Light

Introduction

When you enter into a dark room, nothing is visible. The moment you switch on the light, everything in the room becomes visible. How do we see things with our eyes? When you look at this book, the light falling on the book is reflected and enters your eyes.

Light is a type of energy that helps us to see all the things around us. Light can be detected by the human eye. We all know that light is essential for vision. Let us see more about light in this chapter.

Light is the only source of energy for plants. So, they entirely depend on light. People and animals derive energy from carbohydrates, protein and fat through their food. Plants produce food using the energy from Sun light, carbon-di-oxide and water by the process called as Photosynthesis.

Sources of Light

Objects which are able to emit light are known as light sources. Light rays can come from different sources. There are two types of sources of light.

1. Natural sources of light

2. Artificial sources of light

Natural Sources of light

Sources which emit light naturally are known as natural sources of light. The Sun is the primary and the major source of natural light. Stars also produce light, in the same way as the Sun do. The moon provides light, particularly in the night. Some living organisms have the ability to produce light named by bioluminescence. It is the effect of certain chemical reactions occurring in the organism. Fireflies, jellyfish, glow worm, certain deep sea plants and some microorganisms can emit light naturally.

Artificial Sources of light

Apart from the natural sources, light can also be produced artificially. The different light sources that are able to produce light artificially can be put under three broad categories. Artificial sources are man – made light sources such as flame of candle, incandescent lamp, neon lamp, Sodium lamp etc.

1. Incandescent Sources:

When certain objects are heated to a high temperature, they begin to emit light. The glowing of hot iron rod is a kind of Incandescent light.

Example: Candle, incandescent lamp.

2. Gas Discharge Sources:

Passing electricity through certain gases at a very low pressure (discharging) can produce light .

Example: Neon lamp, Sodium lamp

Properties of light

In this section, we shall examine some properties of light. Light has some fundamental properties as mentioned below

Ø Rectilinear propagation of light

Ø Reflection

Ø Speed

Ø Interaction of light with matter

· Types of material according to permeability

· Formation of shadows

· Plane mirror and images

Ø Spectrum

The path of light

How does light travel?

Ø Have you ever seen the scene of light penetrating through the branches of trees in denser forest?

Ø Have you ever seen the path of sun light entering through the hole of a cement grill building?

Ø Have you ever seen the path of a laser light?

Light travels in straight line, it cannot bend the path itself. This is called as the rectilinear propagation of light. This is one of the most important property of light.

Pinhole Camera

Pinhole camera is a simple device which helps us to understand about the rectilinear propagation of light.

The above picture shows a model of a pinhole camera. O is small hole by a pin. XY is the object and Y'X' is the image of XY. As light travels in straight line, one light ray from X travels along the XO strikes the screen X'.

In similar way, another light ray staring from Y and travels along YO strikes the screen Y'. Similarly, all the rays in between X and Y fall on the screen between Y' and X'. Thus Y'X' becomes the image of XY. The image produced is temporary, if a simple paper is used. The image can be made permanent if the paper is replaced by a photographic plate.

Reflection

A mirror reflects our face. A still water body like a pond reflects the scenery around it. When we see our face in the mirror, we see the light rays from our face bouncing off the surface of the mirror. How the rays of the light are reflected? Take a plane mirror. Cover it with black paper. Cut a small slit as shown in the figure. If you shine light on the mirror from a torch light or sunlight, you will get a small ray of light. We can use this to study the properties of light.

Place a blank white sheet on a level ground out in the open. Choose a place where partly the sheet gets sunlight and partly it is in shadow. Hold the mirror with the slit facing the sun. You can see a straight ray of light reflected from the slit on the paper. Hold another mirror to reflect this ray. Observe well.

The light falling on the mirror is called as incident ray and the light reflected is called reflected ray.

Relationship between the incident ray and reflected ray

Draw a straight line ABC and angles as shown in the figure above. The line 1 is at 60O from BD, 2 at 30O from BD. Now , the line 4 at 60O from BD and line 3 at 30O from BD. The line BD is perpendicular, to ABC. Hold the mirror along the line ABC. Use the mirror with slit and make a ray go along the line 1 and reach the mirror at point B. Observe where the reflected ray is.

Now, try keeping the mirror with slit and make the incident ray go along line 2. Now do we see that the reflected ray is along line 3? Line BD, which is perpendicular the mirror surface is called as normal. The angle between the incident ray and the line BD is called angle of incident. Similarly, the angle between reflected ray and the normal is called as angle of reflection.

Terms used in reflection of light

Incident ray: The ray of light that falls on the surface of the reflection materials. In figure, PO is the incident ray.

Reflected ray: The ray of light that comes from the point when the incident ray falls on the reflection material. In the figure, OQ is the reflected ray.

Point of incidence : The point of which are incident ray strikes the reflecting surface is the point of incidence. In the figure ‘O‘ point of incidence.

Normal : The perpendicular line drawn from the point of incidence to the plane of reflecting surface is called normal. In figure, ON is the normal.

Angle of incidence: The angle formed between the incident ray PO and the normal ‘ON’ is angle of incidence. It is denoted by i.

Angle of reflection: The angle formed between the reflected ray OQ and the normal ON is angle of reflection. It is denoted by r.

Laws of reflection:

1. The angle of incidence is always equal to the angle of reflection. i = r

2. The incident ray, the reflected ray and the normal at the point of incidence lie on the same plane.

Example 1: A light ray strikes a reflective plane surface at an angle of 43° with the plane surface.

i. Find the angle of incidence.

ii. Find the angle of reflection. I

iii. Find the angle between the incident and the reflected ray

iv. Find the angle between the reflected ray and the plane surface.

Solution: We use the diagram shown below to answer the questions.

a) Angle of incidence: i = 90 -43 = 47 °

b) angle of reflection r = i = 47°

c) i + r = 47 + 47 = 94 °

d) x = 90 - r = 90 - 47 = 43 °

Types of reflection

On a mirror we can see our image, but not on the wall.Both the surface reflects light. Only because the reflected light comes to our eyes, we are able to see it. If the wall was not reflecting light, then we cannot see it.

We saw earlier that the light reflects off surfaces in a very predictable manner, in accordance with the law of reflection. The laws of reflection holds good for all surfaces irrespective of the shape. Vertical surfaces, angled surfaces, and even over the curved surfaces, the laws of reflection holds good. As long as we can draw the normal, perpendicular to the surface at the point can be drawn, the angle of incidence at that point will be equal to angle of reflection.

The law of reflection is always observed regardless of the orientation of the surface. If the surface is smooth, and flat, all points on it have the normal in the same direction. Therefore a set of parallel rays striking the surface will be reflected at an angle, but the rays themselves will still remain parallel to each other.

However ,consider a surface which is not smooth. Such as the surface of a wall. Roughness of the wall means that each individual ray meets a surface which has a different orientation. The diagram below depicts the case. Five incident rays labelled as A, B, C, D and E approach a surface. The normal line at each point of incidence is shown in black and labelled with an N.

In each case, the law of reflection is followed, resulting in five reflected rays labelled A', B', C', D', and E'. While the incident rays were parallel to each other, the reflected rays are going in different directions. The result is that the rays of light are incident upon the surface in a concentrated bundle and are diffused upon reflection.

Broadly, we can say that there are two types of reflection. If the surface is smooth then we have specular reflection. The parallel light rays striking the surface gets reflected, yet individual reflected rays remain parallel.

If the surface is rough, then we have diff used reflection. Light rays, after reflection go in many directions.

Types of beam of light

Generally light is not a single ray, but a bundle of rays which are called as a beam of light.

A light beam can be a bundle of parallel rays, convergent rays or divergent rays. Let us look at the light coming from the Sun. The rays of sunlight are parallel. However look at the rays of light coming out of a candle. Light rays go in all directions, from the candle fire. These rays are divergent. Light rays from a flash light is also divergent. Using lenses we can converge light rays. Using a lens, you can focus sunlight at a point. That is what we are making the light rays to converge.

Speed of light:

When lighting a bulb in a dark room, light spreads the whole room quickly. This is because the light travels very fast. Light travels three lakh kilometers per second in air or vacuum. In theory , nothing can travel faster than light

Interaction of light with matter

Take a piece of clear glass, a paper and a metal sheet. Shine a light from one side of each object and see if the light penetrate on the other side. Readily, we can see light enters and comes out of the other end of clear glass, whereas the light is bit dim through a paper. Light does not pass through metal sheet. Depending upon permeability, materials can be classified into three categories.

Transparent Material:

Materials that allow light to pass through completely are known as transparent material.

Example: Eye glasses, clear drinking glass, clear water, face glasses used in buses.

Translucent Material:

Objects that allow light to pass through partially are called translucent material. For example, we cannot see the image of someone who stands behind a rough window glass, because it allows only a part of light from the person.

Opaque Material:

Materials that are not able to allow light to pass through, are called opaque material.

Example: Wall, thick card board, stone, etc.

Shadows

As we saw earlier, light is obstructed by certain materials. Light travels in a straight line. Hence it cannot go around such objects. That is why we see shadow. Shadow is always against, opposite side of light source. It is caused by opaque objects that stop light from propagating.

Parts of shadow

When an opaque object is placed in the path of light from a point source, a uniform dark shadow will appear on the screen. This is shadow is called as umbra. When an opaque object is placed in the path of light coming from a broad source of light, a small umbra will appear on the screen and an illuminated shadow area appears around umbra. This illuminated shadow area is called as penumbra. The penumbra always surrounds the umbra. The umbra is the darkest part of a shadow. In this part, light rays are completely prevented by the opaque object. The lighter shade of shadow is the penumbra.

Properties of shadow

1. All objects do not form shadows. Only opaque objects form shadows

2. Shadows will be formed in the opposite side of light source

3. It cannot be determined the characteristics of an object by its shadow.

4. The shadow will be always darker, whatever may be the color of light rays.

5. Light source, opaque object are shadow all are in a straight line.

6. The size of shadow depends upon the distance between light source and object and the distance between object and the screen.

Eclipses

An eclipse is an incident, when any astronomical object is partially or fully obscured due to the placement of another astronomical object in the presence of light.

Thus, solar and lunar eclipses are occurring that are due to the property of light known as the rectilinear propagation of light.

Solar eclipse

Solar eclipse occurs, when the moon arrives between the sun (S) and the earth(E). The shadow of the moon appears on the earth at A as shown in picture. Hence, those who are at the region A are unable to see the Sun instantly. This is solar eclipse. But, those who are at the region B and C are able to see the sun partially.

Lunar eclipse

Lunar eclipse occurs, when the earth (E) comes between the sun (S) and the moon (M). The earth prevents light coming from the sun and makes shadow on the moon. This is lunar eclipse.

Plane Mirror and Reflection

A polished (or) smooth surface (like glass) which forms image by reflection is known as mirror. A plane mirror is a mirror with a flat reflective surface. A plane mirror makes an Image of objects in front of it.

Ø The image is upright.

Ø The image and the object are the same size.

Ø The image is virtual.

Ø The image is laterally inverted. The boy’s left‑ hand is his image’s right hand.

Ø We can determine the position of an object’s image by drawing light rays.

Ø The distance of the object from the mirror is equal to the distance of the image from the mirror (a=b).

Ø The ray of light AO from the boy head strikes the mirror at point O. AO is called the incident ray.

Ø The incident ray makes an angle of incidence. i. with the perpendicular line ON.

Ø The light ray OA strikes the surface and is reflected as OE. OE is called the refelected ray. This ray makes an angle of reflection r with normal.

Ø The line, ON, perpendicular to the surface is known as the normal.

Real and virtual images

We have seen images being formed in a pinhole camera and a mirror. Firstly, the image of the pinhole camera was formed on a screen. While the image made by the mirror is not obtained on a screen. The images that are obtained on a screen are called ‘real image’ and that which cannot be obtained on a screen ‘virtual image’. Also notice that the image on pinhole camera was upside down. While the mirror image was upright.

Properties of Image formed in a plane mirror

Ø Image formed in a plane mirror is upright

Ø Image formed in a plane mirror is virtual

Ø The image is of the same size as the object

Ø The distance of the image from the plane mirror is equal to the distance of the object from the mirror

Ø Image is laterally inverted.

Colour

Colour of sunlight : Light is a form of energy in the form of a wave that simulates that retina of our eyes. Visible light is a spectrum of a number of waves with different wavelength range from 400nm to 700nm (1nm = 10-9 metre) each wave has a definite wavelength represents a particular color. The band of visible light is VIBGYOR.

V - Violet

I - Indigo

B - Blue

G - Green

Y - Yellow

O - Orange

R - Red

Violet colour has shorter wavelength and red color has longer wavelength.

When light ray of particular wavelength (Colour) strikes the retina of our eye, our brain perceives that specific colour. When all colors of visible light strikes the retina of our eye at the same time, our brain perceives white.

Prism

A prism is an object made up of a transparent material, like glass or plastic that has at least two flat surfaces that from an acute angle (less than 90O degrees).

When white light is passed through a prism as shown in the figure, the colors of the rainbow emerge from the prism.

Newton Disc:

Newton suggested a process of mixing different colors to make white color by setting an arrangement as shown figure below. Newton Disc is a card board disc with seven equal sectors colored red, yellow, orange, green, blue, indigo and violet. When the disc turned quickly, the retina receives the sensation of the spectrum simultaneously and disc appears white. Using this disc, one can explain that white is a combination of VIBGYOR.

Synthesis of colour

Synthesis of colour is the method of creating colour by mixing various proportion of two (or) three distinct colours of light. These distinct colours are Red, Green and Blue called as primary colours.

Ø Equal proportions of two primary colour create a secondary color.

Ø Magenta, Cyan and yellow are called secondary colour.

Ø Equal proportions of all three primary colour create white.