Light Energy – JSS3 Basic Science Lesson Note (Second Term)

Light is a form of energy that travels in straight lines and makes objects visible to us.

Behavioural objectives

By the end of the lesson, students should be able to:

1. explain rectilinear propagation of light and give examples that show it;
2. describe parallel, convergent and divergent beams of light;
3. distinguish between transparent, translucent and opaque objects, and between luminous and non-luminous objects;
4. explain regular and diffuse reflection, and refraction of light;
5. describe dispersion of light and the formation of a rainbow;
6. describe how the eye sees an object.

What is light energy?

Light energy is the form of energy that can do work and lets us see.

• Energy: the capacity to do work.
• Straight-line travel: light moves in straight lines in one medium.
• Reflection: light can be sent back from a surface.
• Refraction: light bends when it passes from one medium to another.
• Dispersion: white light can split into seven colours.
• No medium needed: sunlight reaches the Earth through space.

What is rectilinear propagation of light?

Rectilinear propagation is the travelling of light in a straight line.

• Pinhole camera: it forms an image because light travels in straight lines through the small hole.
• Shadows: an opaque object blocks the straight path of light and leaves a dark area behind it.
• Eclipses: the sun, moon and Earth block each other’s light along straight paths.

What are rays and beams of light?

A ray of light is a straight line along which light travels.

• Ray: a single straight line showing the path of light.
• Beam: a collection of rays moving close together.

There are three types of beam: parallel, convergent and divergent.

1. Parallel beam: the rays travel parallel to each other.
2. Convergent beam: the rays meet at a point.
3. Divergent beam: the rays spread out from a point, away from each other.

What are transparent, translucent and opaque objects?

Objects are transparent, translucent or opaque, depending on how much light passes through them.

• Transparent objects: allow light to pass through, so we see clearly through them, for example glass and clear water.
• Translucent objects: allow some light through, but the source of the light cannot be seen clearly, for example oil paper, wax paper and frosted window glass.
• Opaque objects: do not allow light through. They absorb or reflect it, for example wood, a brick wall, cloth and cardboard.

What are luminous and non-luminous objects?

Luminous objects produce light of their own, while non-luminous objects do not.

• Natural luminous objects: the sun, stars and living things such as glowworms and fireflies.
• Artificial luminous objects: objects made by people that give light, such as electric lamps, kerosene lamps and candles.
• Non-luminous objects: objects that cannot produce light of their own, for example the moon. We see them only when they reflect light from a luminous source.

What is reflection of light?

Reflection is the sending back of light rays when they strike a surface, as in a mirror.

There are two types of reflection: regular and diffuse.

• Regular reflection: parallel rays fall on a smooth, highly polished surface and are reflected in the same direction.
• Diffuse reflection: parallel rays fall on a rough surface and are reflected in different directions, so they scatter.

What are the terms and laws of reflection?

• Normal: a line drawn at a right angle to the surface at the point where the ray strikes.
• Angle of incidence (i): the angle between the incident ray and the normal.
• Angle of reflection (r): the angle between the normal and the reflected ray.

1. The angle of incidence is equal to the angle of reflection (i = r).
2. The incident ray, the reflected ray and the normal lie in the same plane.

What is refraction of light?

Refraction is the change in direction of light when it passes from one medium to another.

• Cause: light changes speed when it moves from one medium to another, so the rays appear bent.
• Air to water: light bends at the surface of a pond or swimming pool.
• Stick or coin in water: it appears raised because of refraction.

What are the laws of refraction?

1. The incident ray and the refracted ray are on opposite sides of the normal. The normal is the line at a right angle to the boundary between the two materials.
2. The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is a constant called the refractive index (n). That is, n = sin i / sin r.

What is apparent depth?

Apparent depth is the depth at which an object under water appears to be when seen from above.

• Coin in a container: viewed from the top, the coin appears raised.
• Apparent depth: it is less than the real depth of the coin at the bottom.
• Clear pool: it looks shallower than it is, so it can be dangerous for amateur swimmers.

What is the difference between reflection and refraction?

Reflection sends light back from a surface, while refraction bends light as it enters another medium.

What is dispersion of light and how does a rainbow form?

Dispersion is the splitting of white light into its seven component colours.

• Spectrum: the band of colours into which white light splits.
• Colours: red, orange, yellow, green, blue, indigo and violet.
• Memory aid: VIBGYOR when read from violet to red.

What is a prism?

A prism is a transparent block of glass, usually triangular, that bends light passing through it.

• Bending: it changes the direction of a light ray moving from air into glass.
• Colours: a triangular prism splits white light into the colours of the rainbow.

How does a prism disperse white light?

1. A ray of white light falls on one face of a triangular prism.
2. The light is refracted as it enters the glass.
3. Each colour bends by a different amount, so the colours separate.
4. The colours leave the prism as a spectrum.

How does a rainbow form?

A rainbow forms when sunlight is dispersed by water droplets in the air.

1. Water droplets are present in the atmosphere, for example after rain.
2. Sun rays enter the droplets and are refracted and split into colours.
3. The light is reflected inside the droplets and refracted again as it leaves.
4. The coloured light reaches the observer’s eyes as a rainbow.

How do we see an object?

We see an object when light from it enters the eye and forms an image on the retina.

1. Light rays from an object enter the eye through the cornea, which refracts them.
2. The rays pass through the aqueous humour, which bends them further, and enter the lens through the pupil.
3. The lens bends the rays again. They pass through the vitreous humour and are focused on the retina, especially on the yellow spot.
4. The rays stimulate the rods and cones and form an inverted image on the retina. The image is smaller than the object.
5. Impulses travel along the optic nerves to the optic lobes of the brain, and then to the visual area of the cerebral hemisphere.
6. The brain interprets the image, so the object is seen in its actual size and colour.

What are the parts of the eye used in seeing?

Evaluation

1. What is rectilinear propagation of light? Give two examples that show it.
2. Explain the difference between transparent, translucent and opaque objects, with one example of each.
3. State the difference between regular reflection and diffuse reflection.
4. What is apparent depth?
5. Give two examples each of luminous and non-luminous objects.

Assignment

Explain why a spoon in a glass of water appears bent and why a clear pool looks shallower than it really is. Use the terms refraction and apparent depth in your answer.