Light Notes 7th Science Lesson 12 Notes in English
7th Science Lesson 12 Notes in English
12] 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 n 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
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:
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. However, as they are much farther away than the Sun, the light from them are too weak. 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.
- 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.
- 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:
- Rectilinear propagation of light
- Reflection
- Speed
- Interaction of light with matter
- Types of material according to permeability
- Formation of shadows
- Plane mirror and images
- Spectrum
The path of light:
How does light travel?
- Have you ever seen the scene of light penetrating through the branches of trees in denser forest?
- Have you ever seen the path of sun light entering through the hole of a cement grill building?
- Have you ever seen the path of a laser light?
Pinhole Camera:
Pin hole camera is a simple device which helps us to understand about the rectilinear propagation of light.
The above picture shows a model of a pin–hole 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.
Is there any 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 60 o 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? Is the reflected ray go along 4?
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.
Can you make out relationship between the angle of incident ray and the angle of reflected ray? Yes. Is it not obvious that the angle of incident is same as the 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 i.
Laws of reflection:
- The angle of incidence is always equal to the angle of reflection. ∟i = ∟r.
- The incident ray, the reflected ray and the normal at the point of incidence lie on the same plane.
Example 1:
In the figure, the incident ray makes 27° with the normal, then find the angle of reflection.
Solution:
Angle of incidence = 27° \ According to the laws of refelection, the angle of refelection = Angle of incidence = 27°
Example 2:
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.
iii. Find the angle between the incident and the reflected ray
iv. F ind 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. Why? 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 fl at, 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. What happen when the light ray hits the rough surface? 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 diff used 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 refl ected rays remain parallel.
If the surface is rough, then we have diff used reflection. Light rays, after reflection go in many directions.
In fact during the day, our class room is illuminated by sunlight . Walls and floors are exposed to diff used reflection. Suppose walls were smooth mirror like. Then sunlight entering through the window will get bounced by the floor at an angle above towards the roof. And it will never get reflected to left or right. That is left and right walls will remain dark. However walls and floors are not smooth surfaces. Therefore, incident light from the window get bounced in all directions that the whole room is illuminated with diffused light.
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. Often the headlight of car gives parallel rays. 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 lights 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.
Shadows:
How are shadows formed?
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 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.
Opaque Material:
Materials that are not able to allow light to pass through, are called opaque material.
Example: Wall, thick card board, stone, etc.
Properties of shadow:
- All objects do not form shadows. Only opaque objects form shadows.
- Shadows will be formed in the opposite side of light source.
- It cannot be determined the characteristics of an object by its shadow.
- The shadow will be always darker; whatever may be the colour of light rays.
- Light source, opaque object are shadow all are in a straight line.
- 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.
Solar Eclipse
Lunar eclipse:
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.
Real and virtual images:
We have seen images being formed in a pinhole camera and a mirror. Can we see what is different in both of these images? 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.
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. This show, white is not a colour at all. But, it is the combination of all the colors of the visible light spectrum. If the entire wavelength (colours) of visible light spectrum give appearance of white similarly, the observe of all there wavelength of visible light, will lead appearance of black.
What is prism?
A prism is an object made up of a transparent material, like glass or plastic that has at least two fl at surfaces that from an acute angle (less than 90o degrees).
| Difference between the images formed in Pinhole camera and Plane mirror | |
| Images formed by Pin hole camera | Images formed in Plane mirror |
| The image is real | The image is virtual |
| The image may not be equal to the size of the object | The image is equal to the size of the object |
| The image is inverted | The image is erect |
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.
We know that white shirt will reflect white light and we have seen that white light consists of different colours. When we look at the white shirt through the yellow gelatin paper, we see it as yellow in color. From this, we can say that the yellow gelatin paper did not allow any other color except yellow to pass through. Similarly, we conclude that red gelatin paper allows only red light and blue gelatin paper allows only the blue light.
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.
- Equal proportions of two primary colour create a secondary color.
- Magenta, Cyan and yellow are called secondary colour.
DO YOU KNOW?
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. Sun light acts a vital role in the process of photosynthesis.
Is the moon a luminous object?
The moon provides light as well, but it cannot produce light by its own. The light emitted by the Moon is the light of the Sun reflected towards the Earth. When we see the Moon, we see only the Moon’s lighted part. Thus, half of the moon is always facing the Sun and receiving light from it. Hence, we receive light from the moon.
We often use a kind of gas discharge lamp that uses fluorescence to produce visible light. The electric current in the gas excites mercury vapour, which produces short-wave ultraviolet light that then causes a phosphor coating on the inside of the lamp to glow in visible light.
Al-hasan-Haytham was a scientific thinker who made important contribution to the understanding of vision, optics and light. He observed that light coming through a tiny hole travelled in straight lines and projected an image onto the opposite wall. Based on such experimentation, he concluded that vision is accomplished by rays coming from external luminous sources and entering the eye, rather than through rays emitted from the eye as was then commonly believed. He is the first one to experiment with light and found important properties like the rectilinear propagation of light.
Before the advancement of camera, Pinhole camera was used to photograph movement of the sun over a
long period of time. This type of photography is known as solo-graphy and also be used for observing and recording solar eclipses. And it was also used to take photograph of stationary objects.
Optical fibre is a device that works on the principle of total internal reflection by which light signals (huge data) can be transmitted from one place to another place with a negligible loss of energy in a very short time. It consists of a cable having one or more thin flexible fibers with a glass core through which light signals can be sent. Optical fiber can be twisted and bent easily. When a light a ray of light is incident at one end of the core of optical fiber, it suffers total internal reflection at the many places inside the fiber and emerges at the other end with negligible loss of energy. The data or information in the form of pulses of light, can be sent through bundles of optical fibers. Optical fibers have become very important in high-speed communications, such as cable TV and high-speed broadband services. Fiber optic cables are able to carry more signals than traditional copper cable telephone lines.
Why is the word “AMBULANCE” written backwards in ambulance vehicle? This is due to lateral inversion .The phenomenon due to which the left side of an object appears to be right side of the object in its image in a reflecting medium (mirror). So that drivers see the word the right way around in their rear-view mirror.
Why danger lights in vehicles are red in colour?
- Red color is scattered the least by air molecules.
- Red color has the highest wavelength of all the other colors. So red color is able to travel the longest distance through air, fog.