaller so less light can get in.


It controls the amount of light that gets into the eye by making the size of the pupil bigger or smaller.

Ciliary Muscles

Ciliary Muscles This lets the eye focus on different things at different distances.

The optic nerves

The optic nerves send signals from the retina to the brain so that the brain can figure out what they mean.

The aqueous humour

Aqueous humour is a clear liquid that fills the space between the eye lens and cornea. It helps light bend in different ways.

vitreous humour

Vitreous humour is a clear liquid that sits between the eye lens and the retina. It looks like jelly. It also helps bend light and keep the shape of the eye.

Dispersion of Light

White light is broken up into different colours during the process of dispersion.

Seven different colours make up white light: red, orange, yellow, green, blue, indigo, and violet. It’s easy to remember by putting the first letter of each colour into an acronym (ROYGBIV). When they are taken apart, they appear in this order.

The spectrum of white light is made up of the seven colours of ROYGBIV. When white light moves from the air or a vacuum into a more dense medium, the speeds of its individual colours change. This causes dispersion, which means that the different colours that make up white light are spread out. If you shine a beam of white light through a triangular prism in front of a white screen, the light will spread out.

Dispersion and Rainbow

When rays of sunlight hit a raindrop, the light bends or “refracts” in a way that makes a rainbow of colours. The red light bends the least, while the violet light bends the most. The rainbow is made up of all the colours that fall between red and violet.

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