LIGHT PART I
| Explain the concept of light |
| Light can carry both energy and information. It both is the way the sun delivers ener gy to the |
| world, making life possible, and it is a very important way that we learn about the world, through |
| Identify sources of light |
| The sun is the natural source of light. On the other hand, torch, candle, kerosene lamp, electric |
| bulb, Florescent lamp, etc are some common examples of artificial sources of light, bodies which |
| give out light are called Luminous bodies . |
| Difference between Luminous and Non-luminous Bodies |
| Distinguish luminous from non-luminous bodies |
| Non-luminous bodies are bodies which do not produce light, non-luminous bodies are seen |
| because they reflect light from luminous bodies in such a way that our eyes receive light from |
| travel in a straight line. Due to transmission of light, we are able to see. |
| Propagation and Transmission of Light |
| The Concept of Rays and Beam of Light |
| Explain the concept of rays and beam of light |
| A straight line showing a path of light is called a ray which is the direction of the path taken by |
| light. Rays of light are represented in diagrams by full straight line with arrow marking the |
| direction in which the light is travelling |
| A beam is a str eam of light rays, and it is usually represented by a number of rays. Rays of light |
| can converge (converging r ays) or they can diver ge (diverging rays) |
| Verification that Light Rays Travel in Straight Line |
| Verify that light rays travels in straight line |
| Aim: To demonstrate the rectilinear propagation of light. |
| The pieces of card board with a hole in each. |
| Source of light, eg. a candle. |
| Put the pieces of cardboard with the holes in a straight line. |
| Arrange them to a distance of about 50cm apart. |
| Place a lighted candle at one end of cardboard A. observe the light from behind cardboard |
| Now displace any one of the pieces of cardboard, eg. Cardboard B. |
| Propagation of light in straight line. |
| When observing from card C, the candle can be seen. Displacing any one of cardboards, the |
| candle can no longer be seen as the holes are no longer in a straight line. This happens due to the |
| fact that light travels in a straight line. |
| Transparent, Translucent and Opaque Materials |
| Identify transparent, translucent and opaque materials |
| Objects can be grouped according to transmission of light through them, ie. The degree passage |
| of light as light rays falls on their surface. |
| These are bodies which do not allow light to pass through them, ie. Stones, wood, concrete walls |
| These are bodies which allow small amount of light falling on them to pass through e.g oiled |
| paper, some plastic materials and tinted glass. |
| These are bodies which allow all light to pass through them, e.g glass, pure water and air. The |
| transmission of light through different bodies can be investigated by performing other |
| Aim: To investigate transmission of light. |
| Which of the materials are opaque? |
| Which of the materials are transparent? |
| The Concept of Reflection of Light |
| Explain the concept of reflection of light |
| Reflections are obtained from hard and highly polished surfaces such as mirrors and sheets of |
| glass than from rougher surfaces, when light falls on the surface at an inclined plane angle, it is |
| sent back into the air. This is also the case when light falls on all highly polished metals. If light |
| falls on a polished surface at an angle 90 |
| , it is sent back into the air on the same path. |
| Difference between Regular and Irregular Reflection of Light |
| Distinguish regular from irregular reflection of light |
| Surface can be classified into two types: |
| Highly polished surfaces, examples are mirror, polished cooking utensil and silvered iron |
| Rough surface, examples are unpolished wooden table, sheet of paper and cobblestone |
| Application of the Laws of Reflection of Light |
| Apply the laws of reflection of light |
| Reflection is guided by some natural laws called “Laws of reflection” the Laws of reflection |
| The incident ray, the reflected ray, and the normal at the point of incidence all lies in the |
| The angle of incidence is equal to the angle of reflection. |
| Image Formed by a Plane Mirror |
| Describe image formed by a plane mirror |
| A clear image is recognised as a result of r eflection of the beam striking the highly polished |
| surfaces. This is called regular reflection. |
| If mirrors were replaced by a piece of sheet of paper, there would be no image to be seen even as |
| a result of diffuse or irregular reflection, this is shown below. |
| Irregular or diffuse reflection. |
| When light rays strike a surface (e.g a mir ror) there are two rays of importance, namely Incident |
| and Reflected rays, see the figure below: |
| Reflection of light rays. |
| MO is a ray of light falling on a mirror; it is called the incident ray. |
| OP is a ray of light leaving the mirr or after reflection; it is called the reflected ray. Line |
| NO is perpendicular to the mirror , it is called the normal. ANGLE MON is called the angle of |
| incidence. (i) this is the angle between the incident ray and the normal , angle NOP is called the |
angle of reflection (t). this is the angle between ref lected ray and the normal.
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