# Light Energy

FIRST TERM E-LEARNING NOTE

SUBJECT: BASIC SCIENCE

CLASS: JSS 3

WEEK EIGHT

LIGHT ENERGY

CONTENT

• Properties of light.
• Lenses and glasses.

Light can be defined as a form of energy which produces electromagnetic radiation capable of causing visual sensation. Light is a kind of electromagnetic wave (waves that can travel in vacuum). Light is the fastest substance known to man. Light travels at a speed of 3×108 m/s (300,000,000 metres per second or 299,792.5 km/s). Light travels in a straight line in what is called Rectilinear propagation of light until it encounters an obstacle. When light cannot pass through an object, shadow is formed. An object that does not allow light to pass through it is an opaque object while those objects that allow light to pass through them are called transparent object.

An object that can produce light on it’s own is called luminous e.g. sun, light bulb, fire. While an object that does not produce light but may reflect it is called non luminous e.g. the moon, mirror, stone, chair etc.

Light waves are also called light rays. A line with an arrowhead is used to represent a ray of light. A collection of light rays is called light beam.

There are there type of beam. They are:

1. Parallel beam: This is the type of beam in which the light rays are parallel to one another.
2. Divergent beam: This is the type of beam in which the ray scatters from a point which is usually the source, a convex mirror or a concave lens.
• Convergent beam: This is the type of beam in which the rays converge or meet at a point called focus.

REFLECTION OF LIGHT

Reflectionoflightis the change in the direction of light after dashing against a surfray. Reflection involves two rays of light: an incoming incident ray and an outgoing reflected ray.

LAW OF REFLECTION

1. The angle of incident ray, the normal and reflected ray at the point of incidence all lie on the same plane.
2. The angle of incidence (i) and the angle of reflection (r) are equal but on opposite sides of the normal. i = r

N.B: The normal is an imaginary line at right angle to the mirror at the point of incidence.

TYPES OF REFLECTION

1. Regular or specular reflection: This is when parallel beam of light is reflected in one direction. Such reflection takes place on smooth, polished surface such as a plane mirror.
2. Diffuse reflection: This is when a parallel beam of light is reflected in different directions. Such reflection takes place from a rough surface, such as water surface or paper surface.

EVALUATION

1. What is reflection?
2. State the laws of reflection
3. What the types of reflection?

REFRACTION

Refraction is the bending or change in the direction of wave when it moves from one medium to another where its speed is different. A medium is a material or space that allows wave pass through it. Refraction is responsible for image formation by lens and the eye.

When you dip your legs into a pool, it disappears bent in the pool. A fish in an aquarium seem to radically change position as it is being viewed from different view points. These are as a result of refraction.

LAWS OF REFRACTION

1. The first law of refraction states that the incident ray, the refracted ray and the normal at the point of incidence all lie on the same plane.
2. The second law states that the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is a constant for a given pair of media.

This is also known as Snell’s law and the ratio of “I” to “r” is called the refractive index.

EVALUATION

1. What is refraction?
2. State the laws of refraction.

LENSES AND GLASS PRISMS

Lenses are curved glasses that disperse or coverge light to a focus.

There are two main types of lenses:

1. Concave lens (diverging lens): This is a lens that bulges outward. A concave lens diverges or scatters light. It is used to correct shortsightedness (myopia)
2. Convex lens (converging lens): This lens bends inward. It converges light to a real focus. It is used to correct longsightedness (hypermetropia).

There are also two types of glass prisms:

(i)  Rectangular glass prisms   (ii)  Triangular glass prisms

EVALUATION

1. What is a lens?
2. Mention the types of lenses.

VISION

Vision is the ability to create an image. The organ of the body that is concerned with vision is the eye. Light enters through a clear covering of the eye called the cornea. It then passes through the adjustable opening in the iris called the pupil. Beyond the pupil is the lens which is a bit soft and flexible. The lens focus image on the retina while like a film or screen at the back of the eye.

The eye and camera are alike in function and arrangement of part. Both are compared for a lot of reasons.

 Part of the eye Similar part in camera Function Cornea A clear covering of the eyes Iris Diaphragm This is coloured part of the eye. It control the amount of light entering into the eyes Lens Lens The lens converges light at the retina for images to be formed Retina Film or Screen The retina is the part of the eyes where images are formed. Pupil Aperture This is a small opening in iris that controls the amount of light that enters into the eyes.

DISPERSION OF LIGHT

This is the splitting or separation of light into different colours. Each colour bends by different degrees when refracted. The colours of light that are gotten after separation are called spectrum. Dispersion of light can be done by passing light through a triangular prism.

RAINBOW

A natural phenomenon that demonstrates the dispersion of light is the the formation of rainbow. The rainbow is an arc of light with an angular radius of 42° centred on a direction which is opposite to that of the sun in sky (I.e it is centered on the direction of propagation of the sun rays). Thus, if the sun low in the sky (I.e close to the horizon) we see almost a full circle. If the sun is higher up in the sky we see a smaller arc. If the sun is more than the angular radius 42° above the horizon, then there will be no rainbow (for viewers on ear surface).

Observers on a hill may see part of the rainbow below the horizontal i.e an arc greater than a semicircle. Passengers on airplane can sometimes see a full circle. The colours of the rainbow vary smoothly from red on the outside and violet on the inside. There are seven colours of the rainbow; red, orange, yellow, green, blue, indigo and violet.

In the dispersion of light, a ray of enters into a triangular glass prism and it is dispersed into colours seven colours written as ROYGBIV.

EVALUATION

1. What is vision?
2. What is dispersion of light?
3. Mention five parts of the eye and the function they perform

GENERAL EVALUATION

1. What is dispersion of light?
2. List five part of the eye.
3. What is genetic counseling?
4. State three factors that affect sense of individual worth.
5. State four communication technique.

WEEKEND ASSIGNMENT

1. Which of the following is not a colour of the rainbow Red B. Blue  C. Brown  D. Indigo
2. The property of light in straight line is Rectitude propagation  B. Rectilinear propagation  C. Resisted promulgation  D. Responsible promulgation
3. The part of the camera that performs the function of the pupil is Aperture  B. Lens  C. Cornea  D. Diaphragm
4. The initial ray of light from the source is called Incident ray  B. Reflected ray  C. Refracted ray  D. Emergent ray.
5. An object that absorbs all blue light will appear Black  B. Blue  C. Yellow  D. White

THEORY

1. What is light?
2. Explain the formation of rainbow
3. Mention the types of lenses
4. What is reflection? Mention the types of reflection
5. Draw a well labelled eye