Light Energy JSS3 Basic Science Lesson Note

Energy is the capacity to do work, Light is a form of energy because it can do work. Light energy travels on a straight line, the phenomenon of light traveling on a straight line is called rectilinear propagation of light. The pinhole camera operates on this principle. Other phenomena that show that light travels on a straight line are the  5 j/formation of shadow and eclipses. A ray of light is a straight line along which a light ray travels. A beam of a collection of parallel rays of light moving close together A beam of light may be

(1)  Parallel – When the rays of light travel parallel

2).  Convergent of rays – when the rays of light meet at a point

3). Divergent rays – When the rays of light spread from a point away from each other

CONCEPT OF LIGHT TRANSMISSION

When light rays fall on a surface it may result in the following

(i)  Reflection_ This is the sending back of rays of light from a smooth or highly polished surface on which it falls e. g mirror or other smooth surface

(ii) Refraction – This is the passage of light rays from one medium to another.  It makes the light rays appear bent. E.g. light rays from air to water. This is the reason why a stick or coin put in a glass of water appears raised.

(iii)Transparent objects are objects that can allow light rays to pass through them. Those objects are said to be transparent, e g glass and clear water.

(iv)Opaque objects are objects that do not allow light to pass through them rather it absorbs the light rays, eg wood, or brick wall. Cloth and cardboard

(v)Translucent objects are objects that allow light to pass through them but the object from which the light comes cannot be seen. For example, oil paper, wax paper, and glass sheets used for certain windows in the house

NATURAL  LUMINOUS  OBJECTS

Luminous objects are objects that produce light of their own. Examples are the sun, stars, and some living creatures such as glowworms and fireflies.

Artificial Luminous objects are objects such as electric lamps, kerosene lamps, and candles.

Non-luminous objects are those objects that cannot produce light of their own, e.g. the moon.

REFLECTION OF LIGHT -When a ray of light strikes a surface is reflected. The reflection may be regular or diffuse.

REGULAR reflection is when a parallel ray of light is reflected in the same direction when falling on a smooth surface or highly polished surface.

DIFFUSE  reflection occurs when a parallel ray of light is reflected in different directions, the rays become scattered or diffuse.

I= Angle of incidence is the angle the incident ray makes with the normal r= angle of reflection is the angle between the normal and reflected ray. Both I and r are equal.

Refraction of light  Refraction is the change in the direction of light when it passes from one medium to another. An example of a medium that can refract a light ray is a pond or swimming pool. 

The law of refraction 

1. The incident ray and the refracted ray are all on the refracted rays are on the opposite side of the normal. The normal is the line drawn to be at a right angle to the boundary or both materials
2. The angle of incident is equal to the angle of refraction
3. The ratio of the sine of the angle of incident (i) to the sine of the angle of refraction (r0) is referred to as refractive index refractive index “n” = sinio/sinro

 APPARENT DEPTH

When a coin is put in a transparent cup or container containing water. When viewed from the top, the coin appears raised, in which case its depth is less than the actual depth of the coin at the bottom of the beaker. The depth at which it appears to be seen by the eye is called Apparent depth.

Dispersion and rainbow When a ray of white light passes a glass prism, especially a triangular prism. The ray of light is split into seven component colors. The phenomenon is called the dispersion of light. The series of colors into which light is split is called a spectrum the component colors are red, orange, yellow, green, blue, indigo, and violet ( VIGBYOR) The Same phenomenon causes the formation of a rainbow it happens when sin rays pass through water droplet in The atmosphere. The seven colors of a rainbow formed in the sky by a droplet of water which hang in the                                                                                               

Color prism is rectangular or triangle blocks of glass that change the direction of a light ray when it passes through them from another medium e. g  air to glass. The prism bends light rays and produces colored images-the rainbow colour.

Mechanism of Seeing

Light rays from an object enter the eye  through  the cornea  which causes  the light rays  to be refracted and pass through the aqueous humour which further

 Bends the light rays before entering the lens through the pupil. The lens further bends the light rays and focuses them on the retina, especially on the yellow spot. The light rays pass through the vitreous humuor . Further bending of the light rays occurs again. The light rays reaching the retina stimulate the rods and cones forming an inverted image on the retina. The inverted image is smaller in size than the object. Impulses are sent through the optic nerves the optic lobes of the brain, and then to the visual cerebral hemisphere where the actual size and colour of the image are interpreted correctly.                                        

 ASSIGNMENTS

1, Which of the following is the reason a clear pool of water is dangerous for amateur swimmers A. It is unsuitable for swimming B. It appears to be shallower than its real depth C. It always contains dangerous fish D. It frightens young swimmers.

2. On which of the following does the normal human eye form its image?  A. lens  B. pupil  C. retina  D. rainbow
3. When light passes through the air to water or glass, there is  A. reflection  B. problem  C. incidence  D. refraction.
4. The separation of white light into various components of colours through a glass prism is known as  A. dispersion  Convergent  Divergent  Spectrum.

5a Explain why a spoon in a glass cup of water appears to be bent

1. How does the eye see objects both far and near

c.Explain the meaning of dispersion of light. How does it relate to the rainbow in the sky?

1. Mention four properties of light
2. State the laws of reflection
3. What is refraction